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

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 semisolid

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.