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 col-onies. 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 neon-icotinoids, 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 neon-icotinoid 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.

Automated summary

Collaborative brood care is crucial for honey bee colonies, and the hypopharyngeal food glands of workers play a vital role in this process. However, the effects of the ectoparasitic mite Varroa destructor and neonicotinoids on the feeding glands are not well understood. In this study, we found that the combined exposure to Varroa destructor and neonicotinoids had an antagonistic interaction on hypopharyngeal gland size. Individually, Varroa destructor parasitism negatively affected emergence body mass, survival, and gland size, while neonicotinoid exposure only reduced gland size. These findings highlight the need to further investigate the interaction between ubiquitous stressors and their impact on honey bees.