The red bollworm attacking cotton in Nigeria has recently been shown to be Diparopsis watersi (Roths.), distinct from D. castanea Hmps., the southern African species previously assumed to be that occurring on cotton throughout Africa. Earlier work on cotton pests, including Diparopsis, in southern Nigeria (1664) is briefly reviewed, and the discovery and definition of the problems of cotton pests in northern Nigeria is outlined, and the climatic and agricultural background of the northern areas described.
The main object of the present work, centred at Samaru, was to determine; the annual cycle of activity of Diparopsis in northern Nigeria. The methods employed are described. The insect passes the dry season in diapause as a pupa in the soil. In late May or June, emergence from these pupae commences, but this is completely interrupted during July-September. In October, after the end of the rains, emergence recommences, and the main attack occurs on the crop during October and November. The proportion of pupae entering diapause rises rapidly from mid-October onwards, and activity ceases in early December.
When diapause pupae were incubated at controlled (86°F.) or indoor air (74–86°F.) temperatures, emergence was unimodal, extending from June to October, with the peak in July or August, respectively. It is concluded that the bimodal emergence in the field is caused by waterlogging, or some other factor restricting aeration of the soil during July-September, when the emergence from diapause pupae is interrupted
Trials of dates of sowing laid down at Samaru and Daudawa showed that early sowings, made in mid-June or early July, may suffer heavier bollworm attack than those made in mid- or late July. The earlier sowings nevertheless outyielded later ones. It is suggested that this results from their benefiting from a longer growing season, in which a greater production of fruiting points more than offsets greater losses from bollworms and other insects.
Elimination of ratoon and standover cotton would prevent breeding by the early emergence of moths from diapause pupae, and consequent red-bollworm attack on early sowings of cotton.
The possibility of producing a unimodal emergence from diapause pupae is worth further attention. This might be achieved by altering the soil conditions during July-September, and if successful would result in the main crop escaping almost entirely any attack by Diparopsis.
There appear to be no fundamental differences biologically between Diparopsis castanea and D. watersi, as illustrated by previous work reported from South Africa and Nyasaland, and the observations recorded here.