PHEROMONAL CONTROL OF BEHAVIOURAL PHASE TRANSITION IN LOCUSTS

Yael HEIFETZ and Shalom W. APPLEBAUM
Department of Entomology, Faculty of Agriculture, The Hebrew University, Rehovot 76100, Israel

Locust phase - solitary and gregarious, is dependent on population density. Climatic and biotic factors together aggregate locusts. Convergence of solitarious adults by seasonal winds, aHractance to one another and to limited food plants, sexual attractance and oviposition in select soils of suitable humidity, lead to the evolvement of dense nymphal populations and maintenance of their cohesion. Gregarious nymphs march in bands and gregaarious adults swarm. Visible transition of nymphal mo~pholoovy and colouration in response to population density are delayed until the following moult, while changes in behaviour are rapid and plastic (e.g., Gillett, 1988; Roessinah and Simpson, 1994; Heifetz et al., 1996). This present report focuses on factors oor*rolling behavioural phase transition in the Desert Locust Schistocerca gregaria, quantifies the response of solitarious nymphs to shortterm exposure to such factors, and describes their chemical nature.

A quantitative bioassay was used to measure typical solitarious and gregarious behaviour of desert locust nymphs, and subsequently, to evaluate the behavioural response of solitarious nymphs to various locust-derived compounds. The relative shin of solitarious nymphal behaviour towards gregarious is presented on a sliding scale of "phase index", derived by discriminant analysis of nymphal activity levels and associative indites. This bioassay, in conjunction with olfactometry, differentiates between volatile factors able to induce and maintain nymphal aggregation, and non-volatile factors directly inducing Dehavioural phase transition.

Volatiles from nymphal faeces attract solitarious nymphs, but lack any direct effect on behavioural phase transition. Cuticular extracts of gregarious nymphs do not attract but conversely, do induce behavioural phase transition. Cuticular surface hydrocarbons specifically affect antennal chemoreceptors, inducing inositol triphosphate (IP3) as the initial second messenger in subcellular antennal preparations (Heifetz et al., unpublished). The kinetics and specificity of response is typical for pheromone receptors.

It is suggested that attractant compounds increase nymphal aggregation and that density, maintained by aggregation pheromones, increases the probability and frequency of intraspecific contact. Consequently, as a distinct process, behavioural phase transition is induced by specific cuticular hydrocarbon contact pheromones.

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