Effect of a pyrethroid-based pesticide on colony survival and behaviour of a tropical non-target ant Diacamma indicum.

The global use of chemical pesticides negatively affects both target and non-target species, significantly impacting biodiversity and ecosystems. While the effects of pesticides have been studied on non-target species like bees, there have been few studies considering ants. Our study focuses on the non-target tropical ponerine ant, Diacamma indicum Santschi, 1920, which inhabits areas near human settlements and is likely to encounter pesticide-contaminated environments. We investigated the short-term effects of field-relevant exposure to alpha-cypermethrin, a widely used synthetic pyrethroid insecticide, by contaminating the area around the nest. Using the recommended dose (RD) and its multiples (4RD, 2RD, 0.5RD, 0.25RD) to reflect realistic field levels, we conducted experiments in the lab via double-blind randomised controlled trials. We found that pesticide exposure for five days did not significantly affect colony size or mortality across all tested doses, but individual behaviour was markedly impacted at every dose. Five pesticide-induced behaviours- paralysis, trembling, appendage shaking, staggering, and twitching- were identified and quantified. Compared to the controls, the first- and second-day post-application showed significantly higher behavioural effects. Maximum effects were observed at 4RD, affecting nearly 20% of the colony. Quantitative behavioural assessment revealed that ants that died after five days exhibited significantly higher levels of sickness, with paralysis serving as a predictor of mortality. Our findings demonstrate the resilience of D. indicum colonies to field-realistic cypermethrin exposure while also highlighting adverse behavioural implications, which can be leveraged in future environmental impact assessments.