Investigations on the Cotton Bollworm, Heliothis armigera, Hübn. (obsoleta, Fabr.)

Abstract

Egg Parasites.—In the Eastern Transvaal, Swaziland and Natal the eggs of Heliothis armigera are parasitized by Trichogramma luteum, Gir., and Telenomus (Phanurus) ullyetti, Nixon. No other egg parasites have been encountered.Representative samples of eggs were collected weekly or twice-weekly over a period of seven years and over wide areas from the various cultivated crops and colonies of wild food-plants which exemplify the annual chain of breeding sites of H. armigera. The collected eggs were incubated to determine the “active” fraction derived after withdrawal of the infertile portion and of eggs exposed for less than 24 hours to parasite action. Observed parasitism was computed against the “active” eggs and is regarded as an index of the parasitism in the field. Running indices of this nature are given graphically to indicate the annual incidence of egg parasitism in percentages, which may be viewed in relation to the incidence of bollworm as evinced by oviposition data in the figures given. Sustained high levels of parasitism are associated mainly with the more abundant egg populations on winter-irrigated vegetable crops grown between July and November.It has been found that Telenomus occurs mainly in the period August to December, and Trichogramma in the period December to March–April. The high parasitism of the winter egg populations is due, therefore, almost solely to Telenomus.Evidence is submitted that the levels of parasitism in certain crops are commonly much higher than in others. Explanations are suggested by consideration of sites of egg deposition, accessibility of eggs, plant habit, vegetative and flowering, and, particularly, the known oviposition-flowering relationship of Heliothis moths. The latter has an important bearing on the persistence of host material and governs opportunity for the intervention of parasite progeny in increasing numbers.Records on the status of egg parasitism have been taken in conjunction with investigations of the extent of larval mortality associated with the various types of crop. The rôle of egg parasites as agents reducing the issue of moths from winter and spring breeding areas is discussed briefly. Comparisons are given to show that the influence wielded is inappreciable even in some cases where parasitism is particularly high.