Farming practices affect nest site availability for native ground nesting bees

Animal-mediated pollination, particularly by bees, contributes to the stability of over half of the world’s most important crops. The availability of both foraging and reproductive habitat potentially limit pollinator range, which in turn limits where pollinator services can be expected. Prior studies focus on how foraging habitat influences bee distribution; few consider nesting limitations. To determine how farming practices that influence soil properties affect nest site selection of ground nesting bees, I examined nesting preferences in experimental plots. In each of four blocks, I prepared five soil types containing combinations of soil factors intended to mimic soil conditions in an agricultural landscape. The soil factors included compaction/tillage, irrigation/no irrigation, and the presence/absence of a commonly used pesticide (esfenvalerate, a synthetic pyrethroid). I used soil emergence traps to determine the number and species of individual female bees nesting in each plot as a measure of preference for that soil treatment. Between May and September 2013, I collected 447 nesting females belonging to five different genera.  When initiating nests, females did not distinguish between compacted or tilled soils, irrigated or nonirrigated soils, or between soils with or without pesticide residues. If the presence of insecticide residues or the practice of tillage affects offspring survival, these results suggest that bees nesting in agricultural areas are faced with an ecological trap that could reduce population sizes in the following year. The results of these findings lay a foundation for creating models predicting native bee abundance and distribution in a complex agricultural landscape.