The Effect of Climate and Weather on the Numbers of the Red Locust, Nomadacris septemfasciata (Serv.), in the Rukwa Valley Outbreak Area

Abstract

An examination of the records from 1930 onwards shows that a very high level of Lake Rukwa is correlated with small numbers of the red locust, Nomadacris septemfasciata (Serv.), in the Rukwa Valley outbreak area in Tanganyika. When the lake level is not very high, the size of the adult locust population (taken as that present in the middle of the dry season) is correlated negatively with the total rainfall of the last but one wet season (broadly, November–May), positively with the size of the preceding parental population (taken as that present at the time of oviposition) and possibly also with the rainfall of the preceding October–December, but not significantly with the date of the drop in temperature that is thought to be associated with the start of ovarian development.Three tentative explanations of the correlation between adult population and rainfall are offered. First, if the water-table is high because of heavy rain in the previous season, the early rains may make the soil unsuitable for successful egg-laying and incubation; second, after a season of heavy rainfall the grass cover may be particularly dense at the end of the following dry season and the consequent reduction of the oviposition sites may make breeding unsuccessful; third, parental mortality may be high during a dry season following heavy rains. These possibilities are being further investigated.It is suggested that by using a multiple regression equation incorporating the correlations that have been established, the size of the adult locust population can be forecast in time for the scale of hopper control measures to be appropriately modified and economies thereby achieved.

Analysis of the development of major plagues of the Australian plague locust Chortoicetes terminifera (Walker) using a simulation model

“…This problem has been overcome in other locust studies by the application of empirical models, in which locust population fluctuations and multiplication have been correlated with various factors, usually climatic ones. These range from simple regressions, such as that between multiplication of the red locust in its outbreak areas and rainfall in the previous wet season (Symmons 1959(Symmons , 1964, to elaborate models such as that developed to predict outbreaks of Oedaleus senegalensis (Krauss) (Launois 1978). The model developed for the analysis of the major plagues of C termintfera (Wright 1983) is, however, the first to be based on the developmental biology ofa locust and to use weather data alone to estimate the likely sequence of locust breeding; it is therefore probably best termed a simulation model (Fig.…”

The Population Dynamics of Grasshoppers and Locusts

“…Dissections showed that few embryos developed past anatrepsis. Catastrophic death-rates from desiccation such as these are, however, probably rare in the Rukwa (Symmons, 1962). I n temperate Acrididae the absorption of water is complicated by the occurrence of a diapause in egg development which ensures that active stages are not present when conditions are unfavourable.…”

“…In other species inhabiting flood plains, the capacity of the site may be reduced periodically by flooding, which makes oviposition sites no longer available, e.g. Nomadacris septemfasciata (Gunn, 1956 ;Symmons, 1959), Locusta migratoria manilensis (Tsao, 1950), Locusta migratoria migratorioides (Remaudiirre, 1954; Besides altering the over-all capacity of an area, changes in the relative extent of the components of the mosaic may lead to crowding of one or other of the stages in the life cycle. Thus drought will cause hoppers to concentrate on the remaining patches of green vegetation and in locusts this may lead to the appearance of the gregarious phase and swarm formation (cf.…”

“…It has an adult diapause during the dry part of the year and breeding is confined to the rainy season, so that lack of moisture rarely limits multiplication. Symmons (1959) has studied the effects of weather on numbers of this species in the outbreak areas around Lake Rukwa in Tanganyika. He showed that small numbers of the locust between 1930 and 1957, were correlated with a very high level of the lake.…”

The Gregarization of Locusts

“…Locusta migratoria migratorioides has its main outbreak area in Mali and, during the season when the main flood plains are inundated, small groups migrate to those parts of the surrounding semi-desert that have well-distributed rainfall during that season, and this migration ensures that more or less continuous breeding can take place (Davey, 1956). Outbreaks of Nomadacris may follow a lessening of the rainfall around Lake Rukwa (Symmons, 1959) which causes the flood plains to start to dry out, thus exposing suitable laying grounds.…”