Acute toxicity and sublethal effects of thiamethoxam on the stingless bee Scaptotrigona postica: Survival, neural morphology, and enzymatic responses

Native and cultivated plants in the Neotropics benefit from the pollination services provided by stingless bees. The use of neonicotinoid insecticides negatively impacts bee health, even though bees are not their primary targets. This study determined the oral mean lethal concentration (LC50) of thiamethoxam (TMX) after 24 h of exposure for the stingless bee Scaptotrigona postica. Based on the LC₅₀ value (0.11 ng a.i./μL) obtained, two fractions of this value (1/10 and 1/100 LC₅₀) were selected to assess survival time (LT₅₀), as well as to conduct neural morphological and enzymatic analyses. The LC₅₀/100 group had a LT₅₀ of 15 days, compared to 17 days in the control group, while the LC₅₀/10 group survived for 8 days. Morphological analyses revealed increased Kenyon cell spacing and pyknosis in the mushroom bodies after 1, 3, and 6 days of exposure, suggesting that thiamethoxam adversely affects the brain of S. postica. Regarding enzymatic activity, comparisons between the control and the two sublethal concentrations revealed that Carboxylesterase and Glutathione S-transferase (GST) activity increased in the abdomens after six days of exposure. GST activity also increased in the bees' heads for the LC₅₀/10 concentration after six days of exposure (Control x TMX group). The enzymatic results suggests that thiamethoxam induces oxidative stress in S. postica. The results presented underscore the necessity of considering stingless bees in regulatory decisions regarding the use of insecticides, ensuring that the risks to this important group of pollinators are adequately assessed.