Colony heating protects honey bee populations from a risk of contact with wide-spectrumBeauveria bassianainsecticides applied in the field

BACKGROUND The safety of fungal insecticides to apiculture is a public concern but remains poorly understood. This study seeks to evaluate whether, how and why wide-spectrumBeauveria bassianainsecticides are safe to honey bees in a novel assessment system. RESULTS Mesonotum dipping with a 10(8)conidia ml(-1)suspension and body contact with conidial suspension in sucrose solution caused high mortalities of adult forager bees at 25 degrees C optimal for conidial germination and hyphal invasion. Intriguingly, colony sizes in the hives contaminated by the forager bees contacting viable and inactivated conidia at two sites (1.2 km in distance), respectively, showed similar increase percentages (31.7% versus 29.2%) during a 4-week summer period of exposure to environment. No sign of fungal infection was found within each of the monitored colonies. Neither was fungal outgrowth observed on surfaces of bee cadavers cleaned from each hive at either site. Hourly counts of cleaned cadavers from videotapes presented no significant difference in colony-cleaning behavior between the two sites. During the period, in-hive temperatures at both sites were persistently stabilized at approximately 35 degrees C, which abolished conidial germination and were far above the out-hive temperature range. CONCLUSION It is colony heating that protects honey bee populations from a risk of forager bees' contact with formulated conidia applied for arthropod pest control. No role was detected for colony self-cleaning behavior in protecting the bee colonies from the risk. (c) 2020 Society of Chemical Industry