Diet type modifies ingestion rates and trophallactic exchanges in leaf-cutting ants

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Abstract

Conventional control of leaf-cutting ants is mainly carried out by application of solid insecticide baits. However, alternative approaches could increase the efficiency of control methods. Here, we investigated the hypothesis that liquid and semi-solid baits are more readily ingested by leaf-cutting ants and are therefore more toxic than solid baits. Furthermore, following increased ingestion rates, ants could perform higher frequencies of trophallactic exchanges with their nest mates, thus increasing dispersal rates within the colony. Each of three diets were offered to Acromyrmex subterraneus subterraneus (Forel) and Atta sexdens rubropilosa Forel (both Hymenoptera: Formicidae) for 1 h under laboratory conditions and subsequently crop volumes were quantified. The highest crop volumes for both species were seen when the diet was offered in liquid form, and the lowest crop volumes were observed after offering solid diets. Survival rates of the ants were monitored following addition of the neonicotinoid insecticide imidacloprid (IMI) to the three diets. The ingestion of liquid and semi-solid diets + IMI by A. subterraneus resulted in 17 and 6% survival, respectively, whereas these diets caused 100% mortality of A. sexdens. Ingestion of solid diets with IMI resulted in 51% survival of A. subterraneus and 23% survival of Atta workers. Twenty-three percent of A. subterraneus which had fed on liquid diet carried out trophallactic exchanges, whereas only 10 and 3% of ants carried out trophallaxis when fed on semi-solid or solid diet, respectively. Lower trophallactic frequencies were observed for A. sexdens, with 13 and 3% of ants that had fed on liquid and semi-solid diets performing trophallaxis, respectively. The low trophallactic exchange rates following ingestion of solid diets would reduce the dispersal of these compounds throughout the colony. Control strategies using solid baits should be re-examined in the light of these findings, which indicate the importance of optimizing insecticide ingestion.

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