Past insecticide exposure reduces bee reproduction and population growth rate

Pesticides are linked to global insect declines, with impacts to
biodiversity and essential ecosystem services. In addition to
well-documented direct impacts of pesticides at the current stage or time,
potential delayed “carryover” effects from past exposure at a different
life stage may augment impacts to individuals and populations. We
investigated the effects of current exposure and the carryover effects of
past insecticide exposure on the individual vital rates and population
growth of the solitary bee, Osmia lignaria. Bees in flight cages
freely foraged on wildflowers, some treated with the common insecticide,
imidacloprid, in a fully crossed-design over two years, with insecticide
exposure or no exposure in each year. Insecticide exposure reduced
reproduction, directly to foraging adults and via carryover effects from
past exposure. Repeated exposure across two years additively impaired
individual performance, leading to a nearly four-fold reduction in bee
population growth. Exposure to even a single insecticide application can
have persistent effects on vital rates and reduce population growth for
multiple generations. Carryover effects had profound implications for
population persistence and must be considered in risk assessment,
conservation, and management decisions for pollinators to mitigate effects
of insecticide exposure.