Comparative toxicology across castes in honey bees

Despite queen failure being cited as one of the most frequent causes of honey bee colony loss, relatively few studies have been devoted to understanding queen health, especially in the context of sublethal agrochemical exposure. Queens are usually thought to be shielded from toxin exposure by the buffering of worker bees, as mated queens almost never leave the colony and worker hypopharyngeal gland secretions contain very low pesticide residues. Resultantly, most studies examining agrochemicals and queen health have focused on how larval exposure causes mortality or affects adult phenotypes. However, growing evidence suggests adult queens can be exposed to agrochemicals and that even sublethal exposures can result in significant effects on queen health and reproduction. Despite this, we lack a basic understanding of how queens respond to common agrochemicals. Here, we generated foundational LD₅₀s of frequently applied agricultural pesticides in honey bee queens and compared them against LD₅₀ values in workers, thereby allowing physiological or gene expression response comparisons between castes. Using these data, we then exposed queens to sublethal agrochemical treatments and measured effects on reproduction and reproductive-related behaviors, including egg laying, egg viability, and worker retinue response. We predict that queens will be able to tolerate significantly higher pesticide loads than workers but will still be highly phenotypically responsive to sublethal doses. These results can thus serve as valuable data for studying other aspects of queen health, such as sequestration ability or resource allocation, both of which can help further our understanding of physiological variation between castes.