Comparison of the annual rates of increase of locusts in relation to the incidence of plagues.
Variations in the incidence of plagues of 4 locust species (Chortoicetes terminifera, Locusta migratoria migratorioides, Nomadacris septemfasciata [Patanga septemfasciata] and Schistocerca gregaria) were found to be independent of any differences in the intrinsic rate of natural increase, r, for each species, estimated from a matrix model, based on data from field populations in Australia and Africa. Behavioural changes in locusts, as density increases, prevent populations exceeding carrying capacity of the habitat, and it is unlikely therefore, that intrinsic population fluctuations, predicted by the logistic model, will contribute to the periodicity of plagues. Small variations in immature survival, rather than natality, were thought to account for most of the differences in the growth rates of field populations and are largely controlled by the direct and indirect effects of rainfall. In 3 of the 4 species, high rates of population growth occur under favourable environmental conditions and are capable of initiating outbreaks in the course of only 2 or 3 generations. Outbreaks are also influenced by the increases in population density associated with concentration. It is concluded that stochastic interactions between meteorological factors and locust populations in the recession and invasion areas control the plague dynamics of the species studied.