Native ants learn how to deal with cues of invasive species : responses to footprints of invasive ants are shaped by experience

Abstract

Invasive ants threaten biodiversity worldwide. They may benefit from being novel if native species fail to show appropriate responses to their cues. Cues include chemical footprints (or ‘home-range markings' in ants) left by all walking insects, which resemble cuticular hydrocarbons (CHCs). Ants are known to rely heavily on such cues to forage and recognise other individuals. We investigated the response of Lasius niger ants in central Europe to cues of two invasive ant species (Lasius neglectus and Linepithema humile), studying whether they were shaped by experience and whether the ants discriminated between cues of different species. Groups of L. niger workers encountered invasive ant workers in a negative (aggressive), positive (food-associated), or neutral (neither aggression nor food) context. Using Y-mazes with one blank arm and one arm with footprints, we investigated how this affected their response to footprints compared to responses of naïve ants. Naïve L. niger avoided footprints of both species, walking on the cue-bearing Y arm less than expected by chance (42.9 ± 0.6% SE versus 50% choosing the cue-bearing arm). Avoidance became stronger after negative encounters (35.3 ± 1.9%). However, their response did not change significantly after neutral encounters, whereas they approached the footprints after positive experience (62 ± 1.3% choosing the cue-bearing arm). Notably, there was no change in response to footprints of a species they had not encountered. Our results show that ants can link individual experience with living ants to their chemical footprints, thus transferring information between contexts. Also, they learn to respond specifically to cues of certain species, but do not generalise their responses to cues of other species. Ants hence might learn to interpret the wealth of chemical cues in their environment and use it to optimise foraging decisions. At least for other ants, invasive ants may not benefit from being novel.