Corpse-associated odours elicit avoidance in invasive ants

Toxic bait abandonment by an invasive ant is driven by aversive memories

Social insects such as ants possess a battery of behavioural mechanisms protecting their colonies against pathogens and toxins. Recently, active abandonment of poisoned food was described in the invasive ant Linepithema humile. During this abandonment, foraging declines by 80% within 6-8 h after baits become toxic-a reduction not due to satiety, diminished motivation, or mortality. Here we explore the mechanisms behind this behaviour, testing two hypotheses: (1) the presence of 'no entry' pheromones near toxic food, and (2) the formation of aversive memories linked to the toxic food site. In field trials, we placed bridges leading to sucrose, nothing, or poisoned sucrose on an active trail. Within hours, 80% of ants abandoned poisoned bait bridges. By swapping bridges strategically, we confirmed that aversive memories formed at toxic bait sites, while no evidence of a 'no entry' pheromone was found. Then, in the laboratory, we asked how ants may be sensing the toxicity of the bait, hypothesising poison-induced malaise. Motility, used as a proxy for malaise, was 29% lower in toxicant-exposed ants after 3 h, linking malaise to abandonment. Developing toxicants with delayed malaise, not just delayed mortality, may improve toxic bait control protocols.

Ants evade harmful food by active abandonment

Invasive ants, such as the Argentine ant, pose a severe economic and ecological threat. Despite advancements in baiting techniques, effectively managing established ant populations remains a daunting challenge, often ending in failure. Ant colonies employ behavioural immunity against pathogens, raising the question of whether ants can collectively respond to toxic baits. This study investigates whether ant colonies actively abandon palatable but harmful food sources. We provided two sucrose feeders, each generating a new foraging trail, with one transitioning to offering toxic food. Six hours later, ant activity on that path decreases, while activity on the non-toxic food and the trunk trail remains unaffected, excluding factors like population decline or satiation as reasons for the activity decline. Laboratory experiments confirmed that ants remained alive six hours after ingesting toxic food. Ant presence remains low on the toxic food path for days, gradually decreasing along the nearest section of the trunk trail. This abandonment behaviour minimises the entry of harmful food into the nest, acting as a protective social mechanism. The evasion of toxic bait-treated areas likely contributes considerably to control failures. Understanding the behavioural response to toxic baits is essential for developing effective strategies to combat invasive ant species.