Megachile rotundata

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. 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. More specifically, it belongs to the genus Megachile, which contains 52 subgenera and 1,478 species. The genus Megachile consists of bees that cut leaf pieces to line their nests.

thumb|Heads-on view of female

Description and identification

thumb|Backside of female, showing such features as the [[thorax, abdomen, wings, and color of the body hairs]]

Megachile rotundata is a European leaf-cutting bee placed in the subgenus Eutricharaea, the "small leaf-cutting bees"; they are 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. 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).

Nest construction

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". 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.

Nest recognition

During nest construction, females transfer olfactory cues onto their nests to facilitate nest recognition. Specifically, they transfer these hormones by running their abdomens along the nest or excreting liquid from the tip of the abdomen. The olfactory cues are especially concentrated around the nest entrance. When these olfactory cues were experimentally removed, females of M. rotundata were unable to identify their own nests, revealing the importance of these chemicals. The chemical composition of these olfactory cues includes hydrocarbons, wax esters, fatty aldehydes, and fatty alcohol acetate esters.

Life cycle

left|thumb|Pupae of (A) female M. rotundata and (B) subfossil male [[Megachile gentilis]]

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 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.

Sexual behavior

Mating

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 females 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 fewer 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.

Interaction with other species

Diet

Megachile rotundata can feed on nectar and pollen from a variety of plants but prefer Medicago sativa.

Parasites

Various families of wasps are the primary parasites of M. rotundata. Some species include Pteromalus venustus, Monodontomerus obscurus, Melittobia chalybii, Diachys confusus, Signs of chalkbrood infection have been observed to occur in the fifth-instar larvae, leading to the milky appearance of the larval hemolymph and development of a pink, tan, or gray cast in their head or abdominal regions. These changes are accompanied by the spread of pink, tan, or gray color throughout the body of the larvae, and dark-colored, fungal cysts appear under the cuticle.

A variety of microorganisms have been isolated from the alimentary canals and frass of M. rotundata. Bacteria include Bacillus firmus, B. licheniformis, B. megaterium, B. pumilus, and Streptomyces spp., and fungi include Trichosporonoides megachiliensis.

Pollination

thumbnail|M. rotundata [[pollinator on alfalfa flower]]Megachile rotundata was accidentally introduced into the United States during the 1940s, and its management as a pollinator leads to increases in seed production of some crops in the U.S. It has been shown to pollinate these plant species:

References

External links

Effectiveness of the Alfalfa Leafcutter Bee as a pollinator of legume forage crops (ISHS membership required)
How to Manage Alfalfa Leafcutting Bees for Wild Blueberry Pollination