Bees are indispensable for pollination—without them, humans would not be able to grow food. The managed bees that farmers and fruit growers use for pollination are different from the wild bees. When a black bee about the size of a honeybee appears on a flowering plant in your yard, there is a fair chance that it is a leafcutter bee. Just like honeybees, leafcutter bees are important pollinators and harmless to your plants. The damage that might cause in your garden is negligible and far outweighs their benefits.
Being able to identify leafcutter bees, and knowing their lifecycle and nesting habits, will help you manage and protect them so that humans and bees can coexist happily in your yard.
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 moderately sized, about the same size as a honeybee; their body however is black and furry.
Another way to identify a leafcutter bee is the way the females transport pollen. Unlike other bees, the leafcutter bees do not carry pollen on their hind legs but on the underside of their abdomen whose hairs are specifically designed for that purpose. The underside of the bee might appear yellow or golden, although upon taking a closer look, you might be able to see that it’s actually the pollen.
The Lifecycle of Leafcutter Bees
Shortly after the adult males and female leafcutter bees emerge in the spring, they mate. 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 her brood on her own.
Starting in late spring and into the late summer, the leafcutter bees build nests 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 bee 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. After the nest has been built, the bee goes back and forth between flowers and the nest many times to supply each of the cells with ball of nectar or pollen that she mixed with her own saliva to feed the larvae. Then she lays a single egg on top of the food in each cell. Finally, she seals the entrance with a thick layer of leaves, then dies.
After the new generation of bees emerges, it feeds on what’s in the nest and grows into adult bees. They remain in the nest in a state of dormancy until the next spring when they chew their way out of the nest and the lifecycle begins anew.
Are Leafcutter Bee Stings Dangerous?
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.
Do Leaf Cutter Bees Cause Garden Problems?
The short answer is no. The circular sections that the bees remove from leaves or petals can be unsightly, but rarely affect the health of the plants. Also, their impact is limited, as the bees take only as much as they need for building their nest, unlike an insect that voraciously feeds on leaves.
The preferred nest-building material of leafcutter bees is a deciduous ornamental with thin, smooth leaves such as roses, azaleas, ash trees, redbud trees, and bougainvillea.
The recommendation is that you simply put up with the limited impact. By no means should you apply an insecticide; it would be an ineffective way to try to get rid of the leafcutter bees as they do not feed on the leaves, and it would also kill a host of other beneficial insects.
If you feel that you absolutely need to protect an ornamental plant from leafcutter bees after you have noticed some chewing, you can protect it with a cheesecloth until the bees are done building their nests. Keep in mind though that a barrier will keep all other pollinators out as well.
Why Leafcutter Bees Are Important Pollinators
Leafcutter bees are important native pollinators of many wildflowers as well as squash, melons, peas, and other summer fruits and vegetables.
Some leafcutter bees are used as managed bees for commercial pollination. Under controlled conditions some leafcutter bee species can undergo several generations per year. Osmia leafcutter bees for instance are used to pollinate 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 nesting sites, either by not removing 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.