Leafcutting bees—Megachile spp.
Leafcutting bees (family Megachilidae) are beneficial pollinators that cut circular pieces from leaves and petals to use as nesting material for their young. At least 75 Megachile species of leafcutting bees occur in California. For example, the introduced alfalfa leafcutting bee, M. rotundata, is of great economic importance because it is the most efficient pollinator of alfalfa.
Megachile adults are generally blackish or gray with white or yellowish hairs on the abdomen that form pale bands that separate the segments. Adults can be up 1/2 inch long, but commonly are somewhat smaller than a honey bee. Unlike with honey bees where pollen is carried on hairs on the legs, leafcutting bees carry most pollen on hairs on the underside of the abdomen. See Family Megachilidae - Leafcutter, Mason, and Resin Bees, and Allies from Iowa State University and Leafcutter Bee, Megachile sp. from UC Irvine for photos of adults of various species.
The pale eggs, larvae, and pupae and dark cocoons occur hidden in cylindrical cells in tubelike cavities and are not commonly observed. Immatures occur individually in cells that adult females form in holes or hollows where the young are reared. Locations of brood cells include abandoned tunnels of wood-boring insects, stem hollows, and tree holes.
The Megachilidae family in California includes at least 9 genera in addition to Megachile. Other genera with the most species are discussed below. All Megachilidae share the traits of carrying pollen on dense hairs (scopae) on the underside of the abdomen and rearing their young in individual chambers formed from plant parts or soil.
Hairy belly bees. Ashmeadiella includes 42 California species and Hoplitis includes 70 California species. These bees line and partition their brood cavities with chewed-up leaf pulp and plant resins. Ashmeadiella are chunky, relatively small, and commonly black with pale hairs in bands on the abdomen. Hoplitis includes species that are black, black and red, or green. Many species have pale hairs on the thorax and bands of hairs on the abdomen. Some species have males with distinctive antennae that are swollen at the base and middle and pointed at the tip.
Mason bees. Osmia includes 95 California species that line their nest cavities in wood with chewed-up leaf pulp, mud, or sand. These stout-bodied bees are mostly dark metallic blue, but some are bright metallic green.
Wool carder bees. Anthidium includes 22 California species that scrape plant hairs from leaf and stem surfaces to line their cavity nests with cottonlike fillings. Their brood chambers are located in hollows in plant stems or wood or holes in the ground. They are stout-bodied, some with black and yellow coloration that mimics yellowjackets; other species are black and white. Males have prominent toothlike projections at the tip of their abdomen.
Bees develop through four life stages: egg, larva, pupa, and adult. Winter is spent as mature larvae (prepupae) in individual cells formed by adult females. In spring to early summer, the bees pupate, emerge as adults, and mate. Adults of most species are active only during spring and early summer.
Unlike honey bees that live in a colony of many bees, most bees including leafcutting bees are solitary species. Adult females seek hollows where they create brood cells lined with cut away plant parts or soil. Within the hollow the female provisions a cell with a mixture of nectar and pollen, lays an egg, then seals the egg in the cell with food. Within the same hole or hollow adult females commonly form a series of cells one after the other.
After a cell is sealed by the mother bee the larva hatches. Each larva over several weeks consumes the bee food, then spins a dark cocoon within its cell. Most species of leafcutting bees have only one generation per year. Most of the life span is spent within the cell as larvae.
Leafcutting bees feed on pollen and nectar and do not otherwise eat plants. Megachile species and many other species of leafcutting bees chew circular to semicircular holes in the margins of blossoms or leaves. This damage can mar plant aesthetics but is harmless to otherwise healthy plants.
Bees are important pollinators and should not be killed. Where these bees have preferred small plants for collecting material, covering them with cheesecloth or other screening prevents further damage. Place out these barriers when leaf cutting is first observed. Barriers can be removed about midsummer when the adults are no longer active. Reducing nesting sites may locally reduce the number of leafcutting bees. For example, fill holes that are 1/8 to 7/8 inch in diameter. Seal the ends of pruned rose canes with wax or a couple drops of white glue.
For more information see The Alfalfa Leafcutting Bee, Megachile rotundata: The World's Most Intensively Managed Solitary Bee; California Bees and Blooms; Common Bees in California Gardens; Common Name: Leafcutting Bees, Scientific Name: Megachilidae (Insecta: Hymenoptera: Megachilidae: Megachilinae); and The Megachiline Bees of California (PDF).