Responses of common bean (Phaseolus Vulgaris L.) cultivars to beanflies (Diptera: Agromyzidae

The population patterns and behaviour of the immature stages and the adults of two species of Ophiomyia, namely Ophiomyia spencerella Greathead and Ophiomyia phaseoli Tryon on seven selected cultivars of common beans (Phaseolus vulgaris L.) were studied under field conditions with a view to determining the resistance/susceptibility of the cultivars to infestation of bean fly. The method adopted for evaluating resistance/ susceptibility of the cultivars involved exposing growing bean plants to natural bean fly populations in the field and recording the number of leaf punctures, eggs, larvae, pupae/puparia, and percent infested plants, percent plant mortality and damage scores of representative sample for each cultivar. On the basis of levels of infestation and damage, Glp 1004 and Glp x-92 appeared to have some resistance while Glp 2, Glp 24, Glp 585, Glp x-1127(a) and Mexican 142 appeared to be susceptible. However, seasonal differences in the response of the cultivars to bean fly infestation tended to obscure this finding. The anomaly in the results was attributed to differences in the size of the initial population of bean fly in the vicinity of the bean crop, being highest in the non cropping season of 1985 and lowest in long rains of 1985, In cases of high bean fly infestation all the bean cultivars, including those that had shown signs of resistance under moderate infestation, were severely damaged. Thus there were difficulties in having a uniform bean fly infestation of sufficient level to which the cultivars were subjected during the growing period. However, natural populations of bean fly on beans planted in a single planting in the late part of the cropping season or the beginning of non cropping season was sufficient to cause observable damage symptoms, which could be reliably used as parameters for resistance. Successive plantings of bean cultivars in the middle part of cropping season could also produce a bean fly population sufficiently high for screening purposes. Under moderate field infestation, determination of percent plant mortality due to 0. spencerella, stem damage scores due to 0. phaseoli and the number of pupae/puparia were more reliable indices of resistance/susceptibility than the number of bean fly leaf punctures, eggs or larvae as parameters for resistance measurement.