The role of food distribution and nutritional quality in behavioural phasechange in the desert locust

The behaviour of herbivorous insects is influenced by their nutritional sta te. Nutrition-induced behavioural changes are often interpreted as adaptive mechanisms for controlling nutrient intake; however, their influence on ot her life history traits has received far less attention. We investigated th e effect of food quality and distribution on the behaviour and phase state of desert locusts, Schistocerca gregaria Forskal (Orthoptera, Acrididae), w hich change from the 'solitarious' to the 'gregarious' phase in response to population density. Phase change involves many morphological, physiologica l and behavioural changes. Solitarious insects are cryptic whereas gregario us locusts aggregate. Individual phase change is stimulated by mechanical c ontact with other locusts. A clumped resource distribution promotes change to the gregarious phase by increasing crowding and contact between individu als. In this study, we found that the effect of food distribution on locust phase depended on the nutritional quality of the food. We used three synth etic food treatments: near optimal, dilute and a choice of two unbalanced b ut complementary foods. Clumped resource distribution led to increased greg arization in the dilute and the complementary diet treatments. This effect was particularly pronounced on the complementary foods, owing to the intera ction of crowding and locomotion. Gregarization was most pronounced in the dilute diet treatment, owing to increased activity. These diet-induced effe cts are explained in terms of behavioural changes in locomotion, quiescence and feeding that are consistent with what is known from earlier work on lo cust feeding behaviour and behavioural phase change. (C) 2000 The Associati on for the Study of Animal Behaviour.