Sub-lethal exposure to a mammalian pesticide bait alters behaviour in an orthopteran

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Pesticides are a crucial tool to control introduced pest species, facilitating restoration of native ecosystems and reducing mortality risk in endemic species. However, non-target species, including many invertebrates, consume pesticides. Although lethal effects on these non-target species are often documented, sub-lethal effects are poorly understood. New Zealand presents a unique case study in this field, with a range of introduced mammalian pest species that threaten a large proportion of endemic fauna. Current research suggests that vertebrate pesticide baits do not cause mortality in invertebrate consumers, but few studies investigate sub-lethal behavioural effects. In this study, we tested whether consumption of a widely dispersed anticoagulant rodenticide, brodifacoum, influenced the expression and repeatability of behaviour in a known consumer of the bait, Wellington tree weta (Hemideina crassidens) over a 40-day treatment period. We compared refuge emergence, activity, refuge-seeking tendencies and defensive responses of brodifacoum-exposed and untreated control weta. Bait-exposed individuals showed increased emergence but decreased activity, boldness and aggression compared to untreated controls. These results show that brodifacoum, although not directly causing mortality, may alter behaviours directly related to foraging, mating, competitive interactions and anti-predator responses, which could be detrimental to fitness and survival. More research should be undertaken to investigate the capacity for H. crassidens to tolerate exposure, and to determine any age- or duration-related responses. These results should also be a catalyst for further research on non-target behavioural effects in other pesticide-exposed insect species.