Small-scale processes in desert locust swarm formation: how vegetation patterns influence gregarization

Desert locusts (Schistocerca gregaria) change phase in response to populati on density: 'solitarious' insects avoid one another, but when crowded they shift to the gregarious phase and aggregate. This individual-level process is the basis for population-level responses that may ultimately include swa rm formation. We have recently developed an individual-based model of locus t behavior in which contagious resource distribution leads to phase change. This model shows how population gregarization can result from simple proce sses operating at the individual level. In the present study, we performed a series of laboratory experiments in which vegetation pattern and locust p hase stale were assigned quantitative, measurable indices. The pattern of d istribution of the resource was represented via fractal dimension; the phas e slate was evaluated using a behavioral assay based on logistic regression analysis. Locusts were exposed to different patterns of food resource in a n artificial arena, after which their behavioral phase state was assayed. T hese experiments showed that when the distribution of the vegetation was pa tchy, locusts were more active, experienced higher levels of crowding, and became more gregarious. These results are consistent with simulation predic tions and held observations, and demonstrate that small-scale vegetation di stribution influences individual behavior and phase state and plays a role in population-level responses.