Effects of starvation and vegetation distribution on locust collective motion

Locusts are able to migrate over long distances across areas with different vegetation structures. This work investigates how the spatial arrangement of vegetation might affect locust collective motion. The behavior of groups of third instar Schistocerca gregaria hoppers that had been fed or starved for 24 h was studied experimentally. Food patches were introduced into a circular arena where locust groups were marching. We tested two different types of food distribution: uniform and clustered. The starved locust groups were used to simulate how encountering vegetation affected locusts after crossing a large bare area. We analyzed the directed motion and mean speed of the locust groups. On the small scale of the arena, the results did not show any effects of vegetation distribution on the oriented motion and mean speed of the groups (fed group). However, encountering vegetation greatly affected the oriented movement and walking speed in the starved group, mimicking the crossing of a large bare area (group starved for 24 h). After feeding, the individuals entered a post-prandial period. That period was significantly longer for the starved hoppers than for the fed hoppers. After the post-prandial period, the hoppers started marching and did not return to feed on the food. It is therefore suggested that the marching activity of hoppers is not directly related to their hunger level. The effect of the spatial distribution of vegetation on the hunger status of hoppers and the implication for barrier treatments used in locust control are further discussed