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

Authors

  • E. Despland,

  • M. Collett,

  • S. J. Simpson


E. Despland, Dept of Zoology, Univ. of Oxford, South Parks Rd, Oxford UK OX1 3PS. – M. Collett, BBSRC-NERC Ecology & Behaviour Group, Dept of Zoology, Univ. of Oxford, South Parks Rd, Oxford, UK OX1 3PS. – S. J. Simpson (correspondence), Dept of Zoology and University Museum, Univ. of Oxford, South Parks Rd, Oxford, UK OX1 3PS (stephen.simpson@zoo.ox.ac.uk).

Abstract

Desert locusts (Schistocerca gregaria) change phase in response to population 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 swarm formation. We have recently developed an individual-based model of locust behavior in which contagious resource distribution leads to phase change. This model shows how population gregarization can result from simple processes operating at the individual level. In the present study, we performed a series of laboratory experiments in which vegetation pattern and locust phase state were assigned quantitative, measurable indices. The pattern of distribution of the resource was represented via fractal dimension; the phase state was evaluated using a behavioral assay based on logistic regression analysis. Locusts were exposed to different patterns of food resource in an artificial arena, after which their behavioral phase state was assayed. These experiments showed that when the distribution of the vegetation was patchy, locusts were more active, experienced higher levels of crowding, and became more gregarious. These results are consistent with simulation predictions and field observations, and demonstrate that small-scale vegetation distribution influences individual behavior and phase state and plays a role in population-level responses.

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