Bees and other beneficial insects experience multiple stressors within agricultural landscapes that act together to impact their health and diminish their ability to deliver the ecosystem services on which human food supplies depend. Disentangling the effects of simultaneous flowering resource scarcity and pesticide exposure is a primary challenge in understanding how to promote their populations and ensure robust pollination and other ecosystem services. Negative effects of systemic pesticides on bee survival and reproduction are well documented but based largely on correlative field and lab studies. We used a crossed design to quantify the individual and combined effects of food resource limitation and pesticide exposure on the survival, nesting, and reproduction of the blue orchard bee Osmia lignaria. We established nesting females in 16 large flight cages using a crossed resource x pesticide design; cages contained spring wildflowers at high or low densities, treated with or without imidacloprid. We monitored nesting activity and brood cell construction daily and used video footage to measure foraging trip times, nest usurpation and defense, and activity patterns across treatments.
Results/Conclusions
Pesticides and resource limitation acted additively to dramatically reduce reproductive fitness in free-flying bees. Of the female O. lignaria that initiated nesting, those exposed to imidacloprid produced 42% fewer surviving offspring than unexposed controls, and bees with low resources produced 26% fewer surviving offspring than bees with abundant resources. In addition to direct effects on reproduction, combined resource and pesticide stressors led to male-biased sex ratios, further limiting reproductive output. Stressed bees also constructed nests slower, spent more time foraging, and nested for fewer days than unstressed bees. We demonstrate that pesticide and resource stressors are additive, which may enable us to make preliminary predictions about the effects of environmental change from univariate experiments. Furthermore, this work directly addresses the lack of field studies examining multiple demographic responses for solitary bees. Our results emphasize the importance of considering multiple drivers to inform population persistence, management, and risk assessment for the long-term sustainability of food production and natural ecosystems.