Social behaviour of immature adults of two species of locust, Schistocerca gregaria (Forskål) Locusta migratoria migratorioides (Reiche and Fairmaire)

Abstract

Experiments were carried out to determine if the social behaviour of immature Schistocerca gregaria adults, like that of hoppers, was affected by the conditions of population density under which they had been reared since'hatching. The conditions of rearing employed were the two extremes of visual and tactile isolation and heavy crowding. By testing the experimental locusts in a uniform environment arena, it was not possible to show any significant difference in the distribution of the isolated and crowded adults.

The isolated locusts were forming as many groups as were the crowded locusts.

The unexpectedly large amount of group formation by the isolated locusts, was considered to be a consequence of the conditions of the present Anti-Locust Research Centre laboratories, where a very high locust population has been reared in a. restricted space for several years. The most likely factor involved was thought to be an olfactory stimulus. To avoid the action of this possible contaminant, a culture of isolated locusts was reared' and bested in an otherwise locust-free laboratory at Edinburgh University Zoology Department. Some of these locusts were tested at 2nd instar and others when adult. They were found to be aggregating less than isolated 2nd instar and adult locusts reared and tested at A.L.R.C.. The most striking difference was found with 2nd instar hoppers. It is believed that some olfactory influence, a pheromone, is acting on these isolated locusts at A.L.R.C. in such a way that they exhibit aggregating behaviour without having had any previous social experience.

Experiments were carried out on Schistocerca gregaria and Locusta migratoria migratorioides to investigate the field report that there is a period of dispersal after fledging. The aggregation was measured of 5th instar hoppers, 5 day old adults, 10 day old adults and mature .adults. There was a drop in aggregation after fledging and again at maturation, but these fluctuations were not significant. This laboratory study indicates that the dispersal observed in the field is probably not inherent to locusts, it is more likely to be a result of climatic conditions.