Thiamethoxam-induced stress responses in Melipona scutellaris: insights into the toxicological effects on Malpighian tubules

The increasing use of pesticides has been identified as a major driver of pollinator decline. Thiamethoxam, a neonicotinoid pesticide, poses a significant threat to neotropical bees by impacting behavior and physiology. Despite the ecological importance of stingless bees, little is known about the effects of pesticides on these species. This study investigated the effects of a sublethal concentration of thiamethoxam on the Malpighian tubules (key organs responsible for excretion and detoxification) in Melipona scutellaris. Using immunolabeling techniques, we analyzed the expression of heat shock proteins (HSP70 and HSP90) and cellular stress biomarkers, and evaluated cell death through the TUNEL method. Results showed an increase in HSP70 expression on day 1 of exposure, followed by a decrease on day 8, while HSP90 exhibited the opposite trend. The observed pattern for HSP70 suggests that the Malpighian tubules initially responded to oxidative stress caused by acute exposure (1 day), while after 8 days, other metabolic responses or pathways may have been activated to counteract cellular stress. In contrast, the pattern for HSP90, which was opposite to that of HSP70, may indicate a compensatory response, with increased HSP90 levels potentially protecting the Malpighian tubules as HSP70 levels declined. Additionally, TUNEL assays revealed minimal DNA fragmentation by the eighth day of exposure. These findings highlight the complex response of M. scutellaris to TMX and underscore the need for further ecotoxicological research on stingless bees to better understand the impacts of pesticides on non-target species.