Comparative analyses of collective movement in two Schistocerca species that vary in expression of behavioral phase plasticity

Phenotypic plasticity is a phenomenon characterized by the expression of multiple phenotypes by one genotype and has been the focus of extensive research among locusts. Locusts express density-dependent behavioral phase polyphenism whereby conspecifics reared at low population densities tend to be sedentary and avoid each other in the “solitarious phase”, but switch upon crowding at high densities to a “gregarious phase” that is much more active and forms aggregations that migrate en mass. We compared the collective movement behavior between a swarming locust known to express extreme density-dependent behavior changes, Schistocerca gregaria, and the related non-swarming grasshopper, Schistocerca americana, which expresses reduced density-dependent plasticity in behavior. Crowd-reared fourth instar juveniles were placed in circular arenas at six different densities with their behavior recorded for six hours. The marching and non-marching behaviors were analyzed using TRex, automated video-tracking software that simultaneously tracks multiple individuals. Collective movement at high local density was observed in both species, although there was some variation in behavior between them. The threshold density for marching appeared to be different as well. Although S. americana rarely if ever swarms in the field and exhibit reduced behavioral plasticity, these results suggest it is nonetheless capable of collective movement similar to that of typical locust species. As such, the lack of swarming by S. americana observed in the field is most likely because it occurs in environments that are not conducive to swarm formation as opposed to a lack of genetic variation for the expression of swarming behavior.