Queen honey bee (Apis mellifera) pheromone and reproductive behavior are affected by pesticide exposure during development

Abstract

Pollinator diversity and abundance in North America have been at a steep decline over the last two decades due to the combinatorial effects of several environmental and anthropogenic stressors. In particular, managed honey bees (Apis mellifera) face multiple health risks including nutritional stress, exposure to pests and pathogens, poor queen quality, and pesticide contamination, which cause problems at the individual and colony levels. One of the gravest problems faced by honey bees is parasitization by the mite Varroa destructor, which is typically controlled through the application of miticides such as tau-fluvalinate, coumaphos, and amitraz. In addition to miticides, colonies are also exposed to pesticides brought by foragers from agricultural settings, including the fungicide chlorothalonil and the insecticide chlorpyrifos. Here, we explored whether exposure of wax to combinations of these pesticides during development affects honey bee queen physiology and worker behavior. To do this, we reared queens in plastic cups coated with molten beeswax that was either pesticide-free or containing field-relevant concentrations of tau-fluvalinate and coumaphos, amitraz, or chlorothalonil and chlorpyrifos. Once queens mated naturally, we placed them in observation hives to measure egg-laying rate and worker retinue size. We then dissected the queens and used the contents of their mandibular glands to measure worker attractiveness in caged bioassays and to analyze their chemical components using GC-MS. Exposure of wax to field-relevant concentrations of the tested pesticides during queen development significantly lowered the adult queens’ egg-laying rate and worker retinue size. Miticide exposure during development also lowered the attractiveness of queen mandibular gland contents to workers and affected the relative amounts of the glands’ chemical components. Our results support the ideas that mandibular gland pheromones act as honest indicators of queen reproductive fitness and that pesticide exposure of wax during bee development is an important and concerning factor impairing honey bee health. Pollinator health is currently being threatened by multiple environmental factors, including poor nutrition, habitat fragmentation, and pesticide contamination. In particular, honey bee health is facing challenges caused by combinations of these factors, leading to an annual mortality of over 30% of all managed colonies in the USA. The conditions in which honey bee queens are reared can cause severe physiological and behavioral effects in adults, which in turn can affect the behavior of workers that tend to those queens, thus affecting colony productivity, growth, and survival. In this study, we show that queens reared in beeswax contaminated with miticides or agrochemicals lay fewer eggs over time and exhibit mandibular gland pheromone profiles that differ from those of queens reared in pesticide-free wax. Workers discern these differences, preferentially investigating the gland contents and forming larger retinues (thus performing more tending behaviors) for queens reared in pesticide-free wax. These results indicate that pesticide contamination of the queen-rearing wax directly affects queen physiology and behavior and further affirm that queen mandibular pheromones seem to act as honest indicators to workers of a queen’s reproductive capacity and overall health.