Mortality in an invasive ant does not scale linearly with fipronil exposure

Controlling eusocial invasive and other pestiferous insects is difficult. A common method of ant control is application of edible bait with delayed toxicity, enabling a horizontal transfer of the toxicant throughout the colony. If a toxicant dose is too high, foragers die before sharing bait with nestmates; if too low, it will be sublethal once shared throughout the colony. We asked how fipronil concentration and forager access influence horizontal transfer efficiency and colony mortality in Anoplolepis gracilipes (yellow crazy ants). We tested these effects by varying toxicant concentration and the number of workers accessing bait, measuring subsequent worker and queen ant mortality. Over 2 weeks, we compared expected mortality to observed mortality, working under the assumption that if horizontal transfer is independent of concentration colony mortality should increase linearly with the total fipronil delivered.

High fipronil doses produced far lower worker mortality than predicted, potentially because exposed ants died before distributing the toxicant. Worker mortality aligned with expectations only at low doses. Only 6 of 40 queens that were exposed to workers fed toxicant died, and surviving workers showed strong feeding aversion 2 weeks after treatment.

Our results suggest that horizontal transfer can fail at high concentrations and that effective suppression of colony workers may be achievable at much lower fipronil rates, potentially also reducing unintended environmental impacts. Feeding aversion post-treatment suggests repeated baiting within short time frames is unlikely to be effective. © 2026 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.