Relocating in contaminated terrains: cypermethrin impacts task performance and nest choice in a non-target ant...

On the face of growing pesticide contamination in the environment, non-target species of insects face serious threats in the ecosystem, ranging from species decline to behavioural changes. Social insects living in colonies are vulnerable to the multimodal effects of pesticide contamination, especially when they are relocating their nests. Nest relocation is a goal-oriented task in eusocial species, which exposes the whole colony to environmental stressors like pesticide contamination. In this context, we investigated the effect of a pyrethroid pesticide, alpha-cypermethrin, on the colony relocation of a non-target ponerine ant, Diacamma indicum. Using field-relevant cypermethrin contamination and simulating natural conditions of nest relocations, we asked if ant colonies would be able to relocate through a contaminated terrain. Furthermore, we tested the effect of prior exposure to pesticide contamination on nest choice during colony relocation using a T-maze set-up. Testing seventy naturally collected D. indicum colonies across five different relocation scenarios, we found cypermethrin impacted behaviour, relocation dynamics and nest choice. Five pesticide-induced behaviours (paralysis, trembling, appendage shaking, staggering and twitching) were identified and quantified. Although the majority of the colonies successfully relocated to the new nest, relocating through a contaminated terrain significantly delayed the process by 2.6 times compared to control colonies. Individual task performance was impaired as only half of the tandem leaders were working in treatment colonies. Interestingly, none of these leaders died, even though an average of 20% of the colony showed either paralysis or mortality a day after exposure. Colonies without prior exposure to pesticides preferred non-contaminated nests over contaminated ones, while colonies living in pesticide-contaminated environment randomly chose either the contaminated or control nest. Our results highlight the disruptive role of pesticide contamination on colony relocation and nest choice in a non-target ant species, and highlight the need to understand the effects of pesticides on aspects of life history other than foraging in both target and non-target species for a comprehensive understanding.