Past exposure to insecticides reduces bee reproduction and population growth rate


The global decline of insects is of deep concern, especially for groups like bees that provide important services to humanity. However, we do not know to what extent recognized factors of decline, such as pesticides, can produce transfer effects that influence reproduction and population dynamics over time. We reveal that pesticide exposure, both directly on foraging bees and via transmission effects from past exposure, significantly reduced bee reproduction, which reduced population growth. Transfer effects reduced bee reproduction by 20% beyond current impacts on forager bees, exacerbating the negative impact on population growth rates. This indicates that bees may take several generations to recover from a single exposure to pesticides; thus, transmission effects should be considered in risk assessment and conservation management.


Pesticides are linked to the global decline of insects, with impacts on biodiversity and essential ecosystem services. In addition to the well-documented direct impacts of pesticides at or current stage, the potential delayed effects of “transfer” from past exposure to a different life stage can increase impacts on individuals and populations. We investigated the effects of current exposure and the carry-over effects of past exposure to insecticides on individual vital rates and population growth of the solitary bee, Osmia lignaria. Bees in flight cages freely fed on wildflowers, some treated with the common insecticide, imidacloprid, in a 2-year fully crossed design, with either exposure to the insecticide or no exposure each year. Exposure to insecticides directly in foraging adults and through the transmission effects of previous exposure reduced reproduction. Repeated exposure over 2 years additively impaired individual performance, resulting in an almost four-fold reduction in bee population growth. Exposure to even a single application of insecticide can have lingering effects on vital rates and may reduce population growth over several generations. Transfer effects have had profound implications for population persistence and must be considered in risk assessment, conservation and management decisions for pollinators in order to mitigate the effects of insecticide exposure. .


    • Accepted October 9, 2021.
  • Author contributions: research designed by CS and NMW; CS has done research; CS analyzed the data; NMW assisted in the interpretation of the data; and CS and NMW wrote the paper.

  • The authors declare no competing interests.

  • This article is a direct PNAS submission.

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