Can nestmate corpses cause bait neglect in invasive ants?

Invasive ants, such as Linepithema humile (the Argentine ant), pose a global threat, necessitating a better understanding of their behaviour in order to improve management strategies. Traditional eradication methods, including baiting, have had limited success, but the causes of control failure are not always clear. Here we propose that ants may learn to avoid toxic baits in part due to their association with ant corpses. Ants were tested on a Y-maze after exposure to scented corpses or dummies. 69% (n = 64) of ants avoided branches bearing the scent of scented corpses. At a collective level, colonies neglected food sources associated with scented ant corpses in favour of a food source with a novel odour, with only 42% (n = 273) of foragers feeding from the corpse-scent associated food source. However, if corpses were produced by feeding ants scented toxicant, focal ants encountering these corpses did not avoid the corpse-associated scent on a Y maze (53%, n = 65). Moreover, in a dual-feeder test, ants did not avoid feeding at food sources scented with an odour associated with conspecific corpses. The study demonstrates that conspecific corpses can act as a negative stimulus for Linepithema humile, leading to avoidance of odours associated with corpses, which can lead to potential avoidance of toxic baits. Why the more realistic Y-maze trial with corpses of ants that had ingested the toxicant elicited no avoidance is unclear: it may be due to weaker odour cues from ingested food, or a counterbalancing of the negative corpse stimulus by the positive presence of food remains on the corpse. Nonetheless, this study demonstrates that conspecific corpses act as a negative stimulus for ants, and this should be kept in mind when planning control efforts. A simple solution to this issue would be adding odours to baits, and cycling baits between treatments. This would disrupt the association between bait odour and corpses.