1. About 3/4 of all wild bee species nest in the soil and spend much of their life cycle underground. These insects require suitable environmental conditions for nest construction and for the development and survival of their offspring. However, there is little quantitative information on the nesting habitat requirements and preferences of ground-nesting bees. Moreover, there are almost no data on the effects of nesting conditions on these bees' fitness. 2. Here, to better understand the factors that influence nest-site selection in ground-nesting bees, we synthesise the literature on the nesting-habitat associations of these important pollinators. We also review techniques that can be used to study the nesting preferences of ground-nesting bees. 3. Our review reveals enormous variation among bee species in their associations with such nesting-habitat attributes as soil texture, compaction, moisture, temperature, ground surface features, and proximity to conspecifics or floral resources. However, more studies-particularly experimental ones-are needed to segregate the influence of each factor on bees' choices of nesting location, since multiple factors are often correlated. It is also unclear whether nesting-habitat associations vary geographically or seasonally within species, or phylogenetically among ground-nesting bee species, partly because we lack information on nesting habitat for many species. 4. We argue that studies using established habitat-selection methods are essential to properly identify nesting-habitat preferences of ground-nesting species. Finally, more research on nesting ecology is needed (especially in agroecosystems) to determine how best to support this diverse group of bees and the vital ecosystem service they provide.

Specialist insect herbivores make up a substantial fraction of Earth's biodiversity; however, they exploit a minority of plant lineages. For instance, in the eastern United States and Canada, ~25% of bee species are pollen specialists, but they are hosted by a small fraction of the native, animal‐pollinated angiosperms in the region: Only 6% of plant genera and 3% of families support pollen‐specialist bees. It is unclear why some plant lineages host specialist bees while others do not. We know that at least some specialist bees use plant taxa that are avoided by generalists, suggesting that specialist bees favor plants with low‐quality pollen, potentially as a strategy to escape competition or obtain protection from natural enemies. There is also evidence that specialist bees prefer superabundant host plants. Here we investigate whether pollen quality and plant abundance predict patterns of host use by specialist bees in eastern North America. Through field observations, we find that plants hosting specialist bees are frequent sources of pollen for generalists, suggesting that their pollen is not generally avoided by bees due to poor pollen quality. In addition, our analysis of a large citizen‐science data set shows that regional abundance strongly predicts which plant genera in the eastern United States host pollen‐specialist bees. Our results show that bees specialize on regionally abundant—but not necessarily low‐quality—plant lineages. These plant lineages may provide more opportunities for the evolution of specialists and lower likelihood of specialist extinction.