Osmia rufa

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Osmia rufa
Osmia rufa couple (aka).jpg
Scientific classification
Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Order: Hymenoptera
Family: Megachilidae
Genus: Osmia
Species: O. rufa
Binomial name
Osmia rufa
(Linnaeus, 1758)

Osmia bicornis

Osmia rufa or Osmia bicornis is a species of mason bee, and is known as the red mason bee due to its covering of dense gingery hair.[1][2][3][4] It is a solitary bee that nests in holes or stems and is polylectic.[5] Despite being classed as solitary, however, these bees can be seen aggregating together.[2] This bee can be found throughout Europe and North Africa,[6] and nests in preexisting hollows, choosing not to excavate their own. These bees are not aggressive; they will only sting if handled very roughly and are safe to be closely observed by children.[2][4] Females only mate once, usually with closely related males, and can determine the sex ratio of their offspring based on their body size, where larger females will invest more in diploid females eggs than small bees. These bees also have trichromatic color vision and are important pollinators in agriculture.

Taxonomy and phylogeny[edit]

This species is part of the order Hymenoptera, which consists of bees, wasps, ants, and sawflies. O. bicornis is the current scientific name for this bee, although it was formerly known as O. rufa.[7] This bee is a member of the family Megachilidae, which mostly consists of solitary bees, and is among 11 species that have been identified in Britain.[8] There are three subspecies of O. rufa, one of which is Osmia rufa cornigera.[6]

Description and identification[edit]

O. rufa maintains approximately the same body size as the honeybee.[5] The female is larger/broader than the male and has two large horns on the head.[3] Specifically, males are 8–10 mm in size, while females are 10–12 mm in size.[9] The reason for the sexual dimorphism is due to provision size mass, in which females consume more pollen than males. Temperature in brood cells affects the body size of O. rufa; body size decreases as temperature increases. Beyond 25 °C, specifically, body growth can be severely truncated, leading to small adult body size or mortality.[6]

The male has a white tuft of hair on its face[1] while a female possesses darker hairs on its head[10] Additionally, the males possess gray, clypeal hairs, which are absent in females.[9]

The maximum foraging distance for Osmia rufa is about 600 m, though generally if there is high plant density around the nest bees can forage closer to the nest and for a shorter duration.[11]

Distribution and habitat[edit]

O. rufa is found in England, southern Scotland (possibly northern Scotland as well), Wales, mainland Europe, Sweden, Norway, North Africa, Georgia, Turkey and Iran.[1] Of the 11 species identified in England, O. rufa is both the largest and most common species present.[8]

O. rufa occupy a variety of nesting sites within nature and in sites of human construction. These bees have been known to nest in key holes, empty snail shells, plant stems and empty beetle hollows.[3] With respect to snail shells, O. rufa specifically occupy the old shells of the following three species: Helix nemoralis, Helix hortensis, and Helix pomatia and the nests of Anthopora. Additionally, these bees make their nests in such sites as sandy banks, decaying trees planted in clay soil like the willow tree, old-mortared walls, flint stone holes, garden shed fifes, and window frame holes and cracks.[8]

Nest structure[edit]

The nest of O. rufa consists of an array of partitioned cylindrical cells[12] in holes in wood or reed tubes.[13] These bees accept a diverse range of pre-existing cavities as nest sites.[14] The cells are arranged linearly within a narrow tube. If the internal diameter of the tube exceeds 12 mm, then this linear arrangement may be forced into two rows instead of one. The length of each cell can vary from 10 to 21 mm.[15] The inner sides of the partitions are rough and convex while the outer sides are smooth and concave. Between the cells and the terminal plug is a space known as the ‘vestibular cell’.[12] The vestibule acts as a form of protection against volatile environmental conditions. The bees whose nests are exposed to the sun and heat build vestibules more frequently.[13] The material used to build the nests is mud mixed with their mandibles,[15] however the sides of the tunnel in which the nests are located are usually not lined with mud, with the exception of some irregularly arranged nests.[12] Females construct around 6 cells per nest on average; however, larger females construct more cells than smaller ones.[16] When it comes time for the females to lay their eggs, they will add pollen to each brood cell and lay one egg in each cell next to the pollen.[17] The sequence of nesting behavior is as follows: cell construction, provisioning, egg-laying and sealing the cell.[12]

A cross-sectional view of an O. rufa nest. Partitioned cells can be seen in a tube.

Cells containing females are typically larger than those containing males, due to the sexual dimorphism of the species. Additionally, cells containing females are situated towards the back of the nest, while those with males are closer to the nest entrance. Because of this, male offspring leave the nest sooner than females. Due to the linear arrangement of cells in the nest, the youngest bee leaves earlier than older ones.[12]

Colony Cycle[edit]

Development from Egg to Adult Stage[edit]

The species is most active during the spring and early summer although it may be seen into late June.[4] Each year, one generation of bees is formed, making an appearance during the spring.[18] Approximately one week after eggs are laid in the brood cells, the eggs hatch and larvae undergo development up until the end of summer.[17] The larvae then enter the pupae stage upon spinning cocoons, in which the anterior collar, nipple area, and outer meshwork of the cocoon are spun simultaneously.[8] The adults then hibernate through the winter in the cocoons and finally emerge as mature bees in the spring.[17] The entire process from egg to the formation of the cocoon is estimated to be about 20 days.[8]

In the development of O. rufa cornigera, freshly laid eggs are white, elongated and possess slightly pointed tips at the anterior end and a reflective surface. These eggs are laid on the upper, outer surfaces of pollen provisions, with the posteroventral side of each egg in contact with the provision and the anterior tap unattached and elevated above the provision's surface. Between 36 and 48 hours after the egg is laid, the egg enters stage 8 of embryogenesis, during which an embryonic membrane and a labral protubrance appear. Further, the egg is oriented anteriorly in the egg chorion. Upon the surface of the membrane are depressions, each of which are 25 micrometers in diameter and separated by a distance of 50-100 micrometers. 24 hours later, the embryo enters stage 9, during which mandibular, maximallary, labial[disambiguation needed] lobes, body segmentation, and anus formation occur. After an additional 24 hours, the embryo begins to move, evidenced by clypeal contractions and lateral head movement, and rotates for 25-30 min. along its long axis. After 15-25 min. of head and abdominal body movement, the terminal body segments and head capsule within the embryo begin to make contact with the embryonic membrane, eventually resulting in rupture and gradual disintegration of the embryonic membrane as contractions continue to occur.[8]

The egg chorion will split along its spinacular line, a process called eclosion, resulting in the emerging larvae breathing air and ingesting chorionic fluid. This larvae is referred to as first instar larva and will then enter a process called ecdysis, which occurs approximately between 16.4 and 24.5 hours after the chorion has split, and transition into the second instar larva stage. The second instar larva feeds for between 12.85 and 25.45 hours before molting and entering the third instar larva stage.Third instar larva will then molt into the fourth instar larva stage. In the final stage, the fifth instar larva stage, the larva will eat and defecate up into the start of cocoon formation.[8]

During cocoon formation, the larvae utilize saliva to encompass the fecal material and cell. The anterior part of the cocoon is composed of a domed collar and a central, domed nipple region, and the larva weaves salivary "silk" threads in a circular pattern in this region. The larva also utilizes its digestive contents to form smears on the cocoon, leading to hardening of the cocoon and a color change to a dark, red-brown.[8] In this stage, in which the organism becomes classified as an imago, the metabolic rate of the imago declines because the organism must have enough food to survive the winter period.[19] Both the imago's body weight and fat body weight decrease.[19]

Male larvae are placed in front of the females within the nest allowing the males to emerge first in the spring.[2] Specifically, female eggs are laid in inner brood cells, while male eggs are laid in the outer brood cells. Upon emergence, females fly around for eight weeks.[18] These bees store mostly pollen moistened with a small amount of nectar,[4] which is eaten by the larvae during the summer before they rest through the winter in a cocoon.


Color Vision[edit]

O. rufa bees possess a trichromatic color system that they use in foraging for pollen from flowers. A similar color system is found in the following bee species: A. mellifera, B. terrestris, B. lapidarius, B. monticola, B. jonellus, P. germanica, and P. vulgaris. Studies have been performed comparing the color systems of O. rufa and A. mellifera. Both species share the same spectral sensitivity functions in ultraviolet and blue receptors while the green receptor in O. rufa is sensitive to longer wavelengths than in A. mellifera [20]

Division of Labor[edit]

No sterile, female workers are present in a colony, and thus all females work alone to produce a nest.[21]

Mating Behavior[edit]

During mating season, male behavior with respect to pursuing females is varied, with some males establishing territories close to nesting sites where females emerge and other males observing flowering sites nearby.[22] Males do not normally engage in intrasexual aggression, though they do inspect each other. When there is a specific mate of interest, however, signs of aggression are evident among males. When several males become aware of a receptive female, all males will try to mount her; the males do not assault each other directly. In some cases, females may escape and not mate with any of the males.[23]

Females are monogamous, mating with one male within a few days post-emergence in the spring. There are, however, difficulties that males encounter in completing successful copulation with females, including male inability to determine where and when females will emerge form. Nests are dispersed widely, increasing the number of sites that new females can emerge from. Additionally, females fly away from the nests as soon as they emerge, increasing the mating challenge for males. To counteract these difficulties, males can increase their mating chances by positioning themselves close to foraging sites. Factors including value, patrolling time, and the number of competing males are taken into account when males roam foraging sites for females.[23]

In male-female interactions, males sense potential mates by observing the body shape of females and, by evaluating the female's sense, determine whether a specific female will be receptive to copulation.[23] Females use such cues as the vibrational bursts of the male thorax, which has been suggested to be a sign of male health and overall fitness, color, and odor to select mates.[24] Successful mating of females does not depend on male body size but on the speed with which males discover female mates.[23] Further, females do not always choose the male with the largest body size, a choice that possibly indicates that there is preference for an optimum male body size; oftentimes, females will choose males with intermediate body sizes.[24] Yet, the sperm supply of each male limits males to only performing 7 copulations in their lifetimes.[23]

Mating Technique[edit]

O. rufa females attract males through sex pheromones,[24] which are localized on the cuticle surface of females. Extracts of the cuticle elicit copulatory behavior in male O. rufa.[25]

During courtship, the male O. rufa stands on the back of the female to try and persuade her to mate. Several indicators of persuasion by the male include vibrating his thorax, rubbing himself against the female, rubbing his antennae over hers, and rubbing his legs over her compound eyes. The female, however, can choose to reject the male and may push him off her back. There are three phases to mating in O. rufa: precopulatory courtship, copulation and postcopulatory embrace. In the spring, when a female first emerges, males in close proximity approach her. When a male establishes his position on the female’s dorsum, other males retreat. During what is called the precopulatory phase, the male rubs the female’s mesothorax with his first two pairs of legs. The male then strokes her antennae with his own in order to persuade her to copulate. Simultaneously, he rubs her eyes with his front legs. Every stroking motion is recognizable to humans as a high-pitched humming sound, which soon turns into buzzing as the male attempts to copulate. In an attempt to copulate, the male moves backward (on the female) and tries to insert his genitalia into the female’s genital chamber, during which he drums on the females face to produce a tremolo. If the female chooses to consent to the male, copulation begins. If she rejects the male, she can bend her abdomen downward to try and shake him off of her. The male will either stop or repeat his attempts at copulation. If the male successfully attains the female, copulation will occur for several minutes. This is followed by a postcopulatory phase which lasts up to 13 minutes. At this time, the male applies an antiaphrodisiac on the female by stroking his abdomen over her in the posterior to anterior direction.[24]

Female O. rufa have a mating plug in their genital chamber after mating. While the mating plug is thought to prevent females from re-mating with other males, its function is not clear in O. rufa as of yet.[25]

Sex Allocation[edit]

Female body size is indicative of the sex allocation of offspring. Specifically, larger females are able to collect more pollen than smaller females, making larger females less prone to open-cell parasitism while away from the nest. To "make the best of a bad job", or counteract the disadvantage they have, smaller females will deliberately produce more male offspring and reduce female offspring body size. These changes occur because the smaller females are obtaining less pollen; investing in offspring that require fewer food provisions - males - therefore allows smaller females to combat their handicap. Larger females, in contrast, had more female offspring. In addition to increased foraging efficiency, females uphold other advantages over small females include increased egg production and longevity. Because it does not benefit males to be larger in size, due to the independence of body size on female mating selection, females normally invest more in female offspring.[16]

Female age also predicts sex allocation in offspring. Older females are less efficient at foraging for pollen in nest construction than younger females. Thus, they produce more male offspring and reduce the size of offspring.[16]


Diapause allows O. rufa to survive harsh winter conditions.[26] Typically in adult insects, reproductive diapause is characterized by a late development of gonads and a buildup of energy reserves. However, diapause in O. rufa is somewhat different. The ovaries of females are not completely inactive during overwintering, as the development of oocytes continues in the vitellarium region.[27] O. rufa begins diapause in November, and diapause termination occurs toward the end of January.10 Diapause typically lasts around 100 days.[26]

There are two phases to overwintering in O. rufa: diapause and postdipause quiescence. During diapause, the values of the supercooling point decreases, but diapause itself is independent of temperature variation. Temperatures of 20 °C will lead to the bees’ death. During postdiapause quiescence, the bees develop normally, but their development is inhibited by temperature variation.[26]

Foraging and diet[edit]

Females spend between 80-95% of their time invested for preparing cells in foraging.[28] O. rufa has shown a strong inclination towards collecting pollen from maple and oak trees, like most other solitary bees. These bees require nectar along with pollen, and while maple provides both, oak provides only pollen. Those females who collect pollen from oak trees must also collect nectar from other plant sources. While the species is polylectic, females temporarily and locally forage on one or two plant species with great pollen abundance in order to maximize pollen mass collected per unit time. This is done to reduce provisioning time for two reasons: to exploit as much pollen as possible in a short period of time during unstable environmental conditions in the spring as well as to reduce the risk of open-cell parasitism. Pollen diversity has shown no effect on the developmental success of O. rufa offspring, hence it is more beneficial for females to maximize pollen mass from a few species than to regard pollen diversity. Protein consumption is one of the major factors influencing the growth of bees. Since maple and oak pollen have similar protein content (with a deviation of up to 5%), larvae reared on the diet of either plant do not differ in cocoon weight – hence the offspring of O. rufa develop equally on the pollen of both zoophilous and anemophilous plants. When oak and maple are no longer in bloom, the bees tend to forage on pollen from poppy and buttercup plants.[7]

Environmental temperature and cocoon weight are negatively related for O. rufa. Larvae decrease their food intake as temperature rises and start cocoon-spinning earlier, resulting in smaller body mass.[7]

Interaction with other species[edit]

Kin selection[edit]

Kin recognition and discrimination[edit]

Research shows that kin recognition is associated to mate selection in O. rufa. Females will select males for mating with whom they are more closely related to. This behavior suggests that females may select males from within their population as opposed to more distance populations. One rationale for this behavior is that males within the same populations as females are better adapted to local environmental conditions than more distant males.[28]


O. rufa feed on pollen, the amount of which affects larva growth.[6] A majority of the pollen these bees comes from Ranunculus acris, R. bulbosu, R. repens, Quercus robur flowering species.[29] Pollen consumption has also been suggested to impact the fitness of individuals in the colony.[6]

These bees also consume nectar. When the nectar supply is limited, however, these bees may consume honeydew.[30]


Parasites of O. rufa include birds, mice, Monodontomerus obscurus Westwood, Chaetodactylus osmiae, Cacoxenus indagator, and Anthrax anthrax.[31] Chaetodactylus osmiae hypopi, specifically, parasitize nests through phoresy and affect both adults and broods.[30] Both Cacoxenus indagator and Anthrax anthrax lay their eggs while O.rufa female adds food to the nest cells.[28] For instance,Cacoxenus indagator, a member of the family Drosophilidae, may be found in nest cells eating pollen. The organism's activity sometimes results in the bee larvae dying from lack of sufficient food.[22]

Human Importance[edit]


Red mason bees are excellent pollinators, particularly of apple trees.[2] For effective utilization of these bees as pollinators of winter rape plantations in Poland, studies reveal that they should be located at least 300 m from entomophilous plants, which will distract the bees from pollinating the plants of interest.[29]


Normally, O. rufa do not sting unless they are threatened and must defend themselves. The female does have a sting, but this sting is much less severe sting than honeybees or wasps.[4] Studies have been performed on the venom within the stinging apparatus and it has been shown that the venom apparatus is like that of the honeybee. However, venom from O. rufa contain less barbs than that of honeybees, possibly explaining why O. rufa venom does not penetrate human skin like that of the honeybee. Protein components in the venom, such as osmin, have been linked to antimicrobial, antifungal and haemolytic activities.[32]


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