Past insecticide exposure reduces bee reproduction and population growth rate

This two year field cage experiment examined the direct and carryover effects of insecticide exposure on two consecutive generations of the solitary mason bee species Osmia lignaria. Pesticide exposure, both directly to foraging bees and via carryover effects from past exposure, had dramatic impacts on individual reproduction, with important implications for population growth over time. This is one of very few studies that quantify the impacts of pesticide exposure across insect life stages or generations, and highlights the critical need for understanding the potential carryover effects of repeated pesticide exposures for insect fitness and population health.

Field cages were planted with a high density wildflower mix. Half the cages received a soil drench of imidacloprid (at 10.5oz/acre, the maximum label rate for herbs and orchard fruit crops) five weeks before full bloom. When flowers approached full bloom, eight newly emerged adult females and 16 males were released into each cage to match the bees’ natural sex-biased ratio. In year one, half of the bees received exposure to imidacloprid via the pollen and nectar of the wildflowers in their cages as well as the moistened soil for nest construction. In year two, half of the females released in each cage were randomly assigned from each of the year one treatments, so each cage had four females with past imidacloprid exposure and four females with no past exposure. This fully crossed design allowed the authors to partition the current and carryover effects of insecticide exposure on individual performance and populations.

Exposure to imidacloprid reduced female reproduction, both when exposure was directly to foraging adults and via carryover effects from past exposure. In year two, bees exposed to imidacloprid as adults provisioned 30% fewer offspring than unexposed controls. Bees exposed chronically through pollen, nectar, and in nest materials as larvae in year 1 provisioned 20% fewer offspring than bees with no past exposure. Bees exposed both years (as larvae and adults) provisioned 44% fewer offspring than bees never exposed – a difference of ~10 offspring. Exposure of adult females to imidacloprid (year 2) also reduced their probability of nesting/producing any offspring by around 4%. Past exposure as larvae did not carry over to affect nesting probability as adults.

In addition to direct effects on reproduction, current exposure to insecticide increased the male-biased sex ratios in foraging adults, with a 49% reduction in the proportion of daughters provisioned by imidacloprid-exposed adult females. Sex ratio affects both the growth rates and the evolutionary trajectories of wild bee populations. Overall, imidacloprid exposure reduced female offspring production by 71%—nesting mothers exposed to imidacloprid in both years provisioned an average of just 1.5 daughters.

This research indicates that carryover effects may have major impacts on individual fitness and population growth. Pollinators are widely and repeatedly exposed to multiple agrochemicals, but sublethal carryover effects from larva to adult and between generations are not currently considered in the pesticide regulatory process and are largely unstudied. This is a major knowledge gap.

Stuligross, C., and N. Williams
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