Evaluation of laboratory bioassays using oral exposure of the neonicotinyl insecticide imidacloprid to honey bees (Apis mellifera) and bumble bees (Bombus impatiens)

Pesticides are debated as a major contributing factor to bee losses worldwide.  Systemic insecticides such as neonicotinoids, which can translocate to the nectar and pollen of treated plants, may be a source of bee toxicity during foraging. Currently, the EPA has proposed a new risk assessment procedure for systemic insecticides and is undergoing a re-evaluation of neonicotinoids and their potential impact on non-target honey bees to be completed in 2016-2018. The first tier of EPA’s proposed risk assessment includes standard laboratory bioassays to determine toxicity based on LD50 values, when bees are fed varying concentrations. The standard quantity for a single or acute oral exposure in toxicity testing is 10 µl per bee. However in the field, foraging honey bees can take up 80-200 µl in their honey stomachs to carry back to their colony. Standard laboratory bioassays may thus underestimate neonicotinoid toxicity to bees. This study examined the important issue of dose quantity (volume consumed by the bee), and the concentration of the imidacloprid a bee consumes. We compared LD50s using the standard bioassay dose (10 µl) to LD50s when honey bees and bumble bees were fed a more field realistic quantity (50 µl) then measured mortality and unresponsiveness in the lab up to 96 hours after acute exposure. This study improves our understanding and benefits future risk-assessments by evaluating field-realistic responses of neonicotinoid exposure on two species of bees.  Our findings will help predict more accurate application rates for neonicotinoids to minimize non-target effects on beneficial bee pollinators.