Nesting habitat of ground-nesting bees: a review

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. 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. 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. 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.