The Biology of the Sudan Bollworm, Diparopsis watersi (Roths.), in the Gash Delta, Sudan

Abstract

Diparopsis watersi (Roths.) is a serious pest of the cotton crop in the Gash Delta of the eastern Sudan. The climate and method of cotton cultivation are described briefly and a general account of the incidence of D. watersi during the season is given. Earlier work in the Sudan on this bollworm was mainly centred in the Northern Province and there is little information on the history of its occurrence in the Gash Delta. The aspects of its biology considered here include its egg-laying and larval habits, and duration of the pupal stage.The majority of eggs were laid on the younger and more accessible plant growth. The period of wandering in search of food after eclosion from the egg was of short duration with normal egg placement, the emerging larvae had little difficulty in finding suitable food, and mortality from failure to do so was low on healthy plants. Depth of pupation did not vary greatly with soil type; the majority of pupae were found within one inch of the soil surface, and none at a depth greater than three inches. Only when the soil was a heavy clay that had baked hard following the rains, did the emerging moths have difficulty in passing through it and ascending to the surface.The proportion of larvae forming diapause pupae increased amongst field-collected larvae as the season advanced, and in successive generations bred in an insectary from the non-diapause fractions of the preceding generations, but at no time was there a complete cessation in the production of non-diapause pupae and infestations persisted as long as there was actively growing cotton. This persistent activity in the Gash Delta is contrasted with the restriction of D. watersi to a six months' season in Nigeria. The emergence of moths from diapause pupae was not completed during the season following that of pupation, but extended for at least two further seasons; approximately 35 per cent, of the total diapause pupal population was still alive and present in the soil after the first season's emergence. In any one season the emergence was bimodal, the two peaks of emergence, which were of similar magnitude, occurring in September-October and mid-November to mid-January. When diapause pupae were kept in the laboratory and thus exposed to less extreme temperatures than in the field, the emergence was distinctly unimodal, with maximum emergence during September. The bimodal emergence observed in the field is considered to be a result of external environmental factors that inhibit pupal development at certain times of year, and emergence data from diapause pupae exposed to different climatic conditions suggest that high soil temperatures are such a factor. The pattern of moth emergence from diapause pupae in the Gash Delta is compared with that found in Nigeria and Nyasaland.Natural mortality of D. watersi is discussed, and certain larval parasites, notably an un-named species of Apanteles of the ultor Reinh. group, and Bracon brevicornis Wesm., are considered to be of importance.The study of the biology of D. watersi has emphasised the difficulties in controlling this bollworm, and stressed the importance of attempting to destroy it in the diapause state, preferably during the “ dead ” season. It is considered that mechanical cultivation, of the soil may provide a means to that end.