Megachilid bees are a family of solitary, native bees that carry pollen only on the underside of the abdomen, never on the hind legs. Large cutting mouthparts allow them to collect pieces of leaves, soil, or plant resins to line their nests. They usually have rather cylindrical, stout bodies.
Most species are dark-colored, and many have pale or yellow bands across the abdomen. Females carry pollen on the underside of the abdomen, within a special clump of hairs called a scopa or pollen brush. This is the only group of bees that has a scopa for carrying pollen. When full of pollen, the underside of the abdomen therefore looks yellow or orange. They do not carry pollen in baskets on their legs.
They have long mouthparts, used for cutting pieces of leaves, soil, or plant resins to line their nests. One sign of the presence of leaf-cutting megachilids (a subgroup called leafcutter bees) is the neatly rounded holes they cut in the leaves of plants.
Length: from about ¼–1 inch (varies with species). Most are about the size of a honeybee.
Habitat and Conservation
As with other bees, most people see megachilids as they are out visiting flowers in gardens, yards, old fields, prairies, and other open places with plenty of flowers.
Nest locations vary. Often, females make nests near where they themselves had lived as larvae.
- Although most nest above ground, some burrow into particular types of soil.
- Others bore tunnels into wood (especially rotting wood) or use preexisting cells made by other insects such as carpenter bees.
- Some construct cells out of mud, clay, pebbles, and/or plant resin, attaching them to the side of a cliff or other rock surface, or onto a tree or plant stem.
- Some use hollow plant stems or reeds.
- Some species even use empty snail shells.
- Many are happy to use artificial nests made for them by people.
Like many other bees, megachilids eat pollen and nectar from flowers, pollinating the flowers as they go. Unlike honeybees, which have so-called pollen baskets on their hind legs, megachilids collect pollen in hairs on the undersides of their abdomens. Female megachilid bees roll the collected pollen into a ball and tuck it into a nest chamber for each egg. The larval megachilid eats the pollen ball its mother created for it.
Megachilids are not aggressive. The various species are important pollinators for a wide variety of plants, both native and cultivated.
Like most other native pollinators, many of these bee species are declining, which is deeply concerning because of their essential role in nature and in agriculture. Learn what you can do to help native bee populations recover.
These are solitary bees: Unlike honeybees, they don't live in hives of closely related individuals, and each female is in charge of laying her own eggs (there are no separate castes of queens or worker bees). But although they are solitary, they often nest in groups, often above the ground, often in cavities in rotting wood. They create multiple chambers and line their nests with soil or plant material. They lay one egg in each chamber of the nest, each with a pollen ball to feed the young when it hatches. Apparently, most species overwinter as newly pupated adults.
The common names of the bees in this family reflect their nest-building material. Mason bees build their nests out of soil. Leafcutter bees build their nests out of leaf fragments. Resin bees use plant resins (sap). Carder and woolcarder bees use plant fibers, such as the downy fuzz or hairs that cover the leaves of some types of plants, or animal hairs. Pebble bees build cells out of pebbles cemented together with plant resin.
A few genera in this family are brood parasites, and these species lack the scopa (pollen basket) under the abdomen. Females of these groups sneak into the nest of a related bee and deposit eggs. Their larvae will eat the larva of the bee that made the nest and then eat its pollen ball.
About 30 percent of U.S. crops rely on native bees for pollination, so it's important to know that we need many more pollinators than just honeybees. Megachilids and other native bees are often specialists that evolved alongside specific types of plants and are the only insects capable of pollinating them.
Megachilid bees pollinate wildflowers, fruits and vegetables, and a variety of other crops.
The plight of wild bees should concern us all. Fortunately, homeowners can easily help bee populations in their yards. Plant flowers that will bloom throughout the season, especially early and late in the season, when fewer pollen and nectar sources are available. Avoid pesticides and any plants or seed treated with neonicotinoids, which are harmful to all insects, including bees (look on plant or seed labels for imidacloprid or acetamiprid). Reduce mowing to allow broadleaf plants in your lawn to flower. Consider putting up a bee house.
Megachilid bees, being solitary, do not have a colony to defend the way social bees and wasps do, so they do not aggressively defend their nests. They only sting if mishandled. At any rate, the sting is said to be less painful than a honeybee's.
Some megachilids are bred commercially as pollinators, such as certain species of mason bees (Osmia spp.), which are especially helpful for pollinating blueberries, alfalfa, onions, carrots, and other fruits and nuts. Providing nesting holes for them, such as hollow plant stalks, drilled blocks of wood, and paper tubes, helps attract them.
On the down side, leafcutting bees are often attracted to landscaping plants such as roses, redbud, and other species with thin, flat, smooth leaves or flower petals. They may chew circles out of the leaves or petals, making them less attractive. We encourage you to tolerate this, because the bees are so beneficial as pollinators.
Several species have been introduced to North America to serve as additional pollinators in our country. One example is the alfalfa leafcutter bee (Megachile rotundata), a native of Europe.
The name is pronounced mega-KILL-id as well as mega-CHILL-id. It means "big-lipped," coming from Greek mégas (big) and kheîlos (lip). It refers to the large mouthparts these insects use in collecting materials for nest construction.
Many insects and other arthropods eat megachilid bees outright, including spiders, assassin bugs, and robber flies. Others are parasitoids of megachilids (as larvae, they are parasites that end up killing their hosts), including certain types of flies, wasps, and beetles. Ants may raid the nests of megachilid bees. There are even some types of megachilid bees that are brood parasites of other megachilids, laying their own eggs in their relatives' laboriously made nests.
Many types of birds eat bees, and many other vertebrate insectivores aren't put off by the idea of a little sting, either.
Native bees play a critical role in the health and integrity of our natural habitats. Amid Missouri's array of woodlands, grasslands, and wetlands is a wide variety of wildflowers and other flowering plants. Most of these are pollinated by various types of bees. Without pollination, they cannot produce seeds; they would not reproduce. The seeds, fruits, and the plants themselves are the food of countless animals. In a very real way, pollination is critical to life as we know it.
Some of the fossil evidence for leafcutter bees includes a leaf fossil from the Middle Eocene (about 45 million years ago) that shows the distinctive rounded holes these bees make.