Evaluating the effectiveness of caffeine-based baits against two pest ant species

The black cocoa ant, Dolichoderus thoracicus (Smith, 1860) (Hymenoptera: Formicidae) is a pest species capable of reaching high population densities in diverse habitats. In Taiwan, infestations are associated with intrusions into residential homes, damage to electrical equipment, and decreased agricultural productivity. Although baiting programs using borax-based or boric acid-based ant baits have been implemented, these efforts often yield unsatisfactory results. This study evaluated the potential of caffeine, a naturally occurring bioactive compound, as an active ingredient in ant baits for controlling D. thoracicus and Anoplolepis gracilipes (Smith, 1857). Laboratory bioassays demonstrated that hydrogel baits containing 1.0% caffeine resulted in 100% worker mortality in D. thoracicus colony fragments within 15 days (mean survival time = 3.06 days) and complete queen mortality within 5 days (mean = 2.89 days), outperforming baits containing 0.5%, 1.5% caffeine, or 3.0% borax. These findings demonstrate that the exceptionally high attractiveness of a 1.0% caffeine bait can induce substantial worker mortality through direct consumption, leading to complete elimination of laboratory colonies of D. thoracicus despite limited trophallaxis. Additionally, the 1.0% caffeine bait was also highly effective against A. gracilipes. In conclusion, ant bait formulated with an intermediate dose of caffeine provides an optimal balance between bait acceptance and toxicity, making it a promising tool for managing D. thoracicus and other pest ant species, although further field validation is warranted.