Past insecticide exposure reduces bee reproduction and population growth rate

Abstract

Pesticides are linked to global insect declines, with impacts on 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 on 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 2 y, with insecticide exposure or no exposure in each year. Insecticide exposure directly to foraging adults and via carryover effects from past exposure reduced reproduction. Repeated exposure across 2 y additively impaired individual performance, leading to a nearly fourfold reduction in bee population growth. Exposure to even a single insecticide application can have persistent effects on vital rates and can 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 the effects of insecticide exposure.

Carryover insecticide exposure reduces bee reproduction across years

“…Despite restriction in some parts of the world ( 4 ), neonicotinoids are among the most widely used insecticides and are commonly applied in many agroecosystems despite evidence that they can harm bees and other pollinators ( 5 ). In PNAS, Stuligross and Williams ( 6 ) use the solitary bee, Osmia lignaria , an important “alternative managed pollinator” with significantly different biology than the more commonly studied European honey bee, to discover an aspect of sublethal chemical exposure. Focusing on a formulation of the neonicotinoid imidacloprid that is commonly applied in California, they parse apart how past and current insecticide exposure affects vital rates and population growth, finding that sublethal exposure can affect insects over time scales spanning months or years ( Fig.…”

“…The following year, these adults arise to repeat the cycle. In their study, Stuligross and Williams ( 6 ) created a controlled habitat for O. lignaria inside of small mesh flight cages, where they planted wildflowers that provide high-quality nutrition for bees. They applied a common commercial imidacloprid soil treatment or a control (no agrochemical) treatment to their enclosures.…”

“…Insecticide exposure in year 2 reduced total overall reproduction, reduced nesting probability, biased the sex ratio toward males, and reduced nesting rate no matter what the bees experienced in year 1 ( 6 ). Bees from the “worst case scenario,” with insecticide exposure both years, performed significantly worse than bees that only experienced exposure as adults.…”

“…So what does this all mean for protecting pollinators, and how do we use studies, like the work of Stuligross and Williams ( 6 ), to inform best management practices, policy decisions, and other actions? One route is to encourage better use of integrated pest management (IPM), that is, using chemical treatments for pest insects only when truly warranted by observing predetermined pest thresholds.…”

“…For example, because “the [pesticide] label is the law” ( 8 ), changes in application rate, timing, etc., on pesticide labeling could reduce impacts on pollinators relatively quickly ( 7 ). Further, as new chemistries or formulations come on the market, Stuligross and Williams’ ( 6 ) work underlines the importance of continuing to observe subtle effects and their causes. Their focal insecticide, imidacloprid, has been registered for use in the United States since 1994, but new products may soon replace it.…”