Osmia bicornis (synonym Osmia rufa) is a species of mason bee, and is known as the red mason bee due to its covering of dense gingery hair.^[2][3][4][5] It is a solitary bee that nests in holes or stems and is polylectic, meaning it forages pollen from various different flowering plants.^[6] These bees can be seen aggregating together^[3] 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.^[3][5] Females only mate once, usually with closely related males. Further, females 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 colour vision^[7] and are important pollinators in agriculture.^[3]

Quick Facts Red mason bee, Conservation status ...

Red mason bee
Conservation status
Least Concern  (IUCN 3.1)[1]
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Insecta
Order:	Hymenoptera
Family:	Megachilidae
Genus:	Osmia
Species:	O. bicornis
Binomial name
Osmia bicornis (Linnaeus, 1758)
Synonyms
Osmia rufa (Linnaeus, 1758) Apis bicornis Linnaeus, 1758 Apis rufa Linnaeus, 1758 Apis agino Harris, 1776 Apis cornigera Rossi, 1790 Apis fronticornis Panzer, 1799 Osmia hedera Smith, 1844 Osmia fracticornis Pérez, 1895 Osmia rufa var. borealis Ducke, 1899

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This species is part of the order Hymenoptera, which consists of bees, wasps, ants, and sawflies, and a member of the family Megachilidae, which mostly consists of solitary bees, and is among 11 species of Osmia identified in Britain.^[8] O. bicornis is the current scientific name for this bee, although it was formerly known as O. rufa.^[9] In 1758, Linnaeus described the male of this species under the name Apis rufa and described the female as a separate species Apis bicornis. In 1802 Kirby recognised that A. bicornis and A. rufa were the same species and he named this species Apis bicornis. Subsequently, the opinion was put forth that A. rufa was the correct name, because it appeared before bicornis in the Systema Naturae. The use of the name rufa does not comply with the International Code of Zoological Nomenclature's rules, however; the concept of "line priority" does not exist in the rules, and the action of the first revising author, Kirby, must be followed. Thus the correct scientific name for the species is Osmia bicornis, although O. rufa is still widely used.^[10] Three subspecies of O. bicornis are often recognized, namely O. b. bicornis,^[7] O. b. cornigera,^[11] and O. b. fracticornis.^[12]

O. bicornis is about the same body size as the honeybee.^[6] Sexual dimorphism is observed in this species; females are larger than males, because the female larvae are provisioned with (and eat) more pollen.^[13] Body size in O. bicornis decreases as temperature in brood cells increases. Beyond 25 °C, body growth can be severely truncated, leading to small adult body size or mortality.^[11] The male and females are also distinguishable by antenna length, with males possessing an additional antenna segment, (characteristic of almost all Hymenoptera).

O. bicornis is found in England, southern Scotland (possibly northern Scotland, as well), Wales, Ireland, mainland Europe, Sweden, Norway, North Africa, Georgia, Turkey, and Iran.^[2] Of the 11 species identified of Osmia in England, O. bicornis is both the largest and most common species present.^[8]

O. bicornis occupies 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.^[4] O. bicornis occupies the old shells of these three species: Helix nemoralis, Helix hortensis, and Helix pomatia and the nests of Anthophora species. 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^{[clarification needed]}, and window frame holes and cracks.^[8]

The maximum foraging distance for O. bicornis is about 600 m, though generally high plant density around the nests allows bees to forage closer to the nest and for a shorter duration.^[14]

The nest of O. bicornis consists of an array of partitioned cylindrical cells^[15] in holes in wood or reed tubes.^[16] These bees accept a diverse range of pre-existing cavities as nest sites.^[17] 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.^[18] 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.^[15] 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.^[16] The material used to build the nests is mud mixed with their mandibles,^[18] but 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.^[15] Females construct around six cells per nest on average; however, larger females construct more cells than smaller ones.^[19] When it is time for females to lay their eggs, they add pollen to each brood cell and lay one egg in each cell next to the pollen.^[20] The sequence of nesting behavior is: cell construction, provisioning, egg-laying, and sealing the cell.^[15]

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.^[15]

Although these bees may be seen into late June, they are most active during the spring and early summer.^[21] Each year, one generation of bees is formed, making an appearance during the spring.^[22] About one week after eggs are laid in the brood cells, the eggs hatch and larvae develop through the summer.^[20] The larvae then enter the pupal 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.^[20] The eggs hatch after about one week; the larvae start spinning a cocoon about one month after hatching.^[8] The bees become adults in the fall, but stay quiescent until next spring.^[8]

During cocoon formation, the larva uses saliva to encompass the faecal 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 uses its digestive contents to form smears on the cocoon, leading to the hardening of the cocoon and a colour 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 it must have enough food to survive the winter.^[23] Both the imago's body weight and fat body weight decrease.^[23]

Male larvae are placed in front of the females within the nest, allowing the males to emerge first in the spring.^[3] 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 about eight weeks.^[22] These bees store mostly pollen moistened with a small amount of nectar,^[5] which is eaten by the larvae during the summer before they rest through the winter in a cocoon.

Diapause allows O. bicornis to survive harsh winter conditions.^[28] Typically in adult insects, reproductive diapause is characterized by a late development of gonads and a buildup of energy reserves. However, diapause in O. bicornis is somewhat different. The ovaries of females are not completely inactive during overwintering, as the development of oocytes continues in the vitellarium region.^[29] O. bicornis begins diapause in November, and diapause termination occurs toward the end of January. Diapause typically lasts around 100 days.^[28]

The two phases of overwintering in O. bicornis are 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 lead to the bees’ death. During postdiapause quiescence, the bees develop normally, but their development is inhibited by temperature variation.^[28]

Females spend between 80 and 95% of their time invested in preparing cells in foraging.^[30] O. bicornis 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 that 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 to maximize pollen mass collected per unit of time. This is done to reduce provisioning time to exploit as much pollen as possible in a short period of time during unstable environmental conditions in the spring and to reduce the risk of open-cell parasitism. Pollen diversity has shown no effect on the developmental success of O. bicornis 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 up to 5%), larvae reared on the diet of either plant do not differ in cocoon weight – hence the offspring of O. bicornis 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.^[9]

Environmental temperature and cocoon weight are negatively related for O. bicornis. Larvae decrease their food intake as the temperature rises and start cocoon-spinning earlier, resulting in smaller body mass.^[9]