How do chlorpyrifos residues and leaf characteristics affect leaf selection by nest building Megachile rotundata?

In California, managed populations of Megachile rotundata are often used for pollination in alfalfa seed production. M. rotundata are highly sensitive to the pesticides used to control major alfalfa pests, so growers typically apply insecticides seven to ten days before releasing bees. Open flowers will have senesced; however treated alfalfa leaves will still be present when bees are released. M. rotundata females nest in cavities and line each cell with leaf cuttings. Pesticide residues on leaves could therefore impact a nesting female in three ways: (1) by affecting her choice in leaf material, (2) by affecting her overall reproductive output through direct exposure, or (3) by leaching into the pollen provision and affecting her offspring’s development. This study focuses on how foliar application of chlorpyrifos, an organophosphate commonly used in alfalfa seed production, affects nesting behavior. To assess leaf selection, nesting females were given a choice between untreated Wisteria sinensis leaves, or leaves that had been sprayed with chlorpyrifos. To assess reproductive performance, five cages were partitioned in half, and half of each cage was sprayed with chlorpyrifos seven days before releasing adult females. The provisioning rate and total number of cells produced per female were measured. In addition, the emergence of the second generation of leafcutter bees was recorded as a measure of offspring performance. 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.