Seasonal changes in distribution of the African armyworm,Spodoptera exempta(Wlk.) (Lep., Noctuidae), with special reference to eastern Africa

Abstract

Emigration of moths of the NoctuidSpodoptera exempta(Wlk.) from their developmental sites occurred within a few hours of emergence, and captive moths have lived in semi-outdoor conditions for several weeks. Flight has been recorded at all times of night in the Kenya highlands, and has shown a very marked association, seasonally, nightly and even hourly, with rain; it has been observed at temperatures down to 13°C, and inferred at temperatures several degrees lower, so that over the greater part of eastern Africa flight activity is very unlikely to be restricted by low temperatures.The breeding season ofS. exemptadiffers markedly and consistently in different parts of eastern Africa; outbreaks of larvae recorded over the past forty years in Kenya, Tanzania and Uganda have been confined to November-July (very largely to December-May), alternating with those recorded over the past twenty years in Ethiopia, which have been confined to March-December (very largely April to August). The progressive establishment of a network of fifty light-traps in these countries since late 1962 has repeatedly recorded the arrival of invading moths before the first infestations of larvae, and has also provided evidence of a strikingly complete seasonal evacuation of much of the region; all traps in Kenya and most of those in Tanzania have revealed a period of several months in every year without a single moth ofS. exempta.Seasonal successions of moth catches and outbreaks of larvae have provided repeated evidence of a progressive northward movement of populations ofS. exempta, developing in March in Tanzania and Kenya, and extending across Ethiopia, in some years as far as northern Eritrea by June. The extent of these inferred displacements is consistent with the evidence so far available on behaviour and other factors affecting range of flight, which has been examined in as much detail as the limited data permit.Evidence of a return migration southwards to Kenya and Tanzania is relatively scanty. There are some indications of a north to south sequence of moth catches and outbreaks; the heaviest infestations in Kenya and Tanzania during the four-year period of detailed study were preceded by the largest catches so far recorded at the relevant time of year in Ethiopia; and in several years the first moths arriving in Kenya and Tanzania, in November-December, have appeared with the onset of the north-east monsoon, representing the southward movement of the intertropical convergence zone. Conversely, appearances of moths towards the northern limits of their inferred seasonal migration, in Ethiopia and around the Gulf of Aden in June, have shown an analogous association with the establishment of the south-west monsoon,i.e., with the northward movement of the intertropical convergence zone. Brief reference is made to evidence of seasonal migration in other parts of Africa; possibly in some years the eastern African populations may be supplemented by migrants from the south.Two recorded moth ‘ swarms ’ have been associated with weather systems characterised by particularly vigorous wind-convergence. It is suggested that wind-convergence may have contributed to the density of the ‘ swarms ’ and of the subsequent infestations of larvae.The overall effect of the seasonal changes in distribution which have been demonstrated is to bring the population into areas experiencing seasonal rains. Such a pattern of seasonal migration is likely to be of particular survival value for grass-eating insects, and such concentration of the insects as may be attributable to the associated wind-convergence may be significant in relation to the status of such insects as pests of cereal crops.Records in the literature of long-range geographical displacements of some other Lepidoptera have been found to be associated in a similar manner with analogous systems of wind and weather in other parts of the world.