Lethal and sublethal effects of chlorpyrifos exposure on mason bee (Osmia cornifrons) larval development

Solitary bees are vital pollinators for many crops, but face decline due to global change drives such as land use change, global warming, pathogens, and pesticides. While much research has focused on social bees and adult solitary bee exposure, limited studies address the impact of pesticide-contaminated pollen on developing bee larvae. Understanding these effects is crucial for pollinator conservation and global food security. This study investigates how four field-realistic concentrations of the insecticide chlorpyrifos impact larval survival, developmental timing, and pupal size in mason bees (Osmia cornifrons). We simulated natural exposure by adding chlorpyrifos to larval diets. We recorded the duration of development, and monitored pupal body size microscopically, followed by weekly X-ray imaging for mason bee larvae and pupae, respectively. A significant reduction in survival was observed among mason bees consuming pollen containing the highest experimental chlorpyrifos dose (830 parts per billion; ppb) but not among treatments at lower doses. Chlorpyrifos exposure also disrupted developmental timing at several stages, including transitions to the 5th instar, silk formation, cocoon formation, and pupation, as well as pupal size, although some of these effects were not dose-dependent. Our findings indicate that chlorpyrifos can adversely affect the development of O. cornifrons at concentrations typically present in field pollen. It underscores the need for more comprehensive assessments of pesticide impacts on diverse bee species, and revision of current pesticide usage recommendations for sustainable agricultural practices.