Nesting habits predict rarity of North American leaf-cutter bees (genus Megachile)

Leaf-cutter bees of the genus Megachile primarily nest in aboveground cavities, but a notable subset of North American species are obligate ground-nesters, occupying preexisting burrows or excavating their own. Prompted by our recent rediscovery of M. parksi—a likely obligate sandy soil nesting species formerly deemed “possibly extinct” by NatureServe—in Oklahoma, we evaluate whether ground-nesting is associated with relative rarity in North American members of this species-rich and ecologically important genus. We collated nesting biology data on individual species from literature accounts, making inferences based on taxonomic association where necessary, and obtained occurrence data sourced from both physical specimens and photograph-based community science records. Although nesting information is unavailable for many species, we found that obligate ground-nesting species are, on average, rarer than those nesting aboveground or opportunistically either above- or belowground. We also found that the use of resin in nest construction (i.e., subgenus Chelostomoides) was another major predictor of rarity. Maintenance of natural habitat with the correct combination of soil characteristics may thus be important for the conservation of some of the rarest Megachile species in North America.