Spatial and temporal changes in the distribution of eggs of Heliothis armiger (Hübner) (Lepidoptera: Noctuidae) on Cotton in the Sudan Gezira

Abstract

AbstractTo determine the extent to which the distribution of ovipositing Heliothis armiger (Hb.) might be ‘clumpy’ rather than random, the numbers of eggs on cotton throughout the 10 000 km2 Sudan Gezira were subjected to analyses of variance. It was found that at any time there was less variability within than between areas, highly significantly less for units of 600–700 km2. Within all areas, the numbers of eggs varied very significantly with time and many-fold increases in numbers of eggs occurred simultaneously over thousands of square kilometres. In every three-day period, there were two or more significantly different levels of infestation, each extending over areas ranging from a few hundred to several thousand square kilometres; the boundaries of such areas changed continuously, and no consistent differences or patterns were found. These findings suggest that redistribution of mobile populations of moths takes place over large distances. At all times there were areas with egg numbers below those considered economically serious; this has been utilised in new control strategy and tactics which have been effectively implemented experimentally. Larval spraying appeared also to reduce oviposition for at least three days and could in part account for some, but certainly not all, of the differences between areas.