Aggregation pheromone complex of the desert locust, Schistocerca Gregaria (Forskal) (Orthoptera: Acrididae): Current Status

Abstract

The most striking feature of the desert locust, Schistocerca gregaria (Forskal), is its ability to reversibly transform between two morphologically, physiologically and behaviourlly distinct phases: solitaria and gregaria. The solitary phase prevails in the recession areas as dispersed individuals in very low densities in scattered populations, feeding on a limited range of desert plants and reproducing only when environmental conditions are sufficiently favourable. The gregarious phase is characterised by highly cohesive behaviour, long distance migratory aptitude, polyphagy, synchronous and accelerated maturation and communal egglaying, making the desert locust such a devastating pest. Pheromones are implicated in the perpetuation of these gregarious characteristics. The aggregation pheromone system of gregarious S. gregaria is derived from the insects themselves and their faeces. This has been demonstrated in GC-EAD, GC-MS and behavioural assays. Behavioural assays show that two distinct sets of releaser pheromones modulate the aggregation behaviour of the desert locust: a juvenile aggregation pheromone specific to the nymphs and an adult aggregation pheromone specific to the adult stages. There was no sexual differentiation in the production or response to nymphal pheromone system. Young adults (immature adults) of either sex did not produce a stimulus with significant activity. Of the older adults (sexually mature adults), only the males produced the aggregation stimulus to which both sexes were equally responsive. The nymphal pheromone system consists of three sets of compounds: aliphatic C6C8-C10 aldehydes and their corresponding acids produced by the insects themselves, augmented by faecal phenols and guaiacol. The adult aggregation pheromone system consists of phenylacetonitrile, guaiacol, phenol and benzaldehyde. Mature solitary desert locust adults were also responsive behaviourally and electrophysiologically to the components of the adult pheromone system. Faecal volatiles are part of the aggregation pheromone complex. The nymphs aggregate in response to their faecal volatiles and to those of young adults but were indifferent to volatiles emitted from adult faeces. The young and older adults even showed less specificity; they were cross-responsive to each other's faecal volatiles and to those of the nymphs. Phenol and guaiacol were the most predominant electrophysiologically active compounds of nymphal and young adult faeces. Faecal volatiles of older adults contained phenylacetonitrile in addition to phenol and guaiacol. A better understanding of the gregarisation-solitarisation process could open up ways of interfering with the gregarisation behaviour of the desert locust and possibly help develop new intervention strategies for biorational management of the insect.



JOURNAL OF THE GHANA SCIENCE ASSOCIATION Volume 1 Number 1, July (1998) pp. 69-83