Local population density and the activation of movement in migratory band-forming Mormon crickets

The Mormon cricket, Anabrus simplex (Orthoptera: Tettigoniidae), is a flightless katydid that undergoes major outbreaks in western North America. Under outbreak conditions, Mormon crickets often form large cohesive migratory bands that can contain millions of individuals moving in unison across the landscape. Density-dependent phase polyphenism has been hypothesized as a behavioural mechanism underlying Mormon cricket band formation and movement based largely on the similarity between bands of Mormon crickets and those of migratory locusts. I tested this hypothesis by quantifying the effects of long-term differences in rearing density versus the short-term presence or absence of conspecifics on Mormon cricket movement behaviour across three different developmental stages. Rearing density and the presence of conspecifics both influenced Mormon cricket movement, but in distinctly different ways. Increases in the expression of walking-related behaviours were mainly induced by the short-term presence of conspecifics. The longer-term effects of high rearing density primarily influenced only turning-related behaviours. Developmental variation in activity levels was observed and indicates that developmental stage should be an important consideration in future analyses of Mormon cricket movement. Overall, these results suggest that interindividual interactions play a greater role in inducing the movement of migratory band-forming Mormon crickets than do endogenous behavioural-phase changes mediated by high local rearing density.