Diet modulates gregarization in the Australian plague locust

Locusts are grasshoppers that exhibit a density-dependent polyphenism, switching between a lone-living solitarious phase and a gregarious phase that forms migratory swarms. Gregarization is promoted by high population density and its primary trigger is through mechanosensory stimulation from other locusts and subsequent species-specific neurohormonal responses. In the lab, gregarious behavior can be elicited by repeated mechanical stimulation with a paintbrush. It is not known however, if environmental factors may modulate the propensity of a locust population outbreak to develop into a migratory swarm by changing locusts’ receptivity to this mechanical stimulation. In many insect species poor host-plant quality promotes migration to enhance growth and survivorship. We tested the effects of diet quality on growth performance and propensity to gregarize in the Australian plague locust, Chortoicetes terminifera. We reared locust nymphs on one of five synthetic diets; all isocaloric, but differing in their macronutrient balance (protein to digestible carbohydrate ratio). We reared locusts in either isolated or crowded conditions. Isolated locusts were further split into two groups. One group received a paint-brush stimulation treatment, and the other did not. In contrast to the migratory hypothesis, locusts exhibited the most gregarious behavior when reared on more optimally-balanced diets. These results suggest that the Australian plague locust, and perhaps other migratory animals, fit an alternative scenario where access to high quality food may be necessary to support long distance migration. These data indicate that diet quality, in addition to population density, may be an important ecological factor influencing the development of locust swarms.