Divergent horizontal transfer of insecticides in a social termite

In social insects, horizontal transfer determines whether insecticides can spread from exposed individuals to untreated nestmates and achieve colony-level effects. However, transfer outcomes may diverge substantially even among insecticides with comparable acute toxicity. In this study, horizontal transfer among six insecticides was evaluated in the social termite Coptotermes formosanus to examine factors potentially associated with transfer divergence. Acute toxicity differed markedly among insecticides, but transfer performance did not follow the same pattern. Chlorpyrifos exhibited the highest acute toxicity yet produced the weakest horizontal transfer, whereas bifenthrin, imidacloprid, and chlorantraniliprole caused greater recipient mortality despite comparatively higher LC₅₀ values. Notably, bifenthrin retained strong transfer despite its relatively high acute toxicity, indicating that acute toxicity alone could not explain transfer outcomes. Regression analysis showed no relationship between LC₅₀ and recipient survival. Donor survival varied among treatments, with chlorpyrifos causing rapid donor mortality, whereas bifenthrin and thiotraniliprole allowed longer survival. Feeding activity and detoxification enzyme responses differed among treatments, suggesting compound-specific differences in donor physiological condition after exposure. Together, these findings demonstrate that horizontal transfer outcomes may diverge beyond what can be explained by acute toxicity alone and may be associated with differences in donor persistence and post-exposure condition. More broadly, the results highlight how individual-level responses to chemical exposure may shape secondary toxicant transfer within social systems, emphasizing the importance of social transmission processes in determining the ecological consequences of insecticide exposure.