Bees are indispensable for pollination. The numbers may vary, however research has shown that bees pollinate up to two-thirds of all the foods we eat. Honeybees that farmers and fruit growers manage and use for pollination are different from wild bees. While the honeybee might be the bee we hear about most often, there are many species of wild bees important to the pollination effort. If you see a black bee about the size of a honeybee on a flowering plant in your yard it is probably a leafcutter bee. Being able to identify leafcutter bees, and knowing their lifecycle and nesting habits, will help you manage and protect this important pollinator.
How to Identify a Leafcutter Bee
Leafcutter bees are solitary bees of the Megachilidae family, and there are about 242 native Megachile species in North America alone. They are about the same size as honeybees; but their bodies are black and furry where honeybees are dark brown to black and yellow striped.
Another way to identify a leafcutter bee is the way the females transport pollen. Other types of bees have unique pollen "baskets" on their rear legs for collecting and carrying pollen. The leafcutter bee has special hairlike structures on the underside of her abdomen used for transporting pollen. The underside of the bee might appear yellow or golden, but look closely and you might see this is actually pollen.
Look for leaf cutter bees on ornamentals with thin, smooth leaves such as roses, azaleas, ash trees, redbud trees, and bougainvillea. You may also find them pollinating wildflowers as well as squash, melons, peas, and other summer fruits and vegetables.
The Lifecycle of Leafcutter Bees
Leafcutter bees mate shortly after emerging in the spring. The males live for a short period of time after mating, whereas the females seek out potential nesting sites. As solitary bees, leafcutter bees don’t live in colonies with a queen. Each bee builds her own nest and raises brood on her own.
Starting in late spring and into late summer, the leafcutter bee builds her nest in wood cavities, typically rotten wood, or in tunnels carved into hollow stems, empty snail shells, dry soil, or holes in concrete walls and other man-made objects.
Depending on the species—the members of the Megachile family vary in size—the cavity can be smaller or larger but on average it’s about the diameter of a pencil.
To build the nest, the female bee chews ¼- to ½- inch circular pieces from leaves or petals which she layers into the shape of a thimble for each cell. A typical nest consists of up to 20 of these cells, packed tightly together. The female then begins the process of collecting nectar and honey which she mixes with her own saliva to create food for her larvae. After placing food in each cell, she lays a single egg in each one. The final task in her life cycle is to close each cell with a seal made of chewed up leaves.
When the larvae emerge, they feed on the food in the nest and pupate over winter. In spring, the new generation of adult bees chew their way out of the nest and the lifecycle begins anew.
Unlike other bees, wasps, and hornets, leafcutter bees won’t become aggressive when you approach their nesting sites, and they will only sting when handled. The sting is much less painful than the sting of a wasp, hornet, or honeybee. The venom content is much lower however if you have had an allergic reaction to stings, seek medical treatment if you have a reaction after being stung by a leafcutter bee.
Do Leaf Cutter Bees Cause Garden Problems?
Many species of leaf cutter bees chew circular sections from the leaves of plants which they use to build nests. Unlike insects that feed on plant leaves, these bees take only what they need for constructing the nest. Holes in the leaves of your favorite rosebush can be unsightly but the plant, itself, is rarely damaged.
Insecticides are not recommended nor are they effective for this insect. Leafcutter bees don't actually feed on the leaves, so most sprays will not work. Because they are such efficient pollinators, these bees are considered a beneficial as opposed to an insect pest. Using insecticides will kill other beneficial insects too, and these are the ones that you need in your garden.
If you want to protect a prize plant in your garden, try a floating row cover, cheesecloth or dense netting until the bees have finished building their nests,
Why Leafcutter Bees Are Important Pollinators
Mason bees, Osmia spp., are one species of leaf cutter popular with gardeners. Nests are constructed in existing or newly excavated hollows, similar to nesting sites for other leafcutter species. The Mason bee lines its nest with mud, rather than leaves. Mason bee houses can be purchased at many garden centers and DIY patterns exist to build them.
Mason bees are even used as managed bees for commercial pollination for alfalfa, blueberries, carrots, and onions. Some non-native leafcutter bees have also been introduced to North America specifically for commercial pollination purposes, such as the alfalfa leafcutting bee Megachile rotundata.
How to Encourage Leafcutter Bees
Instead of eliminating the nesting sites of the bees, consider providing the bees with suitable housing, either by leaving that rotting tree stump in the back corner of your yard, or by building a leafcutter bee hotel from scratch or from a kit. Building a leafcutter bee hotel and watching the bees build their nests is a fun project to do which children.
The only thing you need to watch out for are the parasitoids that may attack the nests of leafcutting bees, such as wasps, beetles, and ants. The Coelioxys is also a leafcutter bee, albeit not a friendly one, that lays her eggs in the nests of other leafcutter bees where her larvae gobble up the pollen.