Alfalfa leafcutting bee
Megachile rotundata, the alfalfa leafcutting bee, is a European bee that has been introduced to various regions around the world. As a solitary bee species, it does not build colonies or store honey, but is a very efficient pollinator of alfalfa, carrots, other vegetables, and some fruits. Because of this, farmers often use M. rotundata as a pollination aid by distributing M. rotundata prepupae around their crops. Each female constructs and provisions her own nest, which is built in old trees or log tunnels. Being a leafcutter bee, these nests are lined with cut leaves. These bees feed on pollen and nectar and display sexual dimorphism. This species has been known to bite and sting, but it poses no overall danger unless it is threatened or harmed, and its sting has been described as half as painful as a honey bee's.
Megachile rotundata is a European leaf-cutting bee placed in the subgenus Eutricharia, the "small leaf-cutting bees"; they are 6–9 mm (0.24–0.35 in) in length. They are partially bivoltine, meaning that under the right conditions they can produce two generations per year. These bees present a sexual dimorphism, in which the males are smaller than the females and differently marked. Megachile rotundata bees are a dark grey color. Females have white hairs all over their bodies, including on their scopae. In contrast, males have white and yellow spots on their abdomens.
Megachile rotundata is currently found on all continents except Antarctica. In North America, the species was deliberately imported to assist in the pollination of food crops, but has now become feral and widespread. M. rotundata was also introduced to New Zealand in 1971 and Australia in 1987 to assist in the pollination of alfalfa (known locally as lucerne).
Megachile rotundata can feed on nectar and pollen from a variety of plants but prefer Medicago sativa. Females will immediately begin feeding after emergence during the maturation period of their eggs. During feeding, the bee will insert its proboscis into the keel of the plant. In the process, pollen is brushed onto its scopa.
Females construct tubular nests in a variety of sites, including rotting wood, flower stems, reeds, and soda straws. In the wild, females also create nests in small holes in the ground or in available cracks/crevices in trees or buildings. The nests are composed of a string of individual cells, as many as the space will allow. When managed for pollination, the females are induced to nest in paper cylinders similar to drinking straws or drilled blocks of wood.
Each cell is made from circular disks cut from plant leaves using the bee's mandibles, hence the name "leafcutter". Females use about 15 leaves per cell, called a concave bottom, overlapping the leaves to produce the thimble-shaped cell. While the bees do not store honey, females do collect pollen and nectar which they store in the cells of their nests. Specifically, females first regurgitate the nectar they have provisioned into the cell and then transfer the pollen that is attached to their scopa on top of the nectar. Each cell contains one pollen and nectar ball, and one egg with each cell containing a 2:1 nectar-to-pollen ratio. The completion of one cell in the nest requires between 15 and 20 provisioning trips. After the female lays her eggs, she seals the cell with circular leaf pieces.
Studies reveal that positioning of male and female progeny in the nest is strategic and that cell size plays a major role in the size of progeny, independent of the mother's size. Females have been observed to lay female eggs in the inner cells and male eggs in the outer cells. With respect to sex ratios, larger cell provisions are correlated with a greater production of female offspring. Two explanations exist for these behaviors in terms of mother's foraging behaviors: 1) The mother brings more provisions to the inner cells because she expects that female progeny will be produced there and 2) the mother chooses to fertilize her egg, and therefore promotes the production of female progeny, because she has to bring larger provision proportions to a larger cell. The sex ratio changes depending on nest size, length, and nesting material. This ratio is controlled by the female. These observations have been made for females that make their nests in tunnels. For example, a 5.5 mm tunnel diameter is associated with a 3:1 ratio and a 6.0 mm tunnel diameter is associated with a 2:1 ratio of males to females. Shorter tunnels, those that are below 5.0 cm long, are less favorable.
As a member of the Hymenoptera order, the alfalfa leafcutter bee is haplodiploid. Adults emerge by the end of the summer through one of two developmental pathways: larvae will develop by the end of one summer and proceed through the a prepupal diapause phase until the next summer; or larvae, known as "second-generation" bees, skip this phase and emerge as adults in the same summer.
The larva transitions through four instar stages before emerging as an adult. During its development, which occurs rapidly, the larva consumes the pollen ball and enters diapause when the pollen is fully consumed. In its progression into the diapause phase, the larva defecates pellets in a ring formation and then spins its cocoon out of silk threads. The next spring, the mature larva pupates, a process that lasts 3–4 weeks, and completes its development. Once the bee is developed, it cuts its way out from the nest by chewing itself out of its cocoon.
Upon emergence, females mate immediately and begin constructing their nests after a week.
Progeny released via the two alternative pathways for emergence display different sex ratios and sizes. Among adults that emerge during the summer of the same year, the sex ratio is biased towards males. Among the "second-generation bees", however, the sex ratio is female biased. Further, spring-emergent adults weigh more than summer-emergent ("second-generation") adults. These differences have been proposed to be attributed to the chances of survival to mating of the two sexes and the metabolic costs involved in development. Bees that undergo diapause and emerge in the spring must endure the long winter, so require more food stores. As a result, they will be larger when they mature. Another explanation has been that smaller bees mature faster, thus are able to mate more quickly when they emerge in the summer to avoid the cold, harsh conditions of the winter.
The sex ratio of the offspring also depends on the distance between nesting and foraging sites. Females have been observed and determined to bias their offspring sex ratio to males at larger flight distances from the nest.
Megachile rotundata has been found to be a monandrous bee species. During the mating season, males attempt to obtain mates by positioning themselves at sites where female are likely to be, including foraging sites and nests. While females can mate several times, they resist male advances by restricting their mating to one sexual interaction and fleeing from these males. This behavior is attributed to female productivity during the construction of her nest and egg-laying. When harassed, females are unable to build their nests efficiently, making less foraging trips and spending a longer time overall in nest construction. Fleeing allows females to avoid being mounted. In the process of fleeing, however, females may lose their nesting materials, such as leaves and must then make an additional trip to make up for the lost materials.
