Collaborative brood care by workers is essential for the functionality of eusocial honey bee, Apis mellifera, 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 both V. destructor and neonicotinoids reduce hypopharyngeal gland size, thereby potentially compromising collaborative brood care in colonies. In a fully-crossed laboratory experiment, the impact of mites and the neonicotinoids thiamethoxam and clothianidin alone and in combination on workers were evaluated. While the neonicotinoids did not impact survival and emergence body mass, the data confirm that V. destructor reduces both. Even though the interactions between both stressors were antagonistic and neutral, the clear detrimental effects of both stressors alone and in combination on worker longevity and food glands are remarkable. Besides reduced worker longevity, impaired brood care provided by workers exposed to V. destructor and neonicotinoids could be detrimental for honey bee colony functionality. Our findings highlight a mechanism to explain honey bee colonies losses globally.

This data set was collected during a semi-field experiment. The data were noted down on paper before being scanned and entered in Excel. 

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