Acute oral and contact toxicity of imidacloprid and thiamethoxam and their commercial products to the neotropical stingless bee <i>Melipona scutellaris</i>

Neotropical stingless bees have frequently been reported to possess high biodiversity, ecological significance, and sensitivity to insecticides. Surprisingly, few studies have been conducted so far to assess their sensitivity to neonicotinoid insecticides, although there are indications that this insecticide class is especially toxic to stingless bees. The aim of this study was therefore to evaluate the acute oral and topical toxicity of two neonicotinoids, imidacloprid and thiamethoxam, to the neotropical stingless bee Melipona scutellaris. Besides these active ingredients, commercial products containing them were also evaluated. The commercial products were more toxic to the bees than the active ingredients, which may be due to direct toxicity of coformulants and indirectly through their higher biological activity and facilitation of uptake by organisms. The neonicotinoids were more toxic through topical contact than oral exposure. This is the opposite trend to that previously reported for honeybees, which is explained through differences in life-history traits with stingless bees. M. scutellaris was more sensitive to the test substances than standard bee test species commonly used in (temperate) toxicity assessments. This thus stresses the need to include stingless bees in neotropical risk assessments. The relatively high mortality occasionally observed in control groups highlights the biological sensitivity of stingless bees to laboratory conditions rather than a methodological flaw. This finding reinforces the importance of refining experimental setups by minimizing handling stress and improving cage microclimate to enhance control survival and ensure even greater robustness in future toxicity assessments involving native species.