Leaf cutting ant Acromyrmex striatus keep foraging even when attacked by an invasive ant

FRANCISCO SYLVESTER; CAROLINA PARIS; ANDREW SUAREZ

Mendoza

Congreso; X Congreso Argentino de Entomología; 2018

Biomass transportation in leaf cutting ants is modulated by abiotic and biotic factors including temperature, the presence of parasitoid flies, and intrinsic factors of individual colonies. We examined foraging behavior of the leaf cutting ant Acromyrmex striatus at vineyards in San Pedro de Yacochuya where the invasive ant Linepithema humile attacks and kills A. striatus workers while transporting leaves. Stridulation by A. striatus workers close to nest entrances or foraging trail is a common response to attacks, and the numbers of ants stridulating is proportional to the intensity of the attack. In 2015, we experimentally excluded L. humile from nest areas (n=8) of A. striatus to investigate if they modify biomass transportation. Exclusion of L. humile was achieved using pheromone dispensers containing synthetic (Z)-9-hexadecenal that disrupts L. humile foraging trails. Pheromone dispensers were deployed every meter, covering the area of each A striatus nest including nest entrances, foraging trails, and refuse piles. During peak hours of leaf harvesting, we quantified the following for each A. striatus nest: 1. foraging activity (number of laden and unladen ants passing a fixed point during 10 min), 2. the biomass transported by A. striatus in each foraging trail during a 10 min period, 3. biomass transportation speed of three laden workers, 4. the number of workers stridulating, 5. the number of L. humile attacking A. striatus workers, and 6. L. humile activity in the nest area of A striatus during 2 min. Soil surface temperature was also measured at each site. All variables were measured before, during, and after pheromone deployment at each nest over 11 days. We found L humile activity decreased by 73% after pheromone deployment; however attacks were not completely eliminated. Stridulation by A. striatus only occurred at nest areas under L humile attack and was positively correlated with the number of attackers. No other measurement varied with L. humile exclusion. Our results suggest that stridulation may be a mechanism for A. striatus to maintain foraging effort when attacked by the invasive ant L humile.