A neurotoxic pesticide changes the outcome of aggressive interactions between native and invasive ants

Abstract

Neurotoxic pesticides, such as neonicotinoids, negatively affect the cognitive capacity and fitness of non-target species, and could also modify interspecific interactions. We tested whether sublethal contamination with neonicotinoid could affect foraging, colony fitness and the outcome of behavioural interactions between a native (Monomorium antarcticum) and an invasive ant species (Linepithema humile). The foraging behaviour of both ants was not affected by neonicotinoid exposure. Colonies of the invasive species exposed to the neonicotinoid produced significantly fewer brood. In interspecific confrontations, individuals of the native species exposed to the neonicotinoid lowered their aggression towards the invasive species, although their survival probability was not affected. Exposed individuals of the invasive species interacting with non-exposed native ants displayed increased aggression and had their survival probability reduced. Non-exposed individuals of the invasive species were less aggressive but more likely to survive when interacting with exposed native ants. These results suggest that non-target exposure of invaders to neonicotinoids could either increase or decrease the probability of survival according to the exposure status of the native species. Given that, in any community, different species have different food preferences, and thus different exposure to pesticides, non-target exposure could potentially change the dynamics of communities and influence invasion success.

Figure 1.

The food discovery probability over time for the native Southern ant (black lines) and the invasive Argentine ant (grey lines). Dashed lines: colonies not exposed to the neonicotinoid (NIC−); solid lines: colonies exposed to sublethal doses of the neonicotinoid (NIC+). The food discovery probability is a one minus transformation of the estimated Kaplan–Meier probability curves. For each treatment, n = 40.

Figure 2.

The number of (a) workers alive and (b) brood produced on colonies of the native Southern ant and the invasive Argentine ant after 61 days of trials (mean ± s.e.). Light grey columns: colonies of both species not exposed to the neonicotinoid (NIC−). Dark grey columns: colonies of both species exposed to sublethal doses of the neonicotinoid (NIC+). The dashed line in (a) is the initial number of workers in each colony for all treatments. For each treatment, n = 5.

Figure 3.

The proportion (mean ± s.e.) of behavioural reactions displayed between groups containing 10 workers of each ant species in different treatment statuses. Groups of interacting ants were not exposed (NIC−) or exposed (NIC+) to sublethal doses of the neonicotinoid. Dark grey columns are aggressive responses. Light grey columns are non-aggressive responses. (a) The proportion of responses displayed by the native Southern ant during interactions with the invasive Argentine ant. (b) The proportion of responses displayed by the invasive Argentine ant during interactions with the native Southern ant. For each treatment, n = 10.

Figure 4.

The survival probability of ants subjected to interspecific interactions in groups containing 10 workers of each ant species in different treatment statuses. Groups of interacting ants were not exposed (NIC−) or exposed (NIC+) to sublethal doses of the neonicotinoid. (a) Survival probability over time of the native Southern ant during interactions with the invasive Argentine ant. (b) Survival probability over time of the invasive Argentine ant during interactions with the native Southern ant. For each treatment, n = 20.