Pesticide use within a pollinator-dependent crop has negative effects on the abundance and species richness of sweat bees, Lasioglossum spp., and on bumble bee colony growth

Abstract

Pesticides are implicated in current bee declines. Wild bees that nest or forage within agroecosystems may be exposed to numerous pesticides applied throughout their life cycles, with potential additive or synergistic effects. In pollinator-dependent crops, pesticides may reduce bee populations, creating trade-offs between pest management and crop pollination. In this three-year study, we examined the effects of pesticides on the abundance and species richness of wild bees within apple orchards of southern Wisconsin. We additionally deployed colonies of Bombus impatiens, a native and common species, in order to relate colony performance to orchard pesticide use. Utilizing grower spray records, we developed “toxicity scores” as a continuous index of pesticide use for each orchard, a measure that incorporated each pesticide’s relative toxicity to bees, its residual activity, and its application rate. While there was no relationship between total wild bee abundance and species richness with toxicity scores, there was a significant, negative effect on sweat bees, Lasioglossum spp. Many of these sweat bees are small-bodied, have short foraging ranges, are social, and have long foraging periods, all traits that could increase bee exposure or sensitivity to orchard pesticides. In addition, sentinel bumble bee colonies at orchards with high toxicity scores produced fewer, and smaller, workers. Bumble bees may also have a greater sensitivity and exposure to orchard pesticides due to their sociality and long foraging periods. Our results demonstrate that certain bee taxa may have a higher exposure or sensitivity to on-farm pesticide applications, and could therefore be vulnerable in agroecosystems.