The Ant Who Cried Wolf? Short-Term Repeated Exposure to Alarm Pheromone Reduces Behavioral Response in Argentine Ants

SIMPLE SUMMARY: A significant challenge of chemical communication between ants is to maintain accurate communication of information in a variety of contexts. Argentine ants use volatile (airborne) compounds for a variety of functions, but one very important function is to elicit alarm via alarm pheromones. Given the importance of accurately responding to this signal, we expected Argentine ants to consistently show an alarm response to repeated exposure of alarm pheromones from their nestmates. However, we instead observed a reduction in their alarm behaviors over time. We speculate that a consistent response to repeated alarm signaling might require reinforcement from an actual alarming stimulus (e.g., the presence of predators or rival colonies). Argentine ants are considered a pest and several integrated pest management regimes use pheromones (i.e., mating disruption, aggregation pheromones, etc.) to reduce pest populations. Our results could be important to consider in the development of such control strategies because if ants habituate to their alarm pheromone over continuous exposure (without actually alarming stimuli) it might prove to be an ineffective strategy to repel them. ABSTRACT: In this study we test whether Argentine ants (Linepithema humile) progressively reduce their response to a salient stimulus (alarm pheromone) with increased exposure over time. First, we used a two-chamber olfactometer to demonstrate three focal behaviors of Argentine ants that indicate an alarmed state in response to conspecific alarm pheromone and pure synthetic iridomyrmecin (a dominant component of L. humile alarm pheromone). We then measured how these behaviors changed after repeated exposure to conspecific alarm pheromone from live ants. In addition, we investigate whether there is a difference in the ants’ behavioral response after “short” (3 min) versus “long” (6 min) intervals between treatments. Our results show that Argentine ants do exhibit reduced responses to their own alarm pheromone, temporarily ceasing their response to it after four or five exposures, and this pattern holds whether exposure is repeated after “short” or “long” intervals. We suggest alarm pheromones may be perceived as false alarms unless threatening stimuli warrant a continued state of alarm. These results should be kept in mind while developing pheromone-based integrated pest management strategies.