Helicoverpa zea, commonly known as the corn earworm, is a species (formerly in the genus Heliothis) in the family Noctuidae. The larva of the moth Helicoverpa zea is a major agricultural pest that causes massive economic costs. Since it is polyphagous (can feed on many different plants) during the larval stage, this species has been given many different common names. When the larva consumes cotton, it is known as the cotton bollworm. When it consumes corn, it is known as the corn earworm. When it consumes tomatoes, it is known as the tomato fruitworm. However, it is known to consume a wide variety of crops. These moths are found in regions of the world that have characteristically mild or tropical temperatures. As nocturnal insects, they have a highly developed defense system that is selected to increase their survival from predators that hunt at night, such as bats.
- 1 Distribution
- 2 Life cycle and Description
- 3 Economic impact
- 4 Survival
- 5 Movement
- 6 Feeding
- 7 Mating
- 8 Gallery
- 9 References
- 10 Literature
- 11 External links
The corn earworm is found in temperate and tropical regions of North America, with the exception of northern Canada and Alaska as it cannot overwinter in these areas. Helicoverpa zea found in the eastern United States also do not overwinter successfully. They live in Kansas, Ohio, Virginia, and southern New Jersey, but survival rate is mainly affected by the severity of the winter. Corn earworm moths regularly migrate from southern regions to northern regions depending on winter conditions. They are also found in Hawaii, the Caribbean islands, and most of South America, including Peru, Argentina, and Brazil.
Cotton earworms have also been reported from China in 2002.
Life cycle and Description
Eggs are individually deposited on leaf hairs and corn silks, and they hatch in about three to four days. The eggs are originally pale green in color, but over time they turn yellowish and then grey. Eggs are 0.5 mm in height and average about 0.55 mm in diameter. They hatch 66 to 72 hours after development. Once larvae have breached the chorion, the protective outer part of the egg, they spend up to 85% of their time emerging from their shell. In this eclosion process, the larvae work to make the exit hole larger than their head. Larvae spend the rest of the time making a silk meshwork around the exit hole in order to both help escape the shell and to help them find their shell so they can feed on it. After feeding on their shell, larvae rest for approximately 3 minutes before they begin feeding on the plant material around them.
Following hatching, larvae feed on the reproductive structures of the plant and usually develop through four to six instars. Initially, the young larva feed together, and this stage is their most destructive stage. Through maturation, older larvae become aggressive and cannibalistic, leaving one or two larvae per feeding site (See Interfamilial Predation). They usually have orange heads, black thorax plates, and body color that is primarily black. Their bodies can also be brown, pink, green, and yellow with many thorny microspines. Mature larvae migrate to the soil, where they pupate for 12 to 16 days.
Larvae pupate 5 to 10 cm below the soil surface. Pupae are brown in color; they measure 5.5 mm wide and 17 to 22 mm long. The biggest environmental factor that affects the pupal developmental rate is temperature, primarily soil temperature. This is because proper insulation facilitates development, and soil temperatures below 0 degrees Celsius correlate to higher pupal mortality. Another factor that influences pupal development is soil moisture. Pupal mortality is high in wet soil, where the moisture level is between 18 and 25 percent. Dehydration can also lead to high death rates among pupae, if soil moisture is as low as 1 to 2 percent.
Adults have forewings that are yellowish brown in color and have a dark spot located in the center of their body. The moths have a wingspan ranging from 32 to 45mm, and live over thirty days in optimal conditions. However, the life span ranges from five to fifteen days on average. They are nocturnal and hide in vegetation during the day. Adult moths collect nectar or other plant exudates from a large number of plants, and live for 12 to 16 days. Females can lay up to 2,500 eggs in their lifetime.
The corn earworm is considered to be a major agricultural pest, with a large host range encompassing corn and many other crop plants. Helicoverpa zea is the second most important economic pest species in North America, next to the codling moth. The estimated annual cost of the damage is more than 100 million US dollars, even though expenditure on insecticide application has reached up to 250 million dollars. The moth’s high fecundity, ability to lay between 500 to 3,000 eggs, polyphagous larval feeding habits, high mobility during migration, and a facultative pupal diapause have led to the success of this pest.
Control measures have been advocated since the 19th century and fall into two categories. One aims at total pest population reduction, while the other is aimed at protection of the particular crop. As of 2013[update] integrated pest management (IPM), an array of techniques and approaches to control pests, was recommended. Practices such as deep ploughing, mechanical destruction, and trap crops are also used to kill different instars. Chemical control is widely successful, and includes the use of applying mineral oil inside the tip of each corn ear, which suffocates the young larvae. Pesticides are one method by which corn earworm populations are controlled; however, since they have been widely used, the insects have become resistant to many pesticides. The use of biological controls, such as the bacterium Bacillus thuringiensis and various forms of nematodes, is also common, although not without its own problems. Corn earworm moths are not always vulnerable to the bacterium, and they are only afflicted by nematodes once the larvae have pupated and dropped to the ground.
More than one hundred insect species prey on Helicoverpa zea, usually feeding on eggs and larvae. The insidious flower bug (Orius insidiosus), a pirate bug, feeds on the eggs of H. zea, thus acting as a biological control agent. Some plants emit a blend of chemicals in response to damage from H. zea, which attract parasitic insects. Cardiochiles nigriceps, a solitary endoparasitoid wasp, makes use of these volatile plant compounds to identify the presence of H. zea. When the wasp finds a damaged host plant, they hover around and then search for the host with their antennae. When the females find their prey, they use their antennae to position themselves and deposits eggs into the host. The braconid wasp Microplitis croceipes, which deposits its eggs inside a living caterpillar, is also an important parasitoid of both H. zea and the related species Heliothis virescens. When larval densities are high, a fungal pathogen, Nomuraea rileyi, can cause an outbreak of disease. However, pupal mortality is high not because of predators, but because of harsh weather conditions, collapsing pupal chambers, and disease.
As the larvae mature, they become increasingly aggressive. Although they have host plants surrounding them, Helicoverpa zea will attack and eat other insects, a behavior documented by the following study. When presented with a second instar larva of Urbanus proteus, the corn earworm larvae will grasp the insect, roll onto its side to form a semicircle, and begin feeding on the insect's posterior end. If the U. proteus begins to bite out of defense, Helicoverpa zea will rotate the larva 180 degrees and use its mandibles to puncture the head capsule, killing the insect. Then, the H. zea larva will rotate the U. proteus back to its original position and continue feeding until the insect is entirely consumed. Even when presented with up to 5 U. proteus larvae, the Helicoverpa zea will engage in the unique behavior, as the larvae have a higher affinity for Lepidopterous prey over plant material. Helicoverpa zea raised in a low moisture environment have lower pupal weight and a longer developmental time than those raised in environments of high moisture, so there is a nutritional benefit to such aggressive feeding behavior under such conditions.
The auditory system of moths, primarily nocturnal ones, is important for flight behavior. Their ability to detect ultrasound aids their ability to avoid natural predators, like bats, that produce ultrasonic signals. Moths such as hawk moths or larger Saturniid moths lack ears and show no response to ultrasound stimuli. This is because they are too big for the average bat and have no survival advantage from auditory warning measures. High-intensity ultrasounds may indicate that the moth is detected by a predator, and causes the moth to engage in evasive maneuvers such as quick turns and dives. The moth will only be safe to land where a predator’s sonar will not be able to detect it. As a result, the moth will spiral downward to land on a nearby surface. The acoustic cell A2 controls this behavior. These cells are involved in the detection of sounds at high intensities, and have a direct link to motorneuron activity. A1 acoustic cells control slower, decision-making behaviors, such as leading the moth to steer away when it identifies a distant sound. There is more steering away movement from lower-intensity ultrasounds because these sound signals indicate that that predator is still distant. This behavior can take the moth far enough out of the predator’s flight path to allow the moth to remain undetected. Moths will scan around, comparing signals between their bilateral ears, to find the correct sound direction before changing their flight path. Ultrasonic communication is also important in courtship behaviors, for these moths produce sounds bursts of 50 kHz by striking their wingtips during flight, combining olfaction and ultrasound in order to find mates (See Flight Behavior).
Helicoverpa zea is a seasonal, nocturnal migrant, and adults disperse when there are poor reproductive conditions and weather permits. In short-range dispersal, the moths move within the crop and low over the foliage. This type of dispersal is mostly independent of wind currents. Long-range dispersal involves adults flying up to 10 meters above the ground and moving downwind from crop to crop. Migratory flights occur up to 1–2 km above the ground and can last for hours. Migration of 400 km is
common for such flights as moths are carried downwind. Helicoverpa zea caterpillars areusually always intercepted on produce transported by air-freight transportation. Most activity is restricted to the night-time. Some moths display vertical take-off flight, which carries them above the flight boundary layer and allows them to undertake migratory movement in upper wind systems. During mating, males engage in high-speed directed flight in search of pheromone plumes (See Pheromone Production).
Pupae have the ability to enter facultative diapause, the state of arrested development and growth in response to a change in the environment. By preparing themselves for a major change in environmental conditions, they can increase reproductive success. Diapause increases with increasing latitude. In tropical conditions, populations breed continuously, and only 2-4% of pupae diapause. In subtropical and temperate regions, most individuals diapause. Individuals who don't enter diapause in these areas emerge in late fall and die without reproducing. Drought-responsive diapause has also been observed in the summer.
Helicoverpa zea has a wide host range, attacking vegetables that include corn, tomato, artichoke, asparagus, cabbage, cantaloupe, collards, cowpea, cucumber, eggplant, lettuce, lima bean, melon, okra, pea, pepper, potato, pumpkin, snap bean, spinach, squash, sweet potato, and watermelon. However, not all of these are good hosts. While corn and lettuce are shown to be great hosts, tomatoes are less beneficial, and broccoli and cantaloupe are poor hosts. Corn and sorghum are most favored by corn earworms. Various signs reveal the presence of these moths. Young maize crops have holes in their leaves, following whorl-feeding on the apical leaf. Eggs can be found on silks on larger plants, and silks display grazing evidence. The soft, milky grains in the top few centimeters of corn cobs are eaten as the corn ears develop. One larva per cob can be observed. Bore holes are observed in cabbage and lettuce hearts, flower heads, cotton bolls, and tomato fruits. Sorghum heads are grazed, and legume pod seeds are eaten.
Helicoverpa zea earns its nickname the corn earworm for its widely known destruction of cornfields. The corn earworm feeds on every part of corn, including the kernels. Severe feeding at the tip of kernels allows entry for diseases and mold growth. Larvae begin feeding on the kernels once they have reached third instar, a developmental stage in insects. Larvae penetrate 9 to 15 cm into the ear, with deeper penetration occurring as the kernels hardened. Larvae don’t eat the hard kernels, but take bites out of many kernels, lowering the corn quality for processing.
Helicoverpa zea is the most common and destructive pest of soybean growth in Virginia. About one-third of Virginia acreage is treated annually with pounds of insecticide, costing farmers around 2 million dollars. The degree of damage varies on the size of the pest infestation, the timing, and the stage of the plant. However, soybean plants are capable of withstanding a large amount of damage without substantial yield loss depending on soil moisture, planting date, and weather. If the damage is early in the plants life, then damage will mostly be to the leaves. Plants compensate for the damage by processes such as increasing seed size in remaining pods. Most damage happens in August, when the plants are flowering. Attacks that happen after August do much less damage because many pods have developed tougher walls that H. zea can't penetrate. Infestations that affect pod formation and seed filling have the potential to reduce yields, and because this happens in the later stages of plants, they have less time to compensate.
Female moths are attracted to flowering soybean fields. The most severe infestations occur between flowering and when pods become fully developed. Large-scale outbreak is associated with time of peak flowering, when most pods are developed, and peak moth flight, for giant. Moths are also attracted to drought stressed soybeans or fields with poor growth. Dry weather leads to quick drying of corn plants, compelling moths to leave and seek other hosts. Heavy rainfall also decreases corn earworm populations because it drowns pupae in their soil chambers, limits moth flight, washes eggs from leaves, and creates favorable conditions for fungal diseases that kill caterpillars.
A hormone produced in the brain of the female moths controls sex pheromones. The hormone is released into the hemolymph to stimulate pheromone production. Pheromone biosynthesis-activating neuropeptide (PBAN) is a peptide used by moths to regulate pheromone production. It acts on the pheromone gland cells using calcium and cAMP. Although the photoperiod regulates the release of PBAN to some extent, the chemical signals from the host plant supersede the affect from the timing of the day. Female Helicoverpa zea in corn fields do not produce pheromones during the night until they encounter corn. Several natural corn silk volatiles like the plant hormone ethylene induces H. zea pheromone production. The presence of the silk from an ear of corn is enough to cause pheromone production, and physical contact between females and corn is unnecessary. This evolutionary mechanism has come about to make sure that moths can coordinate their reproductive behavior with the availability of food. Female moths often become depleted of sex pheromone after mating within 2 hours of separation from the mating pair. The pheromonosematic peptide (PSP), a protein 57 amino acids long, found in male accessory gland is what causes depletion of the female’s sex pheromone. This ability is selected for so that males can increase their reproductive fitness, since other males will not be attracted to a female without a sex pheromone; thus, the female will only harbor the first male's offspring. Males transferring a spermatophore without accessory gland products do not stop female pheromone production, but do stop the females' calling behavior. Intense selection on males to manipulate female reproductive physiology promotes rapid evolution of specific molecules, and male-derived pheromone suppressing factors exhibit positive selection.
Sperm competition and chemicals introduced to females through mating have a negative effect on females and their lifespan. In males, production of the spermatophore, sperm, and secondary chemicals reduces their lifespan. As the number of copulations increase, the rate of mortality also increases in both sexes.
Males must first wait to sense a female’s pheromones before they can locate her. Before males engage in flight to find a female, they warm-up by shivering the major flight muscles to reach thoracic temperature optimal to sustain flight, around 26 degrees Celsius. The thermoregulatory shivering activities of males were measured as they were exposed to different sex-related olfactory cues. Males are found to heat up more quickly in the presence of a female pheromone and take-off at a lower thoracic temperature than males who are exposed to other fragrances. Since heating up to the right temperature leads to better flight performance than flying immediately, there is a trade-off between sub-optimal flight performance and rapid onset of directed flight. Helicoverpa zea males exposed to an attractive pheromone blend thus spend less time shivering and increase their heating rate. Thermoregulatory behavior of unrestrained moths is associated with competition for access to females, showing the ecological trade-off.
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