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.

Authors: Mallinger, Rachel; WERTS, PETER - The Ipm Institute Of North America, Inc; GRATTON, CLAUDIO - University Of Wisconsin

Submitted to: Journal of Insect Conservation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/19/2015
Publication Date: 10/30/2015
Citation: Mallinger, R.E., Werts, P., Gratton, C. 2015. 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. Journal of Insect Conservation. 19:999-1010. doi:10.1007/s10841-015-9816-z.

Interpretive Summary: Pesticides (insecticides, herbicides and fungicides) are believed to contribute to recent declines in bees. Wild bees in crops can be exposed to numerous pesticides which may reduce bee populations, creating a trade-off between pest management and crop pollination. In this three-year study, the effects of pesticide use on the abundance and species richness of wild bees were examined in southern Wisconsin apple orchards. Colonies of common eastern bumble bee were also used to determine if orchard pesticides affected colony performance. Records of pesticide applications were used to develop toxicity scores for each orchard, which considered relative toxicity to bees, persistence of the pesticide, and amount of pesticide active ingredient. Toxicity scores were not related to the total numbers of wild bees or numbers of wild bee species (diversity). However, as toxicity scores in orchards increased, a group of small-bodied bees (so-called “sweat bees”) were reduced in number and diversity. Orchards with high toxicity scores also produced fewer, and smaller, bumble bee workers, though there were no effects on the production of new bumble bee queens. Results show some bees, particularly small-bodied bees, or social bees with long foraging periods, may be more likely to be affected by on-farm pesticide applications.

Technical 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 pest management 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 that forages throughout the entire growing season, in order to relate colony performance to season-long orchard pesticide use. Utilizing 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 small-bodied bees (Lasioglossum (Dialictus) spp.), which may have increased sensitivity and exposure to orchard pesticides due to their small body sizes and short foraging ranges. In addition, orchards with high toxicity scores produced fewer, and smaller, Bombus impatiens workers, though there were no effects on the production of new queens. Our results demonstrate that certain subsets of the bee community, particularly small-bodied bees, or social bees with long foraging periods, may have a higher exposure or sensitivity to on-farm pesticide applications, and could therefore be most vulnerable in agroecosystems.