Conservation and Management of NORTH AMERICAN LEAFCUTTER BEES Bruce E. Young Dale F. Schweitzer Geoffrey A. Hammerson Nicole A. Sears Margaret F. Ormes Adele O. Tomaino Arlington, VA www.natureserve.org January 2016 The views and opinions expressed in this report are those of the authors. This report was produced in partnership with the USDA Forest Service. Citation: Young, B. E., D. F. Schweitzer, G. A. Hammerson, N. A. Sears, M. F. Ormes, and A. O. Tomaino. 2016. Conservation and Management of North American Leafcutter Bees. NatureServe, Arlington, Virginia. © NatureServe 2016 Cover photos: Left: Leafcutter bee (Megachile texana) cutting a leaf / Rollin Coville Middle: Leafcutter bee Megachile( perihirta) nest cell / Rollin Coville Right: Pollen-coated leafcutter bee Megachile( perihirta) / Rollin Coville NatureServe 4600 N. Fairfax Dr., 7th Floor Arlington, VA 22203 703-908-1800 www.natureserve.org EXECUTIVE SUMMARY This report provides a brief overview of the diversity, natural Management and conservation recommendations center history, conservation status, and management of North on providing suitable nesting habitat where bees spend American leafcutter bees of the genus Megachile. Leafcutter most of the year, as well as foraging habitat. Major bees are stingless, solitary bees. Their common name refers recommendations are: to the pieces of leaves or flowers that the females clip off and use to line their nests. • Identify and protect nesting habitat, including suitable decaying plant materials and open sandy areas. Leafcutter bees occur in a wide range of habitats. They are efficient pollinators in some ecosystems but their importance • Ensure availability of suitable plant leaves, petals, and, for in most habitats is poorly known. Most have not been well some species resins, needed for nest construction. studied except for a few species used for crop pollination. • Avoid fires and mowing in potential nesting habitat, or For example, most alfalfa is now pollinated by the alfalfa alternate these management activities on an annual basis. leafcutter bee M.( rotundata), a species introduced from Europe. Leafcutter bees may become increasingly important • Use artificial nest blocks (trap-nests) with discretion. They may pollinators in both agricultural and natural systems in light of be appropriate in some circumstances, but their usefulness in ongoing declines of honeybees and native pollinators. relatively natural habitats is poorly known. Although some leafcutter bee species remain abundant and • Provide abundant and diverse late spring and summer- widespread, up to 62 (47%) of the 131 native species in North blooming plants, especially those in the aster and pea America may be at risk, including 25 species that have not families, for forage. been recorded for several decades. Leafcutter bees have the • Avoid spraying pesticides on crops visited by leafcutters or largest percentage of potentially at-risk species of any insect other bees while these plants are in flower, and avoid using group that has been comprehensively assessed. Threats to systemic pesticides at any time of the year. leafcutter bees include habitat loss and degradation, diseases, pesticides, and climate change. The effects of introduced, non- • Help prevent the spread of pathogens by not introducing native species on leafcutter bees are, with rare exceptions, managed leafcutter bees to regions where they are not native. poorly known. In a few cases, small range size makes a species particularly vulnerable to localized threats. • Where feasible, establish inventory and monitoring programs to better understand the distribution and population trends of native (and non-native) leafcutter bees. 1 Distribution and Diversity Leafcutter bees are a diverse group 7 of stingless insects known for cutting oval pieces of green leaves to line their 9 9 nests. With over 1,500 described species 5 worldwide, this is one of the most species- rich groups of bees (Ascher and Pickering 4 25 2014). They are effective pollinators, and 22 18 15 interest in these bees has increased in 16 4 recent years due to widespread declines Leafcutter 19 8 of honeybees (Apis mellifera) and some Bee Species 29 10 25 19 10 11 10 native pollinators (Burkle et al. 2013, 30 16 4-10 30 19 14 Vanbergen et al. 2013). 24 19 17 40 23 10 11-20 15 16 31 20 22 52 16 65 27 23 11 These bees are present on all continents 21-30 82 62 14 17 28 14 8 22 except Antarctica (Raw 2007). Although 35 4 31-40 17 14 78 60 10 17 more species occur in the tropics than 16 15 32 elsewhere, leafcutter bees can be found 41-50 66 21 almost anywhere that flowering plants 51-90 33 grow. They range from the arctic to southern South America, and from sea Figure 1. Number of native leafcutter bee species known from U.S. states and level to elevations as high as 5,000 m in Canadian provinces. Source: NatureServe Central Databases, December 2015. the Andes (Raw 2007). Leafcutter bees also occur on remote islands, including Hawaii, in some cases transported to of the global diversity of the genus—are generally to aid in crop pollination. these places by ships carrying (or made of) native to North America (Raw 2007, Diversity is much higher in western North wood containing leafcutter bee nests in Integrated Taxonomic Information System America than in the eastern part of abandoned beetle burrows (Raw 2007). (ITIS) 2008, Sheffield et al. 2011; see the continent and is highest in the arid Appendix for complete list). At least four southwest (Figure 1). The distribution Leafcutter bees occur throughout North additional species (M. rotundata, M. of many species is incompletely known, America except in the northernmost apicalis, M. sculpturalis, M. ericetorum), so species counts undoubtedly are regions of Canada and Alaska. and probably also M. centuncularis, conservative in many cases. Some species Approximately 131 species—about 10% have been introduced to the continent, have been collected only a few times. Taxonomy and Identification All leafcutter bees discussed in this report abdomen rather than in pollen baskets on distinguish (Mader et al. 2010). Males and are members of the genus Megachile, which their legs. They range from 10-20 mm (0.4- females may differ so much that at first includes resin bees as well (subgenera 0.8 inches) in length, with broad heads and glance they may not appear to belong to Callomegachile and Chelostomoides). large mandibles for cutting leaves. Leafcutter the same species. In fact, nearly a third of Leafcutter bees, together with the mason bees can often be distinguished from the North American species are known from bees (genus Osmia) and a few other genera, closely related mason bees by their upturned only one gender (Sheffield et al. 2011). make up the large family Megachilidae. abdomen (Mader et al. 2011). Bees seen Prepared specimens are often needed for Most leafcutter bees are too poorly known cutting or carrying oval pieces of leaves are reliable species identification. Difficulty to have common English names and are likely to be leafcutter bees. in identification likely has been a major therefore referred to in this report by their obstacle to research on leafcutter bees scientific names. Species-level identification of most leafcutter (Raw 2007). Identification is easier where bees is difficult because morphological there are relatively few species, such as Leafcutter bees superficially resemble differences between species are subtle in Canada and the eastern United States honeybees, but they carry pollen on their and require a great deal of experience to (Sheffield et al. 2011). 2 Habitat Leafcutter bees require habitats where of North America takes place in late spring to be poor habitats for leafcutter bees they can find substrate for building their and early summer, although some species, (Hanula et al. 2015). Areas without suitable nests, leaves (often from broad-leaved particularly in the southwest, may nest nest sites such as trees and logs or lacking shrubs) or flower petals for lining nests, throughout the summer. leaves suitable for nest construction and flowers for foraging for both pollen support few leafcutter bees (Minckley et and nectar. Nesting substrates can include North American leafcutter bees find suitable al. 1999, Sheffield et al. 2011). Species hollow plant stems, abandoned beetle nesting sites and floral resources in a wide vary in their ability to adapt to disturbed burrows in decaying wood, small cavities, range of habitats, including areas as varied as habitats such as agricultural areas, but niches between stones, or earthen banks. deserts, coastal dunes, prairies, shrublands, many species readily forage at flowers of Leafcutter bees need flowers that bloom gardens, and openings in forests. Dense nonnative plants and use pieces of leaves during the nesting season, which in most forests with reduced understories appear from nonnative plants in their nests. Annual Cycle and Nesting Biology Although the annual cycle has not been whereas the rare M. oenotherae of the ground-nesting M. wheeleri to nest in trap- described for many species, we know Southeast uses tunnels dug by the mining nests by inserting nest tubes at an angle enough to describe a general cycle for bee Andrena macra (Krombein et al. 1979). into the sand (Pimentel 2010). North American species. As far as known all North American leafcutter bees Many species that normally nest in plant Some species usurp nesting sites of other overwinter in the nest as fully fed prepupal cavities readily accept artificial nest sites. In species either in the soil or in other larvae and pupate in early spring. Adults fact, artificial nest sites, or trap-nests, are substrates. The introduced M. apicalis emerge about a month later. As in many commonly used to inventory or otherwise appropriates nests of other leafcutter insects, adult males begin to emerge a study many kinds of bees in the field.