Studies of factors affecting the formation of hopper bands of the red locust (Nomadacrís septemfasciata) in an outbreak area.
Abstract
The following is virtually the author's summary. Studies were carried out in 1958-62 on some of the factors that affect the formation of bands of hoppers of Nomadacrís septemfasciata (Serv.) in the North Rukwa plain of the Rukwa Rift Valley outbreak area in Tanganyika [cf. RAE A 58 872]. The densities of egg-pods and hatching hoppers were generally small in the years of the study, but concentrations occurred in areas of grassland that had been burnt during the previous dry season. They were larger in lawns of short Cynodon forming a mosaic with tall Echinochloa than in other habitats. Hoppers mostly moved downwind, and from short to tall grass areas, during the first two instars. They concentrated along borders between the taller and shorter grasses, where frequent meetings between hoppers were possible, and bands appeared to form within such zones. Hoppers were more aggregated in the mosaic than in extensive uniform Cynodon lawns, while homogeneous, tall stands of Echinochloa and Cyperus were intermediate in this respect. The first bands were formed in a different instar in each of the years 1959-62. The rate of aggregation may be affected by the extent to which the parent locusts were gregarious, the type of grassland burnt and occupied for oviposition, the influence of weather on the incidence and area of oviposition, and the rate of grass growth. Size of population and breeding success did not appear to be important during 1959-62, but might be so if they were much greater or smaller.