  Locusts are significant agricultural pests. Under favorable environmental
conditions flightless juveniles may aggregate into coherent, aligned swarms
referred to as hopper bands. These bands are often observed as a propagating
wave having a dense front with rapidly decreasing density in the wake. A
tantalizing and common observation is that these fronts slow and steepen in the
presence of green vegetation. This suggests the collective motion of the band
is mediated by resource consumption. Our goal is to model and quantify this
effect. We focus on the Australian plague locust, for which excellent field and
experimental data is available. Exploiting the alignment of locusts in hopper
bands, we concentrate solely on the density variation perpendicular to the
front. We develop two models in tandem; an agent-based model that tracks the
position of individuals and a partial differential equation model that
describes locust density. In both these models, locust are either stationary
(and feeding) or moving. Resources decrease with feeding. The rate at which
locusts transition between moving and stationary (and vice versa) is enhanced
(diminished) by resource abundance. This effect proves essential to the
formation, shape, and speed of locust hopper bands in our models. From the
biological literature we estimate ranges for the ten input parameters of our
models. Sobol sensitivity analysis yields insight into how the band's
collective characteristics vary with changes in the input parameters. By
examining 4.4 million parameter combinations, we identify biologically
consistent parameters that reproduce field observations. We thus demonstrate
that resource-dependent behavior can explain the density distribution observed
in locust hopper bands. This work suggests that feeding behaviors should be an
intrinsic part of future modeling efforts.
