Submitted to: Journal of Animal Behavior
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/25/2004
Publication Date: 4/28/2004
Citation: Sword, G.A. 2005. Local population density and the activation of movement in migratory band-forming mormon crickets. Journal of Animal Behavior. 69(2):437-444. Interpretive Summary: The Mormon cricket is a flightless shield-backed katydid widely distributed throughout western North America. Outbreaks of this species can result in the formation of spectacular migratory bands that can be over 10 km long, several kilometers wide, contain dozens of insects per square meter, and travel up to 2.0 km per day. The observed similarity in migratory band formation and movement between locusts and Mormon crickets fueled speculation that Mormon crickets might express density-dependent behavioral changes similar to those known to occur in locusts. In locusts, these behavioral changes are induced when the insects are reared at high local population densities. This study demonstrates that the behavior of field-collected Mormon crickets from band-forming and non-band-forming populations differs in the laboratory, paving the way for future behavioral analyses aimed at identifying the factors underlying this difference. In addition, it is shown that among band-forming Mormon crickets, short-term interactions among individuals play a much greater role in the activation of an individual's movement than do longer-term behavioral changes induced by high rearing density. These results suggest that the behavioral mechanisms underlying band formation and movement in Mormon crickets are quite different from those known to occur in locusts. Rather than being due to behavioral changes induced by high rearing density conditions, the movement of migratory band-forming Mormon crickets is stimulated by the immediate short-term presence of other nearby individuals.
Technical Abstract: 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 than can contain millions of individuals moving in unison across the landscape. This study demonstrates differences in individual movement behaviour between field-collected Mormon crickets from band-forming, outbreak and non-band-forming, non-outbreak populations. In laboratory assays, both walking and turning-related behaviours were expressed to significantly greater degree in band-forming relative to non-band-forming insects. In a separate laboratory experiment, the effects of long-term differences in rearing density versus the short-term presence or absence of conspecifics on Mormon cricket movement behaviour were examined 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 show that inter-individual interactions play a greater role in inducing the movement of migratory band-forming Mormon crickets than do endogenous behavioural changes mediated by high local rearing density.