Australian stingless bees exhibit greater sensitivity to dimethoate than Apis mellifera: implications for pesticide risk assessment.

The evaluation of pesticide risks to pollinators has gained momentum in recent years, particularly in tropical and subtropical regions, due to the high diversity of bee species in these regions. These regions host many stingless bees, which, according to recent studies in the Neotropics, are highly susceptible to pesticides. Currently, it is a standard practice to use the western honey bee (Apis mellifera) as a model organism to predict pesticide risks for other bees. However, while some studies suggest that A. mellifera can adequately represent neotropical stingless bees, others report discrepancies. Meanwhile, similar data are lacking for the numerous stingless bees of the Asia-Pacific region. To date, no study has assessed the effects of pesticides on Australian stingless bees, or whether A. mellifera is a suitable surrogate for predicting these risks. In this study, we compared the acute topical and oral toxicity of the insecticide dimethoate-the standard positive control in A. mellifera toxicity assays-between A. mellifera and three species of Australian stingless bees: Tetragonula carbonaria, Tetragonula hockingsi, and Tetragonula mellipes. We determined the median lethal dose (LD₅₀) and median lethal concentration (LC₅₀) for each species, and calculated sensitivity ratios (R). All three stingless bee species exhibited higher mortality than A. mellifera following both topical and oral exposure to dimethoate. Moreover, applying a 10× safety factor to the LD₅₀/LC₅₀ values of A. mellifera did not reliably predict the sensitivity of these stingless bees. However, when adjusted for fresh body weight, the sensitivities of stingless bees and honey bees were similar for both exposure routes. These findings suggest that body weight should be considered when comparing toxicity data between species. Alternatively, safety factors of at least 10 (topical exposure) and 20 (oral) should be applied when extrapolating A. mellifera results to stingless bees in the absence of taxon-specific data. This research provides a foundational dataset for investigating the effects of pesticides on Australian stingless bees and for assessing broader health and behavioural risks associated with pesticide exposure in these important pollinators.