Wind disrupts trail pheromone communication in the leaf-cutting ant Acromyrmex lobicornis

Abstract

Although wind is a ubiquitous component of ecological systems and might affect pheromone communication, its effects have not been studied in depth. To test whether wind impacts trail pheromone behaviour, we studied the behaviour of the leaf-cutting ant Acromyrmex lobicornis in natural nests. We conducted observations under (1) unaltered scenarios with different wind intensities and (2) manipulative experiments aimed at altering trail-marking pheromone volatilization rates. In our manipulative experiments, we placed filter paper on ant trails, then removed the pheromone-marked paper 24 h later and either exposed or did not expose the marked paper to artificial wind (2 km/h) for 1 or 3 h. We then put the marked paper back on the trail and filmed ants' responses for 1 min. As controls, we filmed ants’ responses to filter paper immediately before it was removed as well as their responses to new unmarked filter paper. We measured ant flux, speed and walking sinuosity of ants over trails and filter paper, the number of ants tapping their gaster against the substrate (an indication of pheromone-marking frequency), the number of head-on encounters among workers (as alternative modes of communication) and ant body size. Wind negatively affected ant traffic by decreasing ant flux and speed. Colonies increased the number of U-turns and head-on encounters among workers, with a higher response as wind exposure time increased, but trail pheromone deposition was constant among treatments. The size frequency distribution of foragers over the treatment area was skewed towards larger ants on trails with unmarked paper and on trails with paper that had been exposed to wind for 1 or 3 h or not exposed to wind but removed for 3 h. This could be the result of small ants focusing on pheromone trail maintenance, while larger ones focused on foraging on the filter paper irrespective of whether it had been marked. Our results suggest that division of labour and behavioural plasticity might allow leaf-cutting ants to mitigate the effects of wind on pheromone communication and continue foraging.