A brief survey of the effects of biotic factors on populations of the desert locust.
The following is based largely on the author's summary of this review of information on natural mortality in Schistocerca gregaria (Forsk.). Little quantitative information is available on natural mortality in thís locust, and it all relates to eastern África. -studies on the. subject are made difficult by the extreme mobility of the locust, its wide range of habitáts and its changes of phase. The nine different types of natural enemies known to affect it are listed. Losses in the egg and the post-embryonic Mayes are discussed separately, and the techniques adopted for their investigation are noted. Egg mortality in solitary and gregarious locusts must be investigated by different methods. Entomophagous insects. particularly siomorhina límala (F.), Systoechus spp. and Trox procerus Har. [cf. RAE A 48 78; 52 18, 63] are important causes of egg mortality of phase gregaria but do little damage lo phase solitaria. The post-embryonici stages are eaten by birds, which can be a major factor in the extinction of a small population. Other predators are too little known for an. assessment of their role. Insect parasites, especially Blaesoxipha spp., may play an important part in controlling small non-gregarious populations. Large gregarious populations are less affected, though Symmictus costatus Lw. [cf. 47 398; 52 183, 481] can parasitíse a large proportion. Mortality due to inability lo moult and to cannibalism is widespread and possíbly universal. Heavy motrtality of first- and second-instar nymphs, which is possíbly a mechanism for the elemination of unsuitable genotypes, is also discussed. The view that weather is the factor ultimately controlling Acridid populations [53 113] is criticised because biotic factors may be more important in tropical áreas, where insects are able to breed continuously, than was believed. An attempt is made to estimate the range of multiplication rates encounte-red. Rates of over 11 for gregaria or 16.5 for solitaria appear unlíkely. It is shown that, if thís is so, quite modérate rates of parasitism and predation are capable of preventing increases in locust populations.