Varietal differences in the susceptibility of cowpea to larvae of the seed moth, Cydia ptychora (Meyrick) (Lepidoptera: Tortricidae)

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Abstract
Glasshouse and field screening showed that several varieties of cowpea (Vigna unguiculata) in the germplasm collection at the International Institute of Tropical Agriculture at Ibadan, Nigeria, had promising levels of resistance to damage by larvae of Cydia ptychora (Meyr.). There was no preference between varieties for oviposition in the glasshouse, but varietal differences in damage to pods and seeds occurred due to differences in the pod-boring and seed-feeding success of larvae. Large numbers of seeds per pod in some varieties diluted the effects of a given larval density and resulted in low percentages of damaged seed. A measure of relative susceptibility was devised that overcame the problem of chance escape from attack by certain pods. Resistance in glasshouse screening was not necessarily related to resistance in the field, where plant growth habits and the ecological requirements of the adult moths are more fully expressed and play an important role in determining the outcome of the plant-insect interaction. The most resistant variety in the glasshouse, Vita-3, was least resistant in the field, partly because it was partially photosensitive and produced few flowers and pods in the early growing season in Ibadan. Two varieties, TVu 946 and TVu 2994, may have escaped severe attack by maturing quickly. The former is small-leaved, non-vigorous, carries its pods on long peduncles well above the foliage and consequently constitutes an unsuitable habitat for daytime sheltering by adults. It was the least damaged variety in the field trial. The latter was most attacked by other pod-feeding pests, and the consequent reduction in seed weight and quality appeared to enhance the level of damage caused by C. ptychora.