The Outbreak Centres of Schistocerca gregaria, Forsk., on the Red Sea Coast of the Sudan

Abstract

1. An ecological description of the Red Sea coast of the Anglo-Egyptian Sudan is given, with special reference to the occurrence of the solitary phase of Schistocerca gregaria in its different parts and zones.2. The greatest numbers of solitary Desert locusts were found amongst raingrown dukhn (Pennisetum typhoideum, Rich.) cultivated in the Suaeda belt of the coast, near the sea.3. The factors responsible for the higher population of solitary locusts among the cultivation are probably the soil consisting of soft loose sand, and the more or less evergreen condition of the weeds on cultivated plots. It appears possible, however, that in some localities with naturally loose sandy soil all areas suitable for vegetation are cultivated, and in this case the cultivation in itself is not of primary importance.4. Incipient outbreaks were observed in the type of cultivated areas described above in 1933–34, and again in 1934–35 winter seasons. Hoppers of the phase congregans were found mostly on a weed, Heliotropium undulatum, which is preferred by young hoppers to other food-plants, and seemed to act as a concentrating factor for the hoppers. No hoppers of the ph. congregans were found on clean cultivation where this weed was absent.5. The chief factor determining the commencement of an outbreak seems to be heavy winter rains (over 240 mm. during the season), starting early (end of October) and continuing late (end of February), which allows for the development of two successive winter generations, as follows:—(a) Early maturation of old adults (November); (b) plentiful oviposition of old adults (November-December); (c) hoppers beginning to emerge (end of November); (d) hoppers beginning to become adult (early January); (e) majority of hoppers adult (end January); (f) these young adults remaining among dukhn in a fairly crowded condition (February); (g) young adults maturing (end February); (h) young adults ovipositing (March); (i) hoppers hatching (end of March-April); (j) young hoppers crowded on Heliotropium, assuming congregans coloration and later forming marching bands (April); (k) emigration of young flying swarms (May).6. The exact cause of the rapid maturation of young adults of the first winter generation has not been discovered, although a short period of very high humidity seems to have some effect. There is no doubt that late rains are necessary for most adults to mature and oviposit in the field.7. The Red Sea coast of the Sudan is distinguished from the interior of that country by its higher air humidity, by winter rains, and by the fact that the rainy season can extend for a longer time than the summer rainy period inland. The northern parts of the coast, north of the latitude 20°N., are less likely to produce outbreaks only because heavy and prolonged rains are unlikely to occur there.8. General conditions in Eritrea, south of the Sudanese border, are such that some outbreak areas should be expected to exist there.9. It appears probable that the Arabian coast of the Red Sea also contains some outbreak areas. As regards inland parts of Arabia, their climate, the distribution and the life-cycle of the solitary locusts there, are too little known to estimate the possibility of the existence of outbreak areas, until a special investigation is made.10. The results of the investigations on the Red Sea coast put under suspicion of harbouring outbreak areas of the Desert locust all those parts of the sea-coasts of Africa and Asia which may obtain in some years winter rainfall of not less than 240 mm. extending for a period sufficiently long to allow for the development of two successive winter generations of the locust.