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Red imported fire ant

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Red imported fire ant
Fire ants02.jpg
Scientific classification
Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Order: Hymenoptera
Family: Formicidae
Subfamily: Myrmicinae
Tribe: Solenopsidini
Genus: Solenopsis
Species: S. invicta
Binomial name
Solenopsis invicta
Buren, 1972
Synonyms[1]
  • Solenopsis saevissima wagneri Santschi, 1916

The red imported fire ant (Solenopsis invicta), or simply RIFA, is one of over 280 species in the widespread genus Solenopsis. Although the red imported fire ant is native to South America, it has become a pest in the southern United States, Australia, the Caribbean, Taiwan, Hong Kong,the southern Chinese provinces of Guangdong, Guangxi and Fujian, and Macau. RIFAs are known to give a painful, persistently irritating sting that often leaves a pustule on the skin.

Etymology

The specific epithet of the red imported fire ant, invicta, is Latin for "invincible" and "unconquered".[2][3] This derives from the phrase Roma invicta ("unconquered Rome"), used as an inspirational quote until the fall of the Western Roman Empire in 476 AD. This symbolic statement was printed on minted coins.[4] The generic name of the red imported fire ant, Solenopsis, translates to "appearance" or "face". The word solen, meaning "pipe" or "channel", derives from Ancient Greek, and opsis, another Greek word, means "appearance" or "sight".[5][6] Aside from its common name "red imported fire ant" (which is abbreviated as RIFA),[7] they are also known as "fire ants", "red ants" or "tramp ants".[8][9] In French and German, they are known as "fourmi de feu" and "rote importierte Feuerameise".[10]

Taxonomy

Paratype specimen of S. invicta collected from Brazil

The red imported fire ant was first described by Swiss entomologist Felix Santschi in a 1916 journal article published by Physis.[11] Originally named Solenopsis saevissima wagneri from a syntype worker he collected in Santiago del Estero, Argentina, Santschi believed the ant was a variant of S. saevissima. It was also named after E.R. Wagner. Santschi includes a brief description about its length, colour, and the shape of the propodeum.[11] The worker is currently held in the Naturhistorisches Museum Basel in Switzerland, but additional specimens may be housed in the Muséum national d'histoire naturelle in Paris.[12] Santschi would later provide another description of the ant in a 1923 paper reviewing the ants of the Neotropics, where he recorded examined material from Bolivia and Paraguay.[13] In 1930, American myrmecologist William Creighton reviewed the genus Solenopsis and reclassified the taxon as Solenopsis saevissima electra wagneri at infrasubspecific rank, noting that he could not collect any workers that referred to Santschi's original description.[14] In 1952, the S. saevissima species complex was examined and together with nine other species-group names, S. saevissima electra wagneri was synonymised with S. saevissima saevissima.[15] This reclassification was accepted in George Ettershank's revision of the genus and in Walter Kempf's 1972 catalogue of Neotropical ants.[16][17]

In 1972, American entomologist William Buren described what he thought was a new species, Solenopsis invicta.[18] Buren provided the first official description of the ant in a 1972 journal article published by the Georgia Entomological Society, where he collected a holotype worker from Cuiabá in Mato Grosso, Brazil. In this article, Buren accidentally misspelled the specific name as invica above the description pages of the species. However, it was clear that the correct name was invicta, owing to the rest of the article using invicta and discussion of its etymology. Therefore, invicta was the intended spelling.[19] The type material is currently housed in the National Museum of Natural History, Washington, D.C.[18] The species was included in the S. saevissima species complex. This group composed of S. interrupta, S. quinquecuspis and S. saevissima.[18] Buren notes that this species and relatives of the complex are distinguished by workers having 10 segments, queens having 11 and males at 12. A two-jointed club at the end of the funiculus in workers and queens is present, and the second and third segments of the funiciulus are twice as long and broad in larger workers. Polymorphism occurs in all species and the mandibles bear four teeth. Pilosity consists of erect hairs in a variety of lengths.[18]

Casent label of S. invicta paratype worker

In a 1991 review of the species complex, American entomologist James Trager synonymised S. saevissima electra wagneri and S. wagneri, moving S. invicta and relatives to the S. geminata species group.[19] Trager incorrectly cites Solenopsis saevissima electra wagneri as the original name, erroneously believing that the name S. wagneri was unavailable, and used Buren's name Solenopsis invicta. Previously, Trager believed that S. invicta was conspecific with S. saevissima until comparing the material with S. wagneri. Trager notes that even though S. wagneri has priority over S. invicta, the name was never used above infrasubspecific rank. The use of the name since Santschi has not been associated with collected specimens, and as a result is nomen nudum.[19] In 1995, English myrmecologist Barry Bolton corrected Trager's error, recognising S. wagneri as the valid name and synonymised Solenopsis invicta.[20] He states that Trager wrongfully classified S. wagneri as an unavailable name and cites S. saevissima electra wagneri as the original taxon. He concludes that S. wagneri is, in fact, the original name and has priority over S. invicta.[20]

In 1999, Steve Shattuck and colleagues proposed the conservation of the name S. invicta.[12] Since the first description of S. invicta, over 1,800 scientific papers were published using the name discussing a wide range of topics about its ecological behaviour, genetics, chemical communication, economic impacts, methods of controlling them, population and physiology. They state that the use of S. wagneri is a "threat" to nomenclatural stability towards scientists and non-scientists; taxonomists may have been able to adapt to such name change, but name confusion may arise if such case occurred. Due to this, Shattuck and colleagues proposed the continued use of S. invicta and not S. wagneri, as this name has been rarely used; between 1995 and 1998, over 100 papers were published using S. invicta and only three for S. wagneri. They requested that the International Commission on Zoological Nomenclature uses plenary powers to suppress S. wagneri in accordance with the Principle of Priority and not with the Principle of Homonymy. Furthermore, they requested that the name S. invicta should be added to the Official List of Specific Names in Zoology and add S. wagneri to the Official Index of Rejected Invalid Specific Names in Zoology.[12] Upon review, the proposal was universally accepted by the entomological community, with the exception of one voter. They note that while they agree with the objective of the proposal, they say that the taxonomy of the group is far from stable and subjective synonymy will most likely occur in the future. They further note that there is no justification suppressing S. wagneri; instead, it would be better to give precedence to S. invicta over S. wagneri whenever an author treated them as conspecific. The ICZN would eventually conserve S. invicta and suppress S. wagneri in a 2001 review.[21]

Genetics

Phenotypic and genetic data suggests that the red imported fire ant and the black imported fire ant (Solenopsis richteri) differentiate from each other, but they do share a close genetic relationship.[22][23] Hybridisation between the two ants occurs in areas they make contact, with the hybrid zone located in Mississippi. Such hybridisation has resulted from secondary contact between these two ants and several decades ago, where they first encountered each other in southern Alabama.[22] A social chromosome is present in the red imported fire ant; this chromosome can differentiate the social organisation of a colony, where it carries one of two variants of a "supergene" (B and Bb) which contains more than 600 genes. For example, colonies exclusively carrying the B variant of this chromosome have been found to accept a single BB queen, but colonies with a number of B variants will accept multiple Bb queens.[24] Differences in another single gene can also decide whether the colony will have single or multiple queens.[25]

Studies show that mitochondrial DNA variation occurs substantially in polygyne societies (nests with multiple queens), but no variation is detected in monogyne societies (nests with a single queen).[26] Triploidy occurs in red imported fire ants at high rates (as high as 12% in non-reproductive females), which is linked to the high frequency of diploid males.[27] The red imported fire ant is the first species shown to possess a green-beard gene, by which natural selection can favour altruistic behaviour. Workers containing this gene are able to distinguish between queens containing this gene and those that do not by apparently using odor cues. The workers kill queens that do not contain the gene.[28] In 2011, scientists announced they had fully sequenced the red imported fire ant genome from a male.[29]

Description

Closeup face view of a worker

Red imported fire ant workers range in size from small to medium, making them polymorphic. Workers measure between 2.4 to 6 millimetres (0.094 to 0.236 in).[30] The head measures 0.66 to 1.41 millimetres (0.026 to 0.056 in) and is 0.65 to 1.43 millimetres (0.026 to 0.056 in) wide. In the larger workers (as in the major workers), their heads measure 1.35 to 1.40 millimetres (0.053 to 0.055 in) and 1.39 to 1.42 millimetres (0.055 to 0.056 in) wide. The antennal scapes measure 0.96 to 1.02 millimetres (0.038 to 0.040 in) and the thoracic length is 1.70 to 1.73 millimetres (0.067 to 0.068 in).[18] The head becomes wider behind the eyes with rounded occipital lobes present, and unlike the similar-looking S. richteri, the lobes peak further than the midline, but the occipital excision is not as crease-like. The scapes in major workers do not extend beyond occipital peak by one or two scape diameters; this feature is more noticeable in S. richteri. In medium-sized workers, the scapes reach the occipital peaks and exceed the rear border in the smallest workers. In small and medium workers, the head tends to have more elliptical sides. The head of the small workers is wider out the front than it is behind.[18] In the major workers, the pronotum does not have any angular shoulders, nor does it have any sunken posteromedian area. The promesonotum is convex and the propodeum base is rounded and also convex. The base and declivity are of equal length. The suture of the promesonotum is either strong or weak in larger workers. The petiole has a thick and blunt scale; if observed behind, the scale is not as rounded above in contrast to S. richteri, and sometimes it may be subtruncate. The postpetiole is large and broad, and in the larger workers, it is broader than its length. The postpetiole tends to be less broad in front and more behind. On the rear side of the dorsal surface, a transverse impression is present. In S. richteri, this feature is also present but much weaker.[18]

Whole body image of an S. invicta queen

The sculpture is very similar to S. richteri.[18] The punctures are where pilosity arises from, and these are often elongated on the dorsal and ventral portions of the head. On the thorax, striae are present, but they are less engraved with fewer punctures than in S. richteri. On the petiole, the punctates are located on the sides. The postpetiole, when viewed above, has a strong shagreen with distinct transverse punctostriae. The sides are covered in deep punctures, where they appear smaller but deeper. In S. richteri, the punctures are larger and more shallow. This gives a more opaque appearance to the surface. In some cases, punctostriae may be present around the rear portion.[18] The pilosity appears similar to that of S. richteri. These hairs are erect and vary in length, appearing long on each side of the pronotum and mesonotum; on the head, the long hairs are seen in longitudinal rows. There are numerous appressed pubescent hairs on the petiolar scale; this is the opposite in S. richteri, as these hairs are sparse.[18] Workers appear red and somewhat yellowish with a brown or completely black gaster.[11] Gastric spots are sometimes seen in larger workers, where they are not as brightly coloured as those in S. richteri. The gastric spot usually covers a small portion of the first gastric tergite. The thorax is concolorous, ranging from light reddish-brown to dark-brown. The legs and coxae are usually lightly shaded. The head has a consistent colour pattern in large workers, with the occiput and vertex appearing brown. Other parts of the head including the front, genae and the central region of the clypeus are either yellowish or yellowish brown. The anterior border of the genae and mandibles are dark-brown; they also both appear to share the same coloured shade with the occiput. The scapes and funiculi range from being the same colour as the head or shares the same shade with the occiput. Light coloured areas of the head in small to medium-sized workers is restricted to only the frontal region, with a dark mark resembling an arrow or rocket being present. On occasions, nests may have a series of different colours. For example, workers may be much darker, and the gastric spot may be completely absent or appear dark-brown.[18]

Whole body image of a male

Queens have a head length of 1.27 to 1.29 millimetres (0.050 to 0.051 in) and a width of 1.32 to 1.33 millimetres (0.052 to 0.052 in).[18] The scapes measure 0.95 to 0.98 millimetres (0.037 to 0.039 in) and the thorax is 2.60 to 2.63 millimetres (0.102 to 0.104 in). The head is almost indistinguishable from S. richteri, but the occipital excision is less crease-like and the scapes are considerably shorter. Its petiolar scale is convex and resembles that of S. richteri. The postpetiole has straight sides that never concave, unlike in S. richteri where they concave. The thorax is almost identical, but the clear space between the metapleural striate area and propodeal spiracles is either a narrow crease or not present. The side portions of the petiole are punctate. The sides of the postpetiole are opaque with punctures present, but no irregular roughening is seen. The anterior of the dorsum is shagreen, and the middle and rear regions bear transverse puncto-striae. All these regions have erect hairs. The anterior portions of both the petiole and postpetiole have appressed pubescence that is also seen on the propodeum. The colour of the queen is similar to that of a worker: the gaster is dark brown and the legs, scapes and thorax are light brown with dark streaks on the mesoscutum. The head is yellowish or yellowish-brown around the central regions, the occiput and mandibles are a similar colour to the thorax, and the wing veins range from colourless to pale brown.[18] Males appear similar to S. richteri, but the upper border of the petiolar scales are more concave. In both species, the postpetiole's and petiole's spiracles strongly project. The whole body of the male is concolorous black, but the antennae are whitish. Like the queen, the wing veins colourless or pale brown.[18]

Whole body image of S. invicta

The red imported ant can be misidentified as the similar looking S. richteri.[18] The two species can be distinguished from each other through morphological examinations of the head, thorax and postpetiole. In S. richteri, the sides of the head are broadly elliptical-shaped and the cordate shape seen in the red imported fire ant is absent. The region of the occipital lobes that are situated nearby the midline and occipital excision appear more crease-like in S. richteri than it does in the red imported fire ant. The scapes of S. richteri are longer than they are in the red imported fire ant, and the pronotum has strong angulate shoulders. Such character is almost absent in the red imported fire ant. A shallow but sunken area is only known in the larger workers of S. richteri, which is located in the posterior region of the dorsum of the pronotum. This feature is completely absent in larger red imported fire ant workers. The red imported fire ant has a promesonotum that is strongly convex, whereas this feature is weakly convex in S. richteri. Upon the examination of the base of the propodeum, it is elongated and straight in S. richteri while convex and shorter in the red imported fire ant. It also has wide postpetiole with either straight or diverging sides posterior. The postpetiole in S. richteri is narrower with converging sides. In S. richteri, the transverse impression on the posterodorsal portion of the postpetiole is strong but weak or absent in the red imported fire ant.[18]

Brood

Eggs are tiny and appear oval-shaped, where they remain the same size for approximately a week. After one week, the egg will assume the shape of an embryo and form as a larva when the egg shell is removed.[31] Larvae measure 3 millimetres (0.12 in).[32] It shows a similar appearance to S. geminata larvae, but they can be distinguished by the integument with spinules on top of the dorsal portion of the posterior somites. The body hairs measure 0.063 to 0.113 millimetres (0.0025 to 0.0044 in) with a denticulate tip. The antennae both have two or three sensilla. The labrum is smaller with two hairs on the anterior surface that are 0.013 millimetres (0.00051 in). The maxilla has a sclerotised band between the cardo and stipes. The labium also has a small sclerotised band.[32] The tubes of the labial glands are known to produce or secrete a proteinaceous substance that has a rich level of digestive enzymes, which includes proteases and amylases that function as an extraintestinal digestion of solid food. The midgut also contains amylases, roteases and upases. The narrow cells in its reservoir have little to no function in secretion.[33] The pupae resemble adults of any caste, except that their legs and antennae are held against the body tightly. They appear white, but over time, the pupae will eventually turn darker when they are almost ready to mature.[34]

Four larval instars have been described based on their morphological characters.[31][35] With the exception of size differences, the larvae of the minor and major workers are almost impossible to distinguish. Major worker 4th instar larvae tend to be larger than minor worker prepupae with a wider width. Only the reproductive larvae were larger than major workers and in general are very robust. Even male 3rd instar larvae tend to be longer and larger than 3rd instar workers. No significant morphological difference can distinguish male and queen larvae, but the internal gonopodal imaginal discs differ. Most larval changes are not significant, but the moulting of 3rd instar larvae and studies of their eclosion support four larval instar stages.[31]

Red imported fire ant brood excavated from a poisoned mound

The larvae in their first instar measure 0.2 millimetres (0.0079 in) and grow up to 0.6 millimetres (0.024 in) at ecdysis. The body is translucent, short and stubby with nine somites.[35] There is no hair present, but small microsetae is found on the mouthparts and head, as well as the anteroventral body region.[31] Spinules are present around the posterior of the anus and the ventral surface. The head is large and twice the diameter of the body and the labrum is short but twice as long as its width.[35] The width of the head is 0.14 to 0.16 millimetres (0.0055 to 0.0063 in), and the antennae have three sensilla. The mouthparts are unsclerotised; in contrast, the mandibles are sclerotised around the lateral margins.[31][35] The length of the mandibles is slightly greater than its width with a distinct apical tooth. The maxillae are concave and the labium is slightly broader than it is long.[35] The maxillary palps are poorly developed but appear to have three sensilla. The presence of the galea is not evident.[31][31] Larvae in their second instar measure 0.64 millimetres (0.025 in) and grow to 0.84 millimetres (0.033 in) by the time they are about to moult.[35] The body is slender and tapers anteriorly towards the prothorax, where somites are noticeable. Hairs are seen all over the body and head, but they are small and simple.[35] Rows of hairs are also found along the ventral and perianal region. The head width increases to 0.16 to 0.19 millimetres (0.0063 to 0.0075 in); the antennae still have three sensilla. The mouthparts are still unsclerotised and the labrum has four dorsal hairs and 6 sensilla. The mandibles appear similar to that of 1st instar larvae, but the apical tooth and subapical tooth are more noticeable. Two small papillae are also seen. The maxilla bears two large microsetae and the palp is poorly developed but noticeable. The galea is present but poorly developed with five sensilla. The labium has large setae, and labial spinules are more prominent.[31]

Larvae in their third instar grow to 1.04 millimetres (0.041 in), with bifid hairs measuring 0.25 millimetres (0.0098 in) and curved hairs at 0.03 millimetres (0.0012 in).[35][35] The head is smaller than the body, but it is as long as its width; the head width increases to 0.21 to 0.25 millimetres (0.0083 to 0.0098 in).[31] The antennae bear three sensilla, both of which have a spinule. The labrum appears the same as it did in 2nd instar; the antennae also appear the same as it did in 2nd instar, but the spinules are long. The mandibles bear three teeth with a strongly sclerotised apical tooth.[31][35] These teeth are sharper and longer with two or three papillae present. The palp and galea are well developed, bearing the same pattern of sensilla seen on the maxillary palps. The palps are similar to the maxillary palps, but they are less prominent.[31] Larvae in their final instar stage reach 1.32 to 1.82 millimetres (0.052 to 0.072 in) or more, and the bifid and curved hairs are even longer: both measure 0.35 millimetres (0.014 in) and 0.04 millimetres (0.0016 in) respectively.[31][35] These hairs share a similar pattern found in the 3rd instar, as well as the posterior spinules. the head width increases to 0.26 to 0.33 millimetres (0.010 to 0.013 in). The only morphological difference with the antenna is that its spinules are more tapered.[31] The labrum is seen with a series of small protruding bumps, the mandibles are partially sclerotised and the maxillae are rounded with palps present.[31][35] The galea, labial palp, labium, maxillae and maxillary palp have a consistent pattern of sensilla and hairs seen in the 3rd instar, but they are larger. The length of the galea is almost greater than its width, and the opening of the sericteries are developed.[31]

Polymorphism

Anatomical differences of red imported fire ant workers. The scale bar is 1 mm.

As discussed above, the red imported fire ant is polymorphic with two different castes of workers, minor workers and major workers (soldiers). Like many ants that exhibit polymorphism, young, smaller ants do not forage and tend to the brood instead, while the larger workers go out and forage.[36] In incipient colonies, polymorphism does not exist, but instead they are occupied by monomorphic workers called "minims" or "nanitics"; the average head-width in tested colonies increases during the first six months of development.[37][38] As a colony grows and is around five years old, the head width of minor workers decreases but for major workers the headwidth remains the same. The total weight of a major worker is twice that of a minor worker when they first arrive, and by 6 months of age, major workers are four times heavier than minor workers. Once major workers develop, they can take up a large portion of the workforce, with as many as 35% being major workers in a single colony.[37] This does not affect colony performance, as polymorphic colonies and nests with small workers produce brood at roughly the same rate, and polymorphism is not an advantage or disadvantage when food sources are limited. However, polymorphic colonies are more energetically efficient, and under conditions where food is limited, polymorphism may provide a small advantage in brood production but this depends on the levels of food stress.[39]

One study shows that as worker ants grow to larger sizes, the shape of the head changes. This is due to the head length growing the same time as the total body length, and the head width may grow by 20%. The length of the antennae only grows slowly; the antennae may only grow 60% longer by the time the body doubles its length, thus the relative antennal length decreases by 20% as the length of the body doubles.[38] All individual legs of the body are isometric with body length. This means that even when the length of the body doubles, the legs will also double. However, not all of the legs are the same length; the prothoracic portion accounts for 29% of leg length, the mesothoracic at 31% and the metathoracic at 41%. The first two pairs of legs are of equal length to one another, whereas the final pair is longer.[38] Overall, the morphological appearance of a worker changes dramatically when it grows larger. The head exhibits the greatest shape change and the height of the alinotum grows quicker than its length, where there is a height/length ratio of 0.27 in minor workers and 0.32 in major workers.[38] Due to this, larger workers tend to have a humped-shape and robust alinotum in contrast to smaller workers. No petiole segment exhibits any change in shape as the size of the body changes. The width of the gaster grows more rapidly than its length, where the width may be 96% of its length but increases to 106%.[38]

Physiology

Photomicrograph of sting apparatus components

Like other insects, the red imported fire ant breathes through a system of gas-filled tubes called tracheae connected to the external environment through spiracles. The terminal tracheal branches (tracheoles) make direct contact with internal organs and tissue. The transport of oxygen to cells (and carbon dioxide out of cells) occurs through diffusion of gases between the tracheoles and the surrounding tissue and is assisted by a discontinuous gas exchange.[40] As with other insects, the direct communication between the tracheal system and tissues eliminates the need for a circulating fluid network to transport O2.[41] Thus, red imported fire ants and other arthropods can have a modest circulatory system though they have highly expensive metabolic demands.[42]

The excretory system consists of three regions. The basal region has three cells found within the posterior portion of the midgut. The anterior and superior cavity are formed by the bases of four malpighian tubules.[43] The superior cavity opens into the lumen of the small intestine. The rectum is a large but thin-walled sac that occupies the posterior fifth of the larvae. The release of waste is controlled by the rectal valves that lead to the anus.[43] Sometimes, the larvae secrete a liquid that consists of uric acid, water and salts.[44] These contents are often carried outside by workers and rejected, but colonies under water stress may consume the contents.[43] In the reproductive system, queens release a pheromone that prevents dealation and oogenesis in virgin females; those tested in colonies without a queen begin oocyte development after dealation and take up the egg-laying role.[45] Flight muscle degeneration is initiated by mating, and juvenile hormones, and prevented by corpus allatectomy.[46][47] Histolysis begins with the dissolution of the myofibril and the slow breakdown of the myofilaments. Such dissolution continues until it reaches the only free Z-line materials, which would also disappear; only the nuclei and lamellar bodies remain.[43] In one study, the amino acids increase in the hemolymph after insemination.[48] The glandular system contains four glands, the mandibular, maxillary, labial and post-pharyngeal glands.[43] The post-pharyngeal is well developed in the queen while the other glands are larger in workers. The post-pharyngeal gland functions as a vacuum to absorb fatty acids and triglycerides, as well as a gastric caecum.[49] The functions of the other glands remain poorly understood. In one study discussing the enzymes of the digestion system of adult ants, lipase activity was found in the mandibular and labial glands, as well as invertase activity. The Dufour's gland is found in the ant and acts as a source of trail pheromones, although scientists believed the poison gland was the source of the queen pheromone.[43][50] The neurohormone pheromone biosynthesis activating neuropeptide (PBAN) is found in the ant that activates the biosynthesis of pheromones from the Dufour's gland.[51] The spermatheca gland is found in queens, which functions in sperm maintenance. Males appear to lack these glands, but those associated with its head are morphologically similar to those found in workers, but these glands may act differently.[49]

Water loss rates of workers and female alates in S. invicta and S. richteri

The ant faces many respiratory challenges due to its highly variable environment, which can cause increased desiccation, hypoxia, and hypercapnia. Hot, humid climates cause an increase in heart rate and respiration which increases energy and water loss.[41][52] Hypoxia and hypercapnia can result from red imported fire ants colonies living in poorly ventilated thermoregulatory mounds and underground nests. Discontinuous gas exchange (DGE) may allow ants to survive the hypercapnic and hypoxic conditions frequently found in their burrows;[40] it is ideal for adapting to these conditions because it allows the ants to increase the period of O2 intake and CO2 expulsion independently through spiracle manipulation. The invasion success of the red imported fire ant may possibly be related to its physiological tolerance to abiotic stress, being more heat tolerant and more adaptable to desiccation stress than S. richteri. This means that the ant is less vulnerable to heat stress and desiccation stress. Although S. richteri has higher water body content than the red imported fire ant, S. richteri was more vulnerable to desiccation stress. The lower sensitivity to desiccation is due to a lower water loss rate.[53] Colonies living in unshaded and warmer sites tend to have a higher heat tolerance than those living in shaded and cooler sites.[54]

Metabolic rate, which indirectly affects respiration, is also influenced by environmental temperature. Peak metabolism occurs at about 32 °C.[55] Metabolism, and therefore respiration rate, increases consistently as temperature increases. DGE stops above 25 °C, although the reason for this is currently unknown.[56] Temperature plays a role in colony growth and development; colony growth ceases below 24 °C and developmental time decreases from 55 days at temperatures of 24 °C to 23 days at 35 °C. The time required for each cycle (egg, larvae and pupae) was independent of temperature.[57]

Respiration rate also appears to be significantly influenced by caste. Males show a considerably higher rate of respiration than females and workers, due, in part, to their capability for flight and higher muscle mass. In general, males have more muscle and less fat, resulting in a higher metabolic O2 demand.[56] While the metabolic rate is highest at 32 °C, colonies often thrive at slightly cooler temperatures (around 25 °C). The high rate of metabolic activity associated with warmer temperatures is a limiting factor on colony growth because the need for food consumption is also increased. As a result, larger colonies tend to be found in cooler conditions because the metabolic demands required to sustain a colony are decreased.[55]

Distribution and habitat

The original, natural range of S. invicta

Red imported fire ants are native to the tropical areas of Central America and South America, where they have an expansive geographical range. This geographical range expands from southeastern Peru to central Argentina, and to the south of Brazil.[58][59][60][61] In contrast to its geographical range in North America, its range in South America is significantly different. It has an extremely long north-south range but a very narrow east-west distribution. The northernmost record of the red imported fire ant is Porto Velho in Brazil and its southernmost record is Resistencia in Argentina; this is a distance of about 3,000 kilometres (1,900 mi). In comparison, the width of its narrow range is about 350 kilometres (220 mi), and this is most likely narrower into southern Argentina and Paraguay and into the northern areas of the Amazon drainage.[62] Most known records of the red imported fire ant are around the Pantanal region; however, the interior of this areas has not been thoroughly examined, but it is undoubted that the species occurs in favourable locations around the Pantanal. The Pantanal region is hypothesised to be the original homeland of the red imported fire ant; hydrochore dispersal via floating ant rafts could easily account for the far south populations around the Paraguay River and Guaporé River.[62] The western extension of its range is not exactly known, but its abundance there may be limited. It may be extensive in easternmost Bolivia, owing to the presence of the Pantanal region. It is also unknown as to why the ant has penetrated farther into the east; in areas where related Solenopsis ants are found, the red imported is excluded from these areas, making these not entirely insalubrious. Red imported fire ants have not been found in the cerrado area to the east of the Pantanal, where scientists have suggested that its absence is due to the lack of moisture during the dry seasons. They have also not been found in the Paraná River while other Solenopsis ants have been, but this may be due to competition with dominant ants such as Pheidole.[62] In areas it is native, red imported fire ants prefer to inhabit rainforests, even if these sites are at risk of inundation.[61]

These ants are native to Argentina, and sources suggest that the red imported fire ant most likely came from here when they first invaded the United States; in particular, populations of these ants have been found in the provinces of Chaco, Corrientes, Formosa, Santiago del Estero, Santa Fe and Tucumán.[19][62][63] The northeastern regions of Argentina are the most credible guess in which the invading ants originate from.[60] In Brazil, they are found in northern Mato Grosso and in Rondônia and São Paulo. The red imported fire ant and S. saevissima are parapatric in Brazil, with contact zones known in Mato Grosso do Sul, Paraná and São Paulo.[59][64] In Paraguay they are found throughout the country, where they have been recorded in Boquerón, Caaguazú, Canindeyú, Central, Guairá, Ñeembucú, Paraguarí and Presidente Hayes; James Trager claims that the ant is distributed in all regions of the country.[65][66] They are also found in a large portion of northeastern Bolivia and, to a lesser extent, in northwestern Uruguay.[62][67]

Exposed red imported fire ant mound

The red imported fire ant is able to dominate altered areas and live in a variety of habitats. It can survive the extreme weather of the South American rain forest, and in disturbed areas, nests are frequently seen alongside roads and buildings.[68] The ant has been frequently observed around the floodplains of the Paraguay River.[69] In areas where water is present, they are commonly found around irrigation channels, lakes, ponds, reservoirs, rivers, streams, riverbanks and mangrove swamps.[65][68] Nests are found in agricultural areas, coastlands, wetlands, coastal dune remnants, deserts, forests, grasslands, natural forests, oak woodland, mesic forest, leaf-litter, beach margins, shrublands, alongside rail and roads and urban areas.[10] In particular, they are found in cultivated land, managed forests and plantations, disturbed areas, intensive livestock production systems and glasshouses.[68] Red imported fire ants have been found to invade buildings, including medical facilities.[70] In urban areas, colonies dwell in open areas, especially if the area is sunny.[70] This includes urban gardens, picnic areas, lawns, playgrounds, schoolyards, parks and golf courses.[65][70] In some areas, there are on average 200 mounds per acre.[71] During winter, colonies move under pavements or into buildings, and newly mated queens move into pastures.[68][70] Red imported fire ants are mostly found at altitudes of between 5 to 145 m (16 to 476 ft) above sea level.[65]

Mounds range from small to large, measuring 10 to 60 cm (3.9 to 23.6 in) in height and 46 cm (18 in) in diameter with no visible entrances.[68][72] Workers are only able to access their nests by a series of subterranean tunnels that protrude from the central region. Constructed from dirt, mounds are built in an oriental fashion, where the long portions of the mound face toward the sun during the early morning and before sunset.[68][72] Mounds tend to be oval-shaped with the long axis of the nest orientating itself in a north-south direction.[73] These ants also spend large amounts of energy in nest construction and transporting brood, which is related with thermoregulation. The brood is transported to areas where temperatures are high; not only does mound temperature increase due to the sun, the temperature is greater than the ground heat if a mound is shaded. With this said, workers track temperature patterns and do not rely on behavioural habits.[74] Inside nests, mounds contain a series of narrow tunnels, with subterranean shafts and nodes reaching grass roots 10 to 20 centimetres (3.9 to 7.9 in) below the surface; these shafts and nodes connect the mound tunnels to the subterranean chambers. These chambers are approximately 5 cm2 (0.77 inch2) and reach depths of 10 to 80 centimetres (3.9 to 31.5 in). The mean size of ants in a single subterranean chamber is around 200 individuals.[75] Aside from mounds, nests may be found under logs, sidewalks, walls in buildings and electric and water utility boxes.[34] Red imported fire ants and S. geminata tend to be difficult to distinguish; both species form large colonies and defend their territories, build galleried mounds, forage through underground tunnels and have workers approximately the same size. S. geminata can be distinguished by its smaller colony population, preference towards wooded areas and differentiated diet.[76] In areas where the red imported fire ant and S. xyloni meet, S. xyloni nests can be distinguished by their irregular shape with obscure entrances and scattered soil around the nest.[34]

Introductions

Worldwide distribution records of the red imported fire ant. Records in India and the Philippines need further confirmation.

Red imported fire ants are invasive, being among the worst in the world.[77] In the United States, the red imported fire ant first arrived in the seaport of Mobile, Alabama by ship between 1933 and 1945.[62][78] The red imported fire ant was only rare at the time, as entomologists were unable to collect any specimens. The earliest observations of these ants were by E.O. Wilson in 1942, and the population expansion most likely occurred after 1937.[79] Since its introduction to the United States, the red imported fire ant has spread throughout the southern states and north-east of Mexico, negatively affecting wildlife and causing economic damage.[58][80][81][82] The expansion of red imported fire ants may be limited since they almost get wiped out during the winters of Tennessee, and thus they may be reaching their northernmost range.[83][84] However, global warming may allow the red imported fire ant to expand its geographical range.[85] Currently, the ant is found in 13 states and occupies over 128 million hectares of land.[86] The Food and Drug Administration (FDA) estimates more than US$5 billion are spent annually on medical treatment, damage, and control in RIFA-infested areas. Further, the ants cause about US$750 million in damage to agricultural assets, including veterinary bills and livestock loss, as well as crop loss.[87] The United States Department of Agriculture estimates that they expand 193 kilometres (120 miles) per year.[70]

Red imported fire ants were first discovered in Queensland, Australia in 2001.[88] The ants were believed to be present in shipping containers arriving at the Port of Brisbane, most likely from North America.[89] Anecdotal evidence suggests fire ants may have been present in Australia for six to eight years prior to formal identification. While the outbreak is restricted to a small (800-km2) region of south-east Queensland in and around Brisbane, the potential social, economic, and ecological damage prompted the Australian government to respond rapidly. The initial emergency response was followed by the formation of the Fire Ant Control Centre in September 2001. A joint state and federal funding of A$175 million was granted for a six-year eradication programme involving the employment of more than 600 staff.[90] Following the completion of the fourth year of the eradication programme, the Fire Ant Control Centre estimated eradication rates of greater than 99% from previously infested properties. The federal budget confirmed the programme will receive extended Commonwealth funding of around A$10 million for at least another two years, until June 2009, to treat the residual infestations found most recently, and to fund validation of the overall treatment and surveillance programme. In areas with present colonies, native cockroaches, reptiles, frogs and birds are greatly affected, and their populations may decline.[91] In December 2014, a nest was identified at Port Botany, Sydney, New South Wales. The port has been quarantined, and a removal operation is in place.[92] Unsuccessful eradication in this area may cost the Australian economy billions in damages annually, based on a Queensland government estimated the cost of $43 billion over 30 years.[93]

Red imported fire ants have spread beyond North America. The ISSG reports the ant inhabiting from three of the Cayman Islands. However, the sources the ISSG cited give no report about them on the island, but recent collections indicate that they are present.[58] In 2001, red imported fire ants were discovered in New Zealand, but they were successfully eradicated several years later.[58][94] Red imported fire ants have been reported from India,[95] Malaysia,[96] the Philippines[97] and Singapore.[58] However, these reports of the ant were found to be incorrect, as the ants collected there were incorrectly identified as the red imported fire ant. In Singapore, the ants were most likely misidentified as well. In India, surveyed ants in Sattur Taluk, India listed the red imported fire ant there in high populations; meanwhile, there were no reports of the ant outside the surveyed area. The reports in the Philippines most likely misidentified collected material as the red imported fire ant, as no populations have been found there.[58] It was discovered in Taiwan in 2003, Hong Kong and mainland China in 2004, where they have spread into several provinces and Macau in 2005.[98][99][100] The ants in China most likely appeared in 1995, where it has quickly spread; these ants most likely originated from the United States. No geographic or climate barriers prevent these ants from further spreading, thus, it may spread throughout the tropical and subtropical regions of Asia.[86][101] In Europe, a single nest was found in the Netherlands in 2002.[61]

Around 1980, red imported fire ants began spreading throughout the West Indies, where they were first reported in Puerto Rico and the U.S. Virgin Islands.[102][103] Between 1991 and 2001, the ant was recorded from Trinidad and Tobago, several areas in the Bahamas, the British Virgin Islands, Antigua and the Turks and Caicos Islands.[58][104][105] Since then, red imported fire ants have been recorded on more islands and regions, with new populations discovered in Anguilla, Saint Martin, Barbuda, Montserrat, Saint Kitts, Nevis, Aruba and Jamaica.[58] The ants recorded from Aruba and Jamaica have only been found on golf courses; these courses import sod from Florida, so such importation may be an important way for the ant to spread throughout the West Indies.[58]

Populations found outside of North America originate from the United States. In 2011, specimens from Australia, China and Taiwan had their DNA analysed, with results showing that they are related to those in the United States.[106] Despite its spread, S. geminata has a greater geographical range than the red imported fire ant, but it can be easily displaced by it. Due to this, almost all of its exotic range in North America has been lost and the ant has almost disappeared there. On roadsides in Florida, 83% of these sites had S. geminata present when the red imported fire ant was absent, but only 7% when it is present.[58] This means that the ant can probably invade many tropical and subtropical regions where S. geminata populations are present.[58]

Behaviour and ecology

 A floating "raft" of red imported fire ants (RIFA) in North Carolina over land that normally forms the bank of a pond. The land had become submerged due to excessive rain and resultant flooding which inundated the nest. The raft is anchored to some blades of grass extending above the water's surface.
A floating "raft" of red imported fire ants (RIFA) in North Carolina over land that normally forms the bank of a pond. The land had become submerged due to excessive rain and resultant flooding which inundated the nest. The raft is anchored to some blades of grass extending above the water's surface.

Red imported fire ants are extremely resilient and have adapted to contend with both flooding and drought conditions. If the ants sense increased water levels in their nests, they link together and form a ball or raft that floats, with the workers on the outside and the queen inside.[107][108] The brood is transported to the highest surface.[109] They are also used as the founding structure of the raft, except for the eggs and smaller larvae. Before submerging, the ants will tip themselves into the water and sever connections with the dry land. In some cases, workers may deliberately remove all males from the raft, resulting in drowning. The longevity of a raft can be as long as 12 days. Ants that are trapped underwater escape by lifting themselves to the surface via the use of bubbles, which are collected from submerged substrate.[109] Owing to their greater vulnerability to predators, red imported fire ants are significantly more aggressive when rafting. Workers tend to deliver higher doses of venom, which reduces the threat of other animals attacking. Due to this, and the fact that a higher workforce of ants is available, rafts are potentially dangerous to those that encounter them.[110]

Necrophoric behaviour occurs in the red imported fire ant. Workers discard uneaten food and other such wastes in a refuse pile. The active component was not identified, but the fatty acids accumulating as a result of decomposition were implicated and bits of paper coated with synthetic oleic acid typically elicited a necrophoric response. The process behind this behaviour in imported red fire ants was confirmed by Blum (1970): unsaturated fats, such as oleic acid, elicit corpse removal behaviour.[111] Workers also show differentiated responses towards dead workers and pupae. Dead workers are usually taken to refuse pile, whereas the pupae may take a day for a necrophoric response to occur. Pupae infected by Metarhizium anisoplia are usually discarded by workers at a higher rate; 47.5% of unaffected corpses are discarded within a day, but for affected corpses this figure is 73.8%.[112]

Foraging and communication

Colonies of the red imported fire ant have tunneling surfaces that potruding out of surfaces where workers forage.[113][114] These areas where the surfaces protrude from tend to be within their own territory, but greater ant colonisation can affect this.[115] The holes exit out of any point within the colony's territory, and foraging workers may need to travel half a metre to reach the surface. Assuming the average forager travels five metres, over 90% of foraging time is inside the tunnels during the day and rarely at night. Workers forage in soil temperatures reaching 27 °C (80 °F) and surface temperatures of 12–51 °C (53–123 °F).[114] Workers exposed to temperatures of 42 °C (107 °F) are at risk of dying from the heat, despite foraging in greater temperatures.[114] The rate of workers foraging rapidly drops by the time it is autumn, and rarely do they emerge during winter. This may be due to the effects of soil temperature, and a decreased preference for food sources. These preferences only decrease when there is low brood production. In the northern regions of the United States, areas are too cold for the ant to forage, but in other areas such as Florida and Texas, foraging may occur all year round. When it is raining, workers also do not forage outside, as exit holes are temporarily blocked, pheromone trails are washed away and foragers may be physically struck by the rain. The moisture of the soil may also effect the foraging behaviour of workers.[114] When workers are foraging, it is characterised in three steps: searching, recruitment and transportation.[116] Workers tend to search for honey in greater times in contrast to other food sources, and the weight of food has no impact on searching time. Workers may only recruit other nestmates if the food they have found is too heavy, taking as much as 30 minutes for the maximum number of recruited workers to arrive. Lighter food sources take less time and are usually rapidly transported.[116] Foraging workers become scouts and solely search for food outside the surface, and may subsequently die two weeks later due to old age.[117]

Workers communicate by a series of semiochemicals and pheromones. These communication methods are used in a variety of activities, such as nestmate recruitment, foraging, attraction and defence; for example, a worker may secrete trail pheromones if a food source it discovered is too large to carry.[118] These pheromones are synthesized by the Dufour's gland and may trail from the discovered food source back to the nest. Trail pheromones are composed of Z,Z,Z-allofarnesenes, (2Z, 4Z, 6Z)-2,6,10-trimethyls-2, 4, 6 and 10-dodecatetraenes.[119][120] These trail pheromones are also species-specific to this ant only, in contrast to other ants with common tail pheromones.[121] The poison sack in this species has been identified being the novel storage site of the queen pheromone; this pheromone is known to elicit orientation in worker individuals, resulting in the deposition of brood.[122] It is also an attractant, where workers aggregrate towards areas where the pheromone has been released.[118] A brood pheromone is possibly present, as workers are able to segregate brood by their age and caste, which is followed by licking, grooming and antennation.[118] If a colony is under attack, workers will release alarm pheromones. These pheromones, however, are poorly developed in workers. Workers can detect pyrazines which are produced by the alates; these pyrazines may be involved in nuptial flight, as well as an alarm response.[123]

Red imported fire ants can distinguish nestmates and non-nestmates through chemical communication and specific colony odours.[118][124] Workers prefer to dig into nest materials from their own colony and not from soil in unnested areas or from other red imported fire ant colonies. One study suggests that as a colony's diet is similar, the only difference between nested and unnested soil was the nesting of the ants themselves. Therefore, it is possible that workers transfer colony odour within the soil.[124] Colony odour can be affected by the environment, as workers in lab-reared colonies are less aggressive than those in the wild.[125] Queen-derived cues are able to regulate nestmate recognition in workers and amine levels. However, these cues do not play a major role in colony-level recognition, but they can serve as a form of caste-recognition within nests.[126][127] Workers living in monogyne societies tend to be extremely aggressive and attack intruders from neighbouring nests, and in queenless colonies, the addition of alien queens or workers does not increase aggression among the population.[128]

Diet

Hatching alligators are very vulnerable from attacks by red imported fire ants

The diet of the red imported fire ant consists of animals such as dead animals, arthropods, insects, earthworms, vertebrates, and solid food matter such as seeds. However, this species prefers liquid food over solid food. The liquid food they collect is sweet substances collected from plants or honeydew-producing Hemipterans.[72][129][130] Arthropod prey may include Dipteran adults, larvae and pupae and termites. The consumption of sugar amino acid are known to affect recruitment of workers to plant nectars, and mimic plants with sugar rarely have workers to feed on them, whereas those with sugar and amino have considerable numbers.[131][132] The habitats they live in may determine the food they collect the most; for example, forage success rate for solid foods are highest in lakeshore sites, while high levels of liquid sources were collected from pasture sites.[133] The Specific diets can also alter the growth of a colony, with laboratory colonies showing high growth if fed honey-water. Colonies that feed on insects and sugar-water can grow exceptionally large in a short period of time whereas those that do not feed on sugar-water grow substantially slower. Colonies that do not feed on insects cease brood production entirely.[130] Altogether, the volume of food digested by nestmates is regulated within colonies.[134] Larvae are able to display independent appetites for sources such as solid proteins, amino acid solutions, and sucrose solutions, and they also prefer these sources over dilute solutions. Such behaviour is due to its capability of communicating its hunger to workers. The rate of consumption depends on the type, concentration and state of the food they feed on. Workers tend to recruit more nestmates to food sources filled with high levels of sucrose than to protein.[134]

Food distribution plays an important role in a colony. This behaviour varies in colonies, with small workers receiving more food than larger workers if a small colony is heavily deprived from food. In larger colonies, however, the larger workers receive more food. Workers can efficiently donate sugar water to other nestmates, with some acting as donors. These "donors" distribute their food sources to recipients, who may also act as donors. Workers may also share a greater portion of their food with other nestmates.[135] In colonies that are not going through starvation, food is still distributed among the workers and larvae. One study shows that honey and soybean oil were fed to the larvae after 12 to 24 hours of being retained by the workers. The ratio distribution of these food sources was 40% towards the larvae and 60% towards the worker for honey, and for soybean oil, this figure was around 30% and 70% respectively.[136] Red imported fire ants also stockpile specific food sources such as insect pieces, rather than consuming them immediately. These pieces are usually transported below the mound surface and in the driest and warmest locations.[137]

This species engages in trophallaxis with the larvae. Regardless of the attributes and conditions of each larva, they are fed roughly the same amount of liquid food. The rate of trophallaxis may increase with larval food deprivation, but such increase depends on the size of each larva. Larvae that are fed regularly tend to be given small amounts. In order to reach satiation, all larvae regardless of their size generally require the equivalent of 8 hours of feeding.[138]

Predators

L. mactans feeding on a captured insect. Red imported fire ants constitute 75% of all prey items captured by this spider

A number of insects, arachnids and birds prey on these ants, especially when queens are trying to establish a new colony.[139] Many species of dragonfly capture the queens while they are in flight, including Anax junius, Pachydiplax longipennis, Somatochlora provocans and Tramea carolina. 16 species of spiders actively kill red imported fire ants; of these include the wolf spider Lycosa timuga and the southern black widow spider (Latrodectus mactans). L. mactans captures all castes of the species (the workers, queens and males) within its web. These ants constitutes 75% of prey captured by the spider. Juvenile L. mactans spiders have also been seen capturing the ants.[139][140] Other invertebrates that prey on red imported fire ants are earwigs (Labidura riparia) and tiger beetles (Cicindella punctulata).[139] Birds that eat these ants include the chimney swift (Chaetura pelagica), the eastern kingbird (Tyrannus tyrannus) and the eastern bobwhite (Colinus virginianus virginianus). The eastern bobwhite attacks these ants by digging out the mounds for young queens.[139]

Many species of ants have been observed attacking queens and killing them. Predatory ants include Ectatomma edentatum, Ephebomyrmex, Lasius neoniger, Pheidole, Pogonomyrmex badius and Conomyrma insana, which is among the most significant.[139][141][142] C. insana ants are known to be effective predators against founding queens in studied areas of Northern Florida. The pressure of attacks initiated by C. insana increase over time, where queens exhibit different reactions including escaping, concealment or defence. Most queens that are attacked by these ants are ultimately killed.[141] Queens that are in groups have higher chances of survival than solitary queens if they are attacked by S. geminata.[143] Ants can attack queens on the ground and invade nests by stinging and dismembering them. Other ants such as P. porcula try to take the head and gaster, and C. clara invade in groups. Also, certain ants try to drag queens out of their nests by pulling on the antennae or legs.[142] Small, monomorphic ants rely on recruitment to kill queens, where they do not attack queens until reinforcements arrive. Aside from killing the queen, some ants may steal the eggs for consumption or emit a repellent that is effective against red imported fire ants.[142] Certain ant species may raid colonies and destroy them.[144] In areas where Solenopsis invicta (red imported fire ants) founded colonies, larger colonies of Tetramorium caespitum (pavement ants) have destroyed them, leading entomologists to conclude that this conflict between the two species may help impede the spread of Solenopsis invicta.[145][146]

Parasites, pathogens and viruses

Flies in the genus Pseudacteon are known to parasitise ants. One species, Pseudacteon obtusus, attacks the ant by landing on the posterioral portion of the head and lays an egg. The location of the egg makes it impossible for the ant to successfully remove.[147] A large variety of pathogens also infects red imported fire ants. Pathogens include Vairimorpha invictae,[148] Tetradonema solenopsis,[149] Caenocholax fenyesi,[150] Metarhizium anisopliae,[151] Myrmecomyces annellisae, Mattesia spp. and a mermithid nematode.[152] Phorid flies with Kneallhazia solenopsae can serve as vectors in transmitting the disease to the ants.[153] Infections from this disease are localised within the body fat, with spores only occurring in adult individuals.[154] The mortality of an infected colony tends to be greater in contrast to those that are healthy.[152]

A virus, SINV-1,[155] has been found in about 20% of fire ant fields, where it appears to cause the slow death of infected colonies. It has proven to be self-sustaining and transmissible. Once introduced, it can eliminate a colony within three months. Researchers[156] believe the virus has potential as a viable biopesticide to control fire ants.[157]

Queen recruitment

Solenopsis queens and workers

Polygynous colonies differ substantially from monogynous colonies in social insects. The former experience reductions in queen fecundity, dispersal, longevity, and nestmate relatedness.[158] Understanding the mechanisms behind queen recruitment is integral to understanding how these differences in fitness are formed. It is unusual that the number of older queens in the colony does not influence new queen recruitment. Levels of queen pheromone, which appears to be related to queen number, play important roles in regulation of reproduction. It would follow that workers would reject new queens when exposed to large quantities of this queen pheromone. Moreover, experimental data support the claim that queens in both populations enter nests at random, without any regard for the number of older queens present.[159] There is no correlation between the number of older queens and the number of newly recruited queens. Three hypotheses have been posited to explain the acceptance of multiple queens into established colonies: mutualism, kin selection, and parasitism.[160] The mutualism hypothesis states that cooperation leads to an increase in the personal fitness of older queens. However, this hypothesis is not consistent with the fact that increasing queen number decreases both queen production and queen longevity.[161] Kin selection also seems unlikely given that queens have been observed to cooperate under circumstances where the queens are statistically unrelated.[162] Therefore, queens experience no gain in personal fitness by allowing new queens into the colony. Parasitism of preexisting nests appears to be the best explanation of polygyny. One theory is that so many queens attempt to enter the colony that the workers get confused and inadvertently allow several queens to join the colony.

Reproduction

Several studies have been conducted on the sex ratios exhibited within colonies of S. invicta. More specifically, the queen was seen to actually control the sex ratios. In an experiment, 24 field colonies were selected with highly biased sex ratios in a monogyne population. Eleven of these colonies were male specialists (numerical proportion of males, range: .77 to 1.0), and 13 were female specialists (numerical proportion of males, range: 0.0 to .09). After exchanging queens, 22 of the 24 colonies accepted the foreign queen, and 21 of these colonies produced a new batch of offspring five weeks later.[163]

Based on the colony from which the queen was originated, the sex ratios of the new colony after the switch could be predicted.[164] For example, after switching, a colony produced predominantly males if the queen came from a male-producing colony, even if the host colony originally produced mainly females. It is not surprising, then, queens that came from a male-favored sex ratio colony produced no significant change in the sex-ratio of another male-favored colony after the switch. The same was true for a queen that came from a female-favored sex ratio colony and switched into another female-favored colony.[163] Therefore, the queen determines the sex ratio, not the workers.

Another study compared the inhibition of the number of sexuals (male and female) produced in a single queen colony and a queenless colony. Freshly killed corpses of functional (egg-laying) queens were added daily to queenless colonies. These effectively inhibited the production of sexuals through the excretion of pheromones, although not as effectively as living queens. Conversely, corpses of queens not laying eggs did not inhibit the production of sexuals when added to queenless colonies. Also, when queens were introduced into queenless colonies that already had developed sexual larvae, workers in the colony executed these larvae. This indicates the queen’s control over the production of sexuals can be enforced retroactively, even after the larvae are sexualized. These results provide evidence that functional queens exert control over the production of sexuals in S. invicta through pheromones that influence the behaviors of workers toward both male and female larvae.[165]

S. invicta also presents a paradox for kin selection theory. In multiple-queen (polygyne) colonies, the egg-laying queens are, on average, unrelated to one another, so the workers appear to raise new sexuals that are no more closely related to them than are random individuals in a population. This was tested by removing worker/queen pairs engaged in trophallaxis with forceps, and then sampling the allele frequency to estimate for the reference population. Frequencies of the most common allele at each locus have been found to conform to Hardy-Weinberg expectations in past studies. Genotypic data were used to estimate relatedness between the workers and the winged-queens they tended, and it was virtually zero. The results indicate S. invicta workers tending queens in polygyne nests do so without respect to the relatedness of those queens.[164][166]

Unrelated queens commonly found a colony cooperatively. This joint effort of the cofoundresses contributes to the growth and survival of the incipient colony.[167] However, such associations are not always stable. The emergence of the first workers instigates queen-queen and queen-worker fighting. The two factors that could affect the survival of individual queens are their relative fighting capabilities and their relative contribution to worker production. Experimentation indicates that size, an indicator of fighting capacity, positively correlates with survival rates. However, manipulation of the queen’s relative contribution to worker production had no correlation with survival rate. It can be assumed that the worker brood cannot favor its mother based on these results.[168]

S. invicta workers not only tend to queens indiscriminately, but they also indiscriminately attack them. Queens producing diploid males reared fewer offspring, but were as likely to survive as queens producing only workers. It would have been assumed that if workers controlled queen mortality, they would be expected to discriminate in favor of their mother, therefore increasing their inclusive fitness. This, however, should favor the queen with the greatest number of daughters during the period of queen execution. The data actually show the fights among queens themselves have a strong role in determining which queen survives—the heavier cofoundress was more likely to win. Thus, queen survival is enhanced by high fighting ability relative to cofoundresses, rather than by the number of offspring she has. Workers respond to these queen differences by attacking the previously injured queen to reinforce the effects of competition among the queens.[169]

Monogyny and polygyny

Recognition and discrimination between conspecifics

Recognition between conspecifics is an essential attribute of ant social behavior for repelling non-nestmates and protecting food resources. RIFAs use olfactory cues produced by queens to discriminate between colony members and conspecific intruders. They also use environmentally derived cues to discriminate between colony members and nonmembers. RIFAs have two distinct forms of colony organization: monogyny and polygyny, distinguishable by the number of reproductive queens, how reproduction is divided among members of the colony, the number of individuals produced, the degree of genetic relatedness, and queens' and workers' behaviors. Different behaviors are correlated with allelic differences at the nuclear gene general protein-9 (Gp-9) that codes for two groups of odor-binding proteins. Queens of monogyne colonies possess B-like alleles (with BB genotype) and are more prolific, heavier, and longer-lived than queens of polygyne colonies. In Argentina, polygyne colonies can be heterozygous (Bb) or homozygous (BB), thus some polygyne workers present b-like alleles.

Monogyne workers kill foreign queens and aggressively defend their territory. However, not all behaviors are universal, primarily because worker behaviors depend on the ecological context in which they develop, and the manipulation of worker genotypes can elicit change in behaviors. Therefore, behaviors of native populations can differ from those of introduced populations.[170] In a study to assess the aggressive behavior of monogyne and polygyne red fire ant workers by studying interaction in neutral arenas, and to develop a reliable ethogram for readily distinguishing between monogyne and polygyne colonies of RIFAs in the field,[171] monogyne and polygyne workers discriminated between nestmates and foreigners as indicated by different behaviors ranging from tolerance to aggression. Monogyne ants always attacked foreign ants independently if they were from monogyne or polygyne colonies, whereas polygyne ants recognized, but did not attack, foreign polygyne ants, mainly by exhibiting postures similar to behaviors assumed after attacks by Pseudacteon phorids. Hostile versus warning behaviors were strongly dependent on the social structure of workers. Therefore, the behavior toward foreign workers was a reliable ethological indicator to characterize monogyne and polygyne colonies of RIFAs.[172]

The monogynous red imported fire ant colony’s territorial area and the mound size are positively correlated, which, in turn, is regulated by the colony’s size (number and biomass of workers), distance from neighboring colonies, prey density, and by the colony's collective competitive ability. In contrast, nestmate discrimination among polygynous colonies is more relaxed as workers tolerate conspecific ants alien to the colony, accept other heterozygote queens, and do not aggressively protect their territory from polygyne conspecifics.[173] These colonies might increase their reproductive output as a result of having many queens and the possibility of exploiting greater territories by means of cooperative recruitment and interconnected mounds.[174] Therefore, polygyne workers displayed low aggressive responses toward polygyne non-nestmates because lower aggression results in higher survival. Consequently, the behavior of workers is another reliable factor to characterize both monogyne and polygyne colonies of red imported fire ant, in addition to considering mean worker sizes, density or distance between mounds, number of queens, or molecular assays.

Countermeasures

Many scientists and agencies are attempting to develop methods to stop the spread of the RIFA. Typically, control has been achieved through pesticide use. From the 1950s into the 1970s, Mirex was extensively used in an attempt to eradicate the species. However, the pesticide inadvertently aided the fire ants' spread by killing numerous native ant species that compete successfully with them.[175] Mirex also caused even broader ecological harm that was often attributed to the fire ants. For example, it was first thought that the ants were linked to the decline of overwintering birds (e.g. the loggerhead shrike), but a later study showed the pesticides were largely to blame.[176] RIFAs have virtually no natural biological control agents native to, or naturalized in, the United States, China, the Philippines, or Australia. Current research is focused on introducing biological control agents from the RIFA's native range.

Physical methods

Researchers have also been experimenting with extreme temperature change to exterminate RIFAs, such as injecting liquid nitrogen or pressurized steam into RIFA nests. Besides using hot steam, pouring boiling water into ant mounds has been found effective in exterminating their nests.[177] Folk remedies have often sought a rapid increase in temperature by soaking the nest in gasoline or kerosene and lighting it on fire, though this is potentially dangerous. Further, the burning of the nest is ineffective due the tendency of queens to be several feet underground. This confusion stems from the observation that fuel vapor has a near instantaneous lethal effect on the ants.[citation needed]

In Brisbane, Australia, colonies are being eradicated or effectively controlled by ground baiting with food laced with contraceptives that render the colony's queen infertile, and toxicants. Mass baiting was undertaken following detection of the ants around the port of Brisbane and in southwestern Brisbane in 2001. Widespread public reporting of suspect colonies (by sending in samples of ants for identification) allowed mapping of the ant's locations. This was combined with satellite imagery to determine the vegetated habitats most likely to be infiltrated by the ants, and the baits were targeted in these areas. Known infested areas were declared high-risk restricted areas, and any material being moved from these areas which could harbour ants (soil, mulch, potted plants, potting mix, hay bales, construction machinery, etc.) had to be inspected prior to disposal or movement, and bulk waste sent to transfer stations for examination, treatment, and disposal. The infestation was initially thought to cover 270 km2, with a density of up to 600,000 colonies/km2 on highly infested sites. As program activity refined data on the infested area, overall size grew to around 80,000 ha by 2006/7. At mid-2007 in the ongoing nationally funded eradication campaign, fewer than 100 active colonies were located in the entire South-East Queensland area between September 2006 and February 2007. The focus of delivering eradication has now switched largely to surveillance, while control and validation measures are expected to continue until 2009. The six-year eradication campaign has cost A$175 million to date, and had secured funding in principle for a minimum of two more years.[178][179]

Stings to animals

The stings of the red imported fire ant in animals are painful and sometimes life-threatening.[180] In dogs, stings from the red imported fire ant can cause pustular dermatosis, a condition where pustules appear in crops as a result from an ant sting.[181] After getting stung, the immediate response consists of erythema and swelling. The pustules remain for approximately 24 hours, whereas in humans they can last for several days.[182] In livestock, red imported ants mostly sting animals in regions with no hair, particularly around the ears, eyes, muzzle, the perineum and ventral portion of the abdomen.[183] Healthy individuals are less likely to be attacked in contrast to weak and sick animals. Red papule and mild swelling occurs, followed by vesicopustule with a red halo developing within 24 to 48 hours. The eyes and eyelids are commonly damaged from the stings; in sheep and goats, ophthalmic ointment containing antibiotics and corticosteroids can be used to treat the eyes of sheep and goats, but this treatment is not recommended on horses. In non-domestic animals, cases of red imported fire ants stings in animals such as ferrets, moles squirrels, white-tailed deer, cottontail rabbits and newborn blackbucks have been reported, as well as lizards and screech owl nestlings.[183] The aftermath of the injuries are like those in domestic animals.

As red imported fire ants do not intend to kill animals as they are too large and their venom cannot fatally injure them, animals may trigger stinging episodes if they irritate or injure the ants on their body, with all individuals participating. These large-scale attacks on an animal are caused through chemical signals.[183] As a result, this explains as to why an animal may suffer from hundreds of individual stings. It is suspected that many victims from the red imported fire ants may be depressed as a result from the effects of the toxin. Some animals may swallow red imported fire ants as they lick or bite around the sites they are stinging on. This can cause additional injuries inside the animal itself, especially in the upper gastrointestinal tract. In suckling white tail deer fawns, sting sites have been found in the esophagus and abomasum; toxins from the ingested ants may cause inflammation of the gastrointestinal lining.[183]

See also

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Cited literature

  • Tschinkel, W. (2006). The Fire Ants. Cambridge, Massachusetts: Belknap Press of Harvard University Press. ISBN 978-0-674-02207-2. 

External links