Negative but antagonistic effects of neonicotinoid insecticides and ectoparasitic mites Varroa destructor on Apis mellifera honey bee food glands.

Collaborative brood care by workers is essential for the functionality of eusocial Apis mellifera honey bee colonies. The hypopharyngeal food glands of workers play a crucial role in this context. Even though there is consensus that ubiquitous ectoparasitic mites Varroa destructor and widespread insecticides, such as neonicotinoids, are major stressors for honey bee health, their impact alone and in combination on the feeding glands of workers is poorly understood. Here, we show that combined exposure to V. destructor and neonicotinoids antagonistically interacted on hypopharyngeal gland size, yet they did not interact on emergence body mass or survival. While the observed effects of the antagonistic interaction were less negative than expected based on the sum of the individual effects, hypopharyngeal gland size was still significantly reduced. Alone, V. destructor parasitism negatively affected emergence body mass, survival, and hypopharyngeal gland size, whereas neonicotinoid exposure reduced hypopharyngeal gland size only. Since size is associated with hypopharyngeal gland functionality, a reduction could result in inadequate brood care. As cooperative brood care is a cornerstone of eusociality, smaller glands could have adverse down-stream effects on inclusive fitness of honey bee colonies. Therefore, our findings highlight the need to further study how ubiquitous stressors like V. destructor and neonicotinoids interact to affect honey bees.