In nature, leafcutter bees nest in cavities like tunnels made by wood-boring insects. The absence of abun- dant and secure nesting sites limits the size of wild populations. Artificial nests can be constructed with blocks of wood or Styrofoam drilled with multiple
holes, bundles of paper straws, or even sections of reed or bamboo to increase their population.
When searching for a suitable nest site, a female bee will repeatedly enter and inspect numerous cavi- ties. Tunnel depth, diameter, and material all influ- ence the appeal of the nests to female bees, as well as the ratio of male to female eggs that she will lay.
After selecting a nest hole, the bee will then fly in a zigzag pattern in front of the nest entrance, memorizing its exact location. During this orienta- tion flight, the bee memorizes adjacent visual land- marks which help her to relocate the nest later. After memorizing the location, the nest cannot be moved without disorienting the bee, often causing her to completely abandon the nest.
After memorizing the tunnel, the bee will then fly off in search of suitable leaf material from which to begin construction. Leafcutter bees prefer soft, flexible leaves for nest construction, such as alfalfa leaves and flower petals, clover, buckwheat, roses, lamb’s-quarters, and numerous other plants (figure 7.4). Upon finding a suitable leaf, the bee will cut out a semicircular section from the leaf edge (figure
Figure 7.3 Male and female alfalfa leafcutter bees. Note the smaller size, rounded abdomen, and longer antennae of the male (below). Also note the row of gray scopa hairs on the underside of the female’s abdo- men.
Eric Mader
Figure 7.4
As the name suggests, leafcutter bees cut leaf pieces with their mandibles (jaws). They use the leaf pieces to construct the inner par- titions of their nest.
Figure 7.5 Circular sections cut from the edges of leaves are a sign of local leafcutter bee populations. Bees tend to prefer cer- tain plant species over others for leaf harvesting. Favored plants include buckwheat, wild grape, bindweed, wild senna, rose, and tartarian honeysuckle (pictured).
Eric Mader
7.5) with her scissor-like mandibles, resuming flight at the exact moment she finishes cutting out the leaf section. The bee then flies back to the nest with the leaf section held between her legs (figure 7.6). The leaf section may weigh close to one-fourth the weight of the bee herself.
Back at the nest, the leaf piece is transferred to the mandibles as it is carried to the back of the tun- nel. Several other half-circle shaped pieces will then be gathered and placed in the back of the tunnel in overlapping layers, creating a cup-shaped concave cell cemented together with salivary secretions and leaf resins. In commercially available nests, 15/64 to ¼ inch in diameter, (~6 to 6.4 millimeters), roughly 15 leaf pieces are required to construct this initial cell. Within this cell the female bee will deposit a provi- sion of pollen and nectar upon which she will lay an egg. The female will then close off the open end of the cell with two or three additional leaf pieces.
In gathering provisions for the nest, the female bee collects pollen which gets trapped among the hairs covering her body. Periodically while foraging, she will groom herself, transferring pollen from the front of her body backward with her forelegs, to the middle legs, and finally to the hind legs which scrape the pollen onto the scopa—the rows of long stiff hairs on the base of the abdomen that hold the pol- len granules during the flight back to the nest. The bee also collects nectar while foraging by sticking
her tongue into the corolla of flowers and sucking the nectar into her crop, or “honey stomach.” Upon returning to the nest the bee will enter headfirst and regurgitate the nectar at the back of the cell. She will then somersault inside the nest if there is sufficient room, or she will back out of the nest, turn around and back in. She then scrapes the pollen from her scopa using her back legs and tamps the pollen-nec- tar paste into position using the tip of her abdomen (see sidebar, page 79).
A total of 15 to 30 trips may be required to gather the necessary pollen and nectar to provision one cell. Initially the mother bee will collect more pollen than nectar, later increasing the amount of nectar col- lected until the final ration consists of around two- thirds nectar to one-third pollen. Like the leaf pieces from which she constructs the nest cell, these pollen provisions can weigh almost a quarter of the bee’s total body weight. Under warm, clear conditions with unlimited forage, a bee may visit up to 25 flow- ers a minute and complete a single cell in five hours. Prior to closing off the cell, the mother bee lays an egg which is inserted into the pollen-nectar paste. After closing this cell, she will then immediately begin construction of a second cell, repeating the process until the entire tunnel is almost filled. Depending on the length of the tunnel, 8 to 12 cells may be con- structed in a single cavity. When the tunnel is nearly filled with cells, the bee will then collect 10 to 50 circular pieces of leaf which are deposited individu- ally into the nest entrance. These circular pieces are cemented together forming a solid plug which is flush with the hole entrance(figure 7.9). This plug serves as a barrier against rain, predators, and parasites.
Under favorable conditions a female bee may fin- ish two to four tunnels in her lifetime, with a rough average of around 30 eggs—although 50 or more eggs have been documented in some cases. Of course during this nesting period the bee is exposed to pre- dation by birds, insects, and other animals, inclem- ent weather, pesticides, and a host of other condi- tions that can halt nesting activities at any moment. Female bees spend the evening inside the nest, face inwards. Under overcast conditions, during rain, or when temperatures drop below 64°F (17.8°C), females will remain inside the nest facing outward, guarding against intrusion by other insects.
Figure 7.6 Returning bees carrying cut leaf sections that will be used to line the nest interior.
Eric Mader
Mated female bees store sperm in a specialized organ called the spermatheca. The spermatheca allows the mother bee to determine the sex of her offspring by either fertilizing an egg, resulting in a female offspring—or not fertilizing an egg, resulting in a male offspring. Female nest cells are larger and are provisioned with more food. Normally female eggs are laid in the innermost cells, and male eggs are laid in the outermost cells closest to the entrance where parasite and predation rates may be higher because they are more expendable. Sex ratio is deter- mined in large part by the depth and diameter of the nest cavity, with deeper tunnels favoring the pro- duction of more female bees. Under optimal condi- tions, sex ratios should be close to 1.5 to 2 males per female. Many commercially available leafcutter nests favor higher male bee ratios.
A
lfalfa has an unusual flower for which the alfalfa leafcutter bee is especially well adapted. Like many other legumes, an alfalfa flower con- sists of one large “standard” petal, two “wing” petals on the sides of the flower, and two fused bottom petalswhich are called the keel (for their resemblance to a boat keel).
The keel petals enclose the stamen column under tension. When these fused petals are slightly sepa- rated by an insect probing the flower, the column is released, springing forward and slamming into the standard petal. Once released, or “tripped,” the sta-