Chronic exposure of thiamethoxam in Melipona scutellaris: Ultrastructural effects in the midgut

Apis mellifera is the standard organism in pesticide Risk Assessments (RA) schemes, however, in view of the great biodiversity of bees in the neotropical region, studies addressing toxicological approach on native bees have been required for RA purposes. Therefore, here we assessed the sublethal effects of the thiamethoxam neonicotinoid on morphological parameters of Melipona scutellaris, which is included in the list of priority species for RA studies in neotropics. Thus, we evaluated the midguts of foragers bees of M. scutellaris when exposed to a sublethal concentration of thiamethoxam through ultrastructural analyzes. Ninety bees were collected from 3 unrelated colonies and exposed to the LC_50/100 (0.000543 ng a.i./μL diet). Six midguts from each sample group were collected after 1, 4, and 8 days and processed for analysis in transmission electron microscopy. Among the observed effects, only in the eighth day of exposure, it was possible to observe the slightly irregular nuclear envelope and the disorganized basal labyrinth. Compared to other organs, the few changes observed may be related to the regenerative capacity and to the low concentration assessed. Moreover, although the gut is the first organ to have contact with the insecticide due to the oral exposure, it is not the target of thiamethoxam. However, prolonged exposure to thiamethoxam affects other non-target-organs of the mechanism of action of thiamethoxam, such as the gut.