A novel mechanism of pesticide exposure to a cavity-nesting solitary bee: Can residues in nesting material affect female selectivity and reproductive performance?

Background/Question/Methods

Recent pollinator population trends have led to many studies investigating the effects of pesticides on foraging bees, but few consider the potentially unique effects of pesticide residues in nesting material. Megachile rotundata, a solitary cavity-nesting bee, is often managed for pollination in alfalfa seed production. Growers typically apply insecticides seven to ten days before releasing bees, by which time flowers will have senesced.  However, treated alfalfa leaves, which females use to line nests, will still be present. Pesticide residues on leaves could impact a nesting female in three ways: (1) affecting choice of leaf material, (2) affecting overall reproductive output through direct exposure, or (3) leaching into the pollen provision and affecting offspring development.

Two cage studies, a choice assay and a reproductive performance assay, were conducted to examine how foliar application of pesticide affects nesting behaviors. To assess leaf selection, nesting females were given a choice between untreated Wisteria sinensis leaves, or leaves that had been sprayed with chlorpyrifos, an organophosphate used in alfalfa seed production. To assess reproductive performance, nesting behaviors of females were observed in cages placed over flowering alfalfa that was either untreated, or treated with chlorpyrifos seven days prior.

Results/Conclusions

The choice assays revealed that chlorpyrifos residues did not affect female selectivity of leaf material. Other leaf properties such as age, toughness, and surface area were positively correlated to leaf selection. The presence of chlorpyrifos, did, however, affect the results of the nesting performance assays. While on average, measures of female productivity (provisioning rate, total cells provisioned, female:male offspring ratio, and offspring body weight) were higher in untreated over treated cages, these differences were small. The main effects of chlorpyrifos residues were on the development of the offspring. A significantly higher proportion of the offspring of females nesting with chlorpyrifos-treated leaves emerged as adults within the same season, rather than overwintering in diapause. This could be due to a metabolite or additive of the pesticide interfering with the diapause process, or due to a greater proportion of the offspring being too small to survive diapause. Same-year emergence of adults is not desirable for managed populations, since they usually emerge after alfalfa pollination has ceased and have no forage available. Pesticide residues in nesting material present a novel mode of exposure. In this way, cavity-nesting bees, such as M. rotundata, may be more susceptible to pesticide applications than Apis mellifera.