PNW 692 • December 2016 A Pacific Northwest Extension Publication Megachilid Bees in the Pacific Northwest: An Introduction S. M. Roof, S. J. DeBano Figure 1. An alfalfa leafcutting bee (Megachile roduntata) foraging on alfalfa Photo: USDA Agricultural Research Service Bee Biology and Systematic Laboratory lthough the European honey bee (Apis mel- plants and economically important crops, knowing lifera) is perhaps the most well-known more about them and how to protect and encourage Apollinator of home gardens and commercial them can benefit gardeners and farmers alike. farms, over 4,000 other species of bees are also found The Megachilidae family in North America. The Pacific Northwest boasts a wide diversity of these lesser-known bees, including Species in the Megachilidae family are found bumble bees, sweat bees, mason bees, and leafcut- throughout the world. There are over 200 different ting bees. These other species can be more efficient species in the Pacific Northwest alone. Megachilid than honey bees at pollinating certain flowers, such bees are solitary, meaning they do not share nests, as alfalfa (Figure 1) or red clover, and are estimated and there is no social hierarchy or division of labor to pollinate approximately $3 billion worth of fruits among individuals. Rather, each female mates, builds and vegetables in the United States. This publica- her own nest, and is in charge of providing food for tion describes the Megachilidae family of bees, a her larvae. While females of some megachilid species large and diverse group of bees found in the Pacific Northwest, and highlights two important members Samantha M. Roof, Research Assistant, and Sandra J. DeBano, of the family: leafcutting bees (genus Megachile) and Associate Professor, Invertebrate Ecology; both of the mason bees (genus Osmia). Because these bees play Department of Fisheries and Wildlife, Oregon State University, Hermiston Agricultural Research and Extension Center a major role in pollinating both common garden OREGON STATE UNIVERSITY n WASHINGTON STATE UNIVERSITY n UNIVERSITY OF IDAHO 1 c A B C Figure 2. All megachilid bees are solitary, but not all are “loners”—some like close neighbors and feel right at home in human constructed nests, like this orchard bee nest (A) or leafcutting bee nest (B). Many megachilids are opportunists that may also use natural holes, such as this Megachile mendica female provisioning her nest in a dead maple snag (C). Photos: (A) Dale Calder, CC BY-NC-SA 2.0, (B) Sandra DeBano, © Oregon State University , and (C) Randy Tindall to lay their eggs. Some also build their own nests above ground. The female builds the cell walls of the nest using materials found in the environment. Depending on the species, she may use dirt, pebbles, leaves, petals, or resin. This practice is different from other families of bees that secrete their own sub- stances to build their cells. The female megachilid lays a single egg in its own cell. Inside each cell, she adds a loaf of pollen and nectar collected from flowers to feed the larva. Another characteristic that separates the family Megachilidae from other bees is that females do not collect pollen on their legs (Figure 3). Instead, pollen Figure 3. In contrast to megachilids, most bees, like this is collected in long hairs, called scopa, on the under- Stenotritus pubescens, collect pollen on their legs. side of their abdomen (Figure 4, page 3). There are Photo: USGS Bee Inventory and Montoring Lab a few megachilid species that do not have scopa, but this is because they parasitize other bees’ nests and do not need to collect pollen for their own larvae. dislike having close neighbors, females of other These females lay their eggs in the nesting cells of species may nest near each other or share the same a different bee species. The parasitic larvae develop nest entrance (Figure 2). However, regardless of faster than the larvae of the host species and eat the the proximity of their nest to neighbors, all females food intended for the host species. build and provide food for their own nest cells. Leafcutting bees The way the megachilid female builds her nest depends on the species. Unlike many other solitary Leafcutting bees (genus Megachile) get their name bees that dig nesting tunnels underground, megach- because females of many leafcutting bee ilid bees find existing cavities in wood, plant stems, species line their nests with leaf circles that they or human-made structures, such as walls or pipes, cut from plants (Figure 5, page 3) using their sharp, 2 toothed mandibles (Figure 6, page 4). There are approximately 1,100 species of leafcutting bees, and they are found all around the world in both A temperate and tropical regions. Approximately 40 species are found in the Pacific Northwest. Life cycle Although the life cycles of leafcutting bees vary depending on the species, many share a similar pattern. As soon as they emerge as an adult, male leafcutting bees often patrol flowers or nesting sites in search of a mate. After mating, the female finds a convenient hole that can be used as a nesting site. Once she chooses her nest, she begins collecting the bits of leaves necessary to make her nest cells. She is rather picky, choosing thick leaves that are hairless B on at least one side. When she finds a suitable leaf, she cuts a circular piece out with her strong jaws (Figures 7 and 8, page 4), and carries it back to her nest. If her leaf source is nearby, this might take no more than a minute. She will keep making trips back and forth until there are enough leaf pieces to cover the walls, floor, and ceiling of the hole. Different shapes are required to completely cover the entire area, but the female keeps track of where she is in the building process and cuts each leaf piece accordingly. She glues the pieces together by smashing the edges Figure 4. Many species in the Megachilidae family have so the plant juice comes out. If the leaves are hairy, long hairs on the underside of their abdomen called she places them so the smooth side faces inwards. scopa (A) which they use to collect pollen (B). Photos: USGS Bee Inventory and Monitoring Lab Next, in each cell, the female makes a loaf of pollen and nectar, and lays a single egg on top of it. Then, she closes the cell with a few more leaf pieces c Figure 5. The nest of an alfalfa leafcutting bee showing the leaf wall exposed (top). By using an X-ray, the larvae can be seen inside (bottom). Photos: USDA Agricultural Research Service Bee Biology and Systematic Laboratory 3 Figure 6. The mandibles on this female Megachile Figure 7. A female leafcutting bee chewing off a leaf circle melanophaea are enlarged and toothed to cut through leaves. Photo: Colby Francoeur, USGS Bee Inventory and Monitoring Lab and begins work on the next cell. Interestingly, the female lays the eggs of her daughters in the cells farthest from the nest entrance. This way, males can emerge first and search out females to mate with immediately and, in the process, not disturb the slower-developing females. Most species make approximately 6 to 10 cells per nest. Once the last cell is sealed, the female moves on to make one or two more nests before she dies. Alfalfa leafcutting bees Although many different bee species are impor- Figure 8: Once cut, the leaves are carried back to her nest, tant for crop pollination, alfalfa leafcutting bees leaving a distinctive cut pattern on the leaf. (Megachile rotundata, Figure 1, page 1) are one of Photos (Figures 7 and 8): Erica Siegel, ©2010 the most important species of solitary bees to be bred commercially and used by growers to pol- linate alfalfa grown for seed. This seed is the prime alfalfa fields (Figure 9, page 5). Managed populations source of the 25 million acres of alfalfa planted in the of alfalfa leafcutting bees are estimated to increase United States every year. Though not native to North alfalfa seed production by 50 percent. America, alfalfa leafcutting bees are commonly raised in Canada and then transferred to alfalfa seed How to identify leafcutting bees fields in the United States because neither honey Most leafcutting bees are about the size of a honey bee nor native bee populations are large enough to bee (Figure 10, page 5). A good hint that you are sustain the yields growers require to produce an looking at a leafcutting bee is to check for the scopa economically viable crop. Alfalfa leafcutting bees on the bottom of the abdomen. Often, the hairs are that are used for agriculture are encouraged to nest very long and thick, and may be white, yellow, or in “domiciles” or “houses” that consist of wood or reddish-brown (Figure 4, page 3). When the hairs polystyrene boards drilled with appropriately sized are loaded with pollen, they are noticeable when the holes (95 to 150 mm deep, 5 to 7 mm in diameter). bee is visiting flowers; this aids greatly in identifica- The domiciles or houses are placed in blooming tion (Figure 10, page 5). Another excellent feature 4 Figure 9. Domiciles or houses are placed in blooming alfalfa fields. Managed populations of alfalfa leafcutting bees are estimated to increase alfalfa seed production by 50 percent. Photos: Rusty Burlew for identifying leafcutter bees is the size of their jaws (Figure 6, page 4). Because leafcutting bees need to chew through leaves, their mandibles are very large and sharply toothed. Some species also have slightly upturned abdomens in comparison with other bees, which helps them collect pollen on their scopa (Figure 4, page 3).