Effects of thiamethoxam and spinosad on the survival and hypopharyngeal glands of the African honey bee (Apis mellifera intermissa)

Insecticides can affect development and survival of non-target and beneficial arthropods like honey bees (Apis mellifera L.). Thiamethoxam and spinosad are widely used as pesticides in agriculture but they have become an important concern for beekeepers and researchers focusing on bee health; multiple reports stressed adverse effects on bees, notably on honey bees. The present study aims to evaluate the impact of these two insecticides on the development of the HPGs and on the survival of Apis mellifera intermissa a native African subspecies of honey bee present in Algeria. Newly emerged workers were acutely and chronically exposed to thiamethoxam and spinosad through sugar syrup and pollen pastry. The effects of these insecticides were assessed by measuring the size of HPGs acini and the total head protein content. The survival of the workers was also evaluated over 60 days when they were chronically exposed to the insecticides at concentrations corresponding to LC25 and LC10. We found that the insecticide-treated workers, after both acute and chronic exposure, exhibited smaller and irregularly shaped HPG acini. The total head protein content also decreased in treated individuals with the two concentrations of insecticides at day 6 and 9 compared to the respective controls. While the control group exhibited an LC25 (i.e. the time needed to kill 50% of the tested workers) of 22 days, the LC25 was only 3 days for the workers exposed to the LC25 of spinosad and all workers were dead at day 17. In contrast, thiamethoxam exposure at LC25 had no significant detrimental effect on honey bee survival. This study demonstrated the toxicity of thiamethoxam and spinosad to workers of A. mellifera intermissa and highlighted potential detrimental effects of the bioinsecticide spinosad on HPGs and survival of the bee workers.