Chronic contact with imidacloprid during development may decrease female solitary bee foraging ability and increase male competitive ability for mates

Environmentally persistent xenobiotics, such as neonicotinoid insecticides, are thought to contribute to insect declines. Much of what is known about the non-target effects on bees comes from oral exposure in eusocial species. However, most bee species are solitary and nest below ground. For them, contaminated nesting resources may represent an important, yet understudied, route of exposure. We examined the effect of chronic contact exposure with realistic soil concentrations of the neonicotinoid imidacloprid (0, 7.5, 15, or 100 ppb) during immature development on adult locomotion (movement speed and distance) and brain development of the solitary bees Osmia lignaria and Megachile rotundata. Adult locomotion and mushroom body morphology were characterized 2 (females) or 4 (males) and 14 (both sexes) days after emergence. Unlike the 0 and 7.5 ppb groups, female O. lignaria treated with 15 and 100 ppb did not move faster with age. If movement speed is associated with foraging or nest-building ability, this could result in a 25% reduction in nest provisioning efficiency over the first 14 days. Young male M. rotundata moved more quickly (7.5 and 100 ppb) and farther (100 ppb) when treated with imidacloprid, potentially increasing their ability to compete for more receptive female bees. We did not detect an effect of imidacloprid on the relative volumes of the neuropil and Kenyon cell subregions. We discuss how an environmentally persistent xenobiotic has the potential to alter population dynamics through changes in adult locomotion, even in the absence of a detectable effect on gross brain morphology.