Because Rebecca had a question about the leafcutter bees in my last post, I thought I would expand a bit on their biology.
Leafcutter bees (genus Megachile) are solitary bees that are important pollinators. As with most solitary bees, each female constructs its own nest.
The leafcutter bees prefer to nest in pre-formed holes about the size of a pencil (roughly) in diameter. They often nest in wood or hollow stems, although they will also use sites like holes between bricks or even screw holes in patio furniture.
They then carry the pieces back to the nest hole they chose and use the leaves to line the cavity and construct chambers to lay their eggs in.
Below is a completed nest that I found between two cement blocks:
Inside this bundle will be several chambers separated by neat walls of leaf. Within each chamber will be a ball of bee bread, a mixture of nectar and pollen (see sweat bee post for what this looks like). On the ball will be a single white egg, which will hatch into a larva and consume the bee bread. It will pupate within the chamber and later emerge as an adult.
Leafcutter bees harvest leaves from relatively few species of plants. In addition to the pomegranate, they will also use the bracts of bougainvillea, creating a lovely bright pink or red nest. The plants they seem to prefer most are roses, which gets them into trouble with rosarians.
As with many solitary bees, leafcutters are important pollinators. The females carry the pollen in the bristly hairs on the underside of their abdomen, which are called scopae.
Leafcutter bees are managed commercially for pollination of alfalfa for seed production. The idea is that honey bees avoid the elaborate tripping mechanism of the alfalfa flower, but the leafcutter will readily use and pollinate the flowers. The alfalfa leafcutter bee is an introduced species.
The farm were I worked once brought in leafcutters to pollinate an alfalfa crop. Although they are solitary, these leafcutters will nest in aggregations. The farmer brought in a smallish trailer piled high with leafcutter nests, each with an active female. During the day there was a cloud of bees flying to and from the trailer. It was actually very cool, and I wish I had taken a few pictures.
Have you ever seen leafcutters in action?
Places to find out more about solitary bees: