Leafcutter bees, members of the genus Megachile, have an interesting life history. You may have seen their plant damage and wondered who did it because they are seldom caught in the act. Even though the nearly circular holes left on the edges of leaves are pretty good-sized at nearly a half inch across in some cases, it only takes the female bee a matter of seconds (one report says about 10) to chew it out. Commonly used plants include rose, lilac, Virginia creeper, azalea, redbud, ash and others. Leaves that aren’t too thick or waxy are preferred.
These solitary bees are not eating the leaf segments they remove, they are using them for building their nests. Nests are created in long narrow cavities such as hollow stems or crevices. As solitary bees, the female does all the work of nest building, foraging for food and egg-laying. While each queen will build and provision her own nest, some species will be found creating numerous nests near each other. A female may even enter the wrong nest by mistake but she will exit and not work on it or lay an egg there.
Leafcutter bees vary somewhat in size but in general are similar in size to a honeybee. They are mostly black on the back or top and fuzzy golden to orange on the lower side of the abdomen. Adults forage for both nectar and pollen and blend them in combination with saliva to form a ‘bee loaf’ for each individual cell. Formation of the complete bee loaf requires many trips between the nest and the flowers. Once the loaf is completed, she lays a single egg on it. Each cell is lined with leaf pieces and after the egg is laid, the cell is sealed with chewed leaves. A completed nest, depending on species and nest size, may have six to twelve cells/eggs.
Nest building and egg laying occur during early to mid summer. References differ on how the new generation of bees overwinter. Some say they overwinter as larvae while others say they mature into adults and then remain dormant within the cells until the following spring. It may vary by species but in any case there is one generation per year. Adults emerge in the spring once it is suitably warm. They mate over the course of a couple of weeks and the males soon die. Females die once they complete egg laying later in the summer.
Leaf cutting bees are fantastic pollinators. This is because of the way they forage and carry the pollen. Unlike honeybees, who moisten the pollen for carrying on their hind legs, leafcutter bees carry the pollen dry. When they are visiting a flower, they are active, resulting in quite a bit of pollen adhering to their hairy abdomens. Because it’s carried dry, it is easily and abundantly dislodged and distributed to subsequent flowers visited. Early in the twentieth century, pollination of alfalfa, an important livestock food plant, was decreased due to a lack of pollinator habitat as agriculture and land clearing expanded and alfalfa seed shortages resulted. The alfalfa leafcutter bee was introduced to the U.S. and pretty much saved the alfalfa seed industry. It is still relied upon for this purpose today.
Because of their effective role as pollinators, it’s beneficial to encourage leafcutter bees. Another positive characteristic is that they are not aggressive and, while they have stingers, they are only used when the bees feel very threatened or confined. The sting is also minor compared to most. Nesting sites or ‘houses’ can be purchased or constructed from common materials to attract and maintain leafcutter bee populations to your yard or area. A very simple nest unit can be made by drilling numerous ¼” holes (or about the diameter of a pencil) 6” deep into a block of wood. Face the side with the holes to receive early morning sun. One source recommends bringing the nest into an unheated shelter such as a garage or shed once cell construction is complete to protect the overwintering leafcutter bees from pests and predators.
There are many native leafcutter bees and they have the potential to be major contributors to pollination in gardens and on small farms. They don’t forage far from the nest for provisions or for leaf pieces (about 300 ft.) and that is why they are less effective on large expanses of crops.