Notes on the Bionomics of Cotton Stainers (Dysdercus) in Nigeria

Abstract

(1) There are two forms of D. superstitiosus, F., in Nigeria, which are designated banded and spotted forms in the present paper. The banded form appears on cotton earlier in the season than the spotted, and it remains on the crop longer. Migration to cotton is gradual in the case of the banded form, while a sudden influx of migrants of the spotted form commences in October. Many spotted adults migrate to Bombax and Sterculia between January and March, whereas the banded form remains on the cotton until it is uprooted at the end of March. The spotted form is far more numerous than the banded on maize and guinea corn.(2) There is considerable variation in the colouration of adults of the spotted form; at Ibadan about 99 per cent. of the migrants to cotton are yellow or orange in colour, while the adults of the filial generations produced on cotton are almost always red in colour (a few individuals are deep orange-red). These colour variations are thought to be of seasonal origin.(3) D. superstitiosus is the most prevalent species on cotton at Ibadan, Ilorin, Oyo, Zaria and Kano; melanoderes is of equal or greater importance in the forest zone at Ilugun and occasionally is numerous at Ibadan; fasciatus is of importance in savannah country at Oyo; haemorrhoidalis is rare on cotton. No evidence has been obtained that any one of these species is a varietal form of another species. Both forms of superstitiosus are widely distributed in Nigeria.(4) Migration to cotton is induced by the flowering of that crop. The maximum weekly migration has occurred between the weeks ending 15th October and 7th November in five localities in the last three years; viz., at the time of, or soon after, the maximum flowering of the cotton crop. There are indications that there is a tendency for stainers to migrate in a southerly direction, probably as a result of the desiccating harmattan wind from the Sahara.(5) The principal alternative food-plants of stainers in the Ibadan district are: Bombax sp., Sterculia sp., Hibiscus esculentus, H. sabdariffa, Urena lobata, and Abutilon zanzibaricum. At Ilorin, silk-cotton trees (Bombax), H. esculentus, H. lunariifolius and U. lobata are the most important alternative food-plants.(6) A rhythm exists in the weekly proportions of the sexes during the cotton season; males predominate among the early migrants to cotton and among the first individuals to mature of each generation. This rhythm appears to be the result of shorter period of post-embryonic development in the case of males than in that of females derived from the same batch of ova.