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


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.

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We thank Peter Ritchie, Alison MacDiarmid, Melinda Divers and Otari-Wilton’s Bush for allowing us to put wētā motels on their properties, and for aiding in the collection process. We thank the Behavioural Ecology & Evolution Group at Otago and Chris Green from the Department of Conservation for helpful comments that improved this manuscript. This research was funded by the University of Otago Department of Zoology Master’s research fund, and the manuscript was prepared with funding from the University of Otago postgraduate publishing bursary. PMW was supported by a Rutherford Discovery Fellowship from the Royal Society of New Zealand (LCR-14-001).

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Correspondence to Adele Parli.

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As per New Zealand animal welfare legislation, no animal ethics approval was required to complete the research outlined in this manuscript. Despite this, all efforts were taken to provide wētā with optimal living conditions. All animals were provided with adequate food and water to sustain healthy growth, which was measured every 5 days by weighing each wētā. Signs of abnormal behaviour (emergence in the light, no emergence for 3 + days, no sign of eating or defecating) were monitored over the entire duration of the study. If any individual appeared stressed or unwell (including loss of 20% or more of their body weight, or abnormal behaviours), they would have been removed from the trial and humanely euthanised; however, this did not occur during the study. Wētā were provided with shelter (a flax refuge), native leaves, and a climbing stick which also served as a site for immature wētā to hang during moulting. Conditions in the animal holding room were set to match natural conditions at the collection site as closely as possible. Wētā were not released back into the wild at the end of the study period, as they were collected from a different area than they were tested in. Instead, they were maintained in the University of Otago Department of Zoology animal containment facility, under similar conditions as they were kept during the experiment.

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Parli, A., Besson, A., Wehi, P. et al. Sub-lethal exposure to a mammalian pesticide bait alters behaviour in an orthopteran. J Insect Conserv 24, 535–546 (2020).

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