Female pheromonal chorusing and aggregation in an arctiid moth, Utetheisa ornatrix

I report a unique finding of communal sexual display and aggregation mediated by female sex advertisement pheromone in Utetheisa ornatrix (Lepidoptera: Arctiidae). Female U. ornatrix adjust the temporal aspects of their pheromone signaling behavior in response to the long-distance pheromone released from neighboring females during a nightly activity period. The perception of the female pheromone is directly responsible for induction and adjustment of pheromone signaling by conspecific females. Additionally, some females even initiate pheromone signaling during photophase in response to the pheromone and its individual components. However, contrary to the predictions that rhythmic abdominal pumping serves to increase the peak pheromone release rate and heighten response in perceiving females, U. ornatrix females did not respond more strongly to the pulsed pheromone stimulus than to the constant release of pheromone at the same average rate. They were also found to move in response to the female pheromone and to orient toward the higher concentrations with a sensitivity and reaction intensity equivalent to that exhibited by males in the laboratory. Mating pairs tended to settle in the vicinity of pheromone signaling females as opposed to other locations in the natural habitat. Nonetheless, a laboratory test failed to reveal a higher per capita mating rate by females of aggregation, and a field test did not demonstrate actual attraction of females to female signalers or sex pheromone. These findings differing from the usual form of sexual communication in moths may be influenced by mating system of this species. At mating, the male U. ornatrix transfer a large spermatophore that may enhance female reproductive success. This action results in an extended post-mating male refractory period leading to a female-biased operational sex ratio. I argue that this biased sex ratio generates intrasexual competition among females in which they respond by elevating signaling effort such that the likelihood of at least matching their neighbors' signals is increased. I also suggest that females can benefit from aggregation, which create a concentrated pheromone plume that attracts many males and higher opportunities for remating and multiple spermatophores.