Pollinator of the Week: Leaf Cutting Bees

A female leafcutter bee collecting pollen. Image courtesy of Jim McCulloch.

This article is excerpted from Leaf Cutting Bees (Megachile spp.) by Beatriz Moisset, USDA Forest Service.

There are about 242 species of Megachile bees or leaf cutting bees in North America. They belong to a larger group that includes also other leaf cutting as well as mason bees; these are all very good pollinators with very interesting habits.

Megachile bees are black and furry. They vary in size, on average about the same size as a honeybee. Most bees carry pollen in baskets on their legs. However, Megachile is different; the underside of the female’s abdomen is particularly furry and is used for this purpose. If you see a black bee, about the size of a honeybee, with a yellow belly, you probably saw a Megachile.

A female megachile bee working over a purple coneflower (Echinacea sp.). Image courtesy of Laura Duncan.

Both adult males and females emerge in the spring and soon mate. The males do not live much longer but the females start to look for suitable sites for raising a family. They don’t live in large colonies as honey bees do; most are solitary, meaning that each mother takes care of her own brood; a few form small colonies, but they are not truly social; they merely share the entrance to their respective nests. They nest in a variety of cavities in wood or hollow stems. There are even some that nest underground. The cavity is usually about the size of a pencil; it can be a little wider or narrower depending on the species. Many people build nest houses to help the bees and to be able to watch them. 

The mother megachile bee brings pollen to the nest and some nectar in her crop. She kneads the mixture into a bee loaf, adding some of her saliva, which may contain antibacterial and fungicidal substances. It takes many loads to build up a bee loaf large enough to feed one grub from egg to mature size. She diligently visits numerous flowers on her quest to gather the necessary pollen and nectar. When there is enough food, she lays an egg on top. Then she seals that small chamber with chewed up leaves. If you notice nearly perfect round holes in the leaves of your rose bushes, do not begrudge them that little material that they need to raise their families. She repeats this process of making bee loaves, laying eggs and building partitions until the entire nest hole is full. Then, she builds a final, thicker wall. Shortly afterwards she dies.

The next generation feeds on the pollen and nectar bee loaf, grows, and metamorphoses into an adult that remains dormant until the next spring when it chews its way out, mates and is ready to start the circle of life all over again.

In addition to all the native Megachile bees there are a few non-native ones such as the alfalfa leaf cutting bee (Megachile rotundata) which was introduced intentionally to pollinate alfalfa and the giant resin bee (Megachile sculpturalis), which arrived from Asia in recent years; no one knows how.