Infestations of phesolous vulgaris by beanfly ophiomia spp.(Diptera;Acromizidae) and its management by cultural practices

Abstract

Cultural practices as management strategy for beanfly control were examined over four cropping seasons in 1991 and 1992 under farmers developed field conditions at Oyugis , in H o m a B a y District of W e s t e r n Kenya. In many parts of East and Central A fr ic a, the bean fly is major constraint to the production of the bean crop (P h a s e o l u s v u l g a r i s ) , Its i ncidence as it losses averaging 47-87%, Control methods used against the pest are mostly insecticides based. Cultural control as a pest management s trategy is a less considered d e r e d ar ea of research which needs to be studied since its the first line of defence , against pest p o p u l a t i o n s and results in little or no added cost ^ For this reason studies on five cultural practices (soil fertility, intercropping, weeding regimes, plant spacing and planting time) on beanfly infestation were undertaken as possible control methods, Incre ase in ni tr o g e n levels i n c r e a s e d be a n f l y i n f e s t a t i o n by 12-66%. Phosphorus served as catalyst for nitrogen assimilation. The fertilized plants were more succulent, tender and had more nutrients and therefore offered better conditions for beanfly penetration into bean stems, fecundity and development. However, the in fe st ed plants in f e r t i l i z e d so il s we r e able to c o m p e n s a t e for the damage caused to them and grew quickly to pass the critical stages. Thus the beanfly infestation had little effect on grain yield. The effect of beanfly infestation on yield when no n i t r o g e n and p h o s p h o r u s w e r e a p p l i e d , w a s a 4 8 % r e d u c t i o n in yield, Therefore, the use of nitrogen and phosphorus fertilizers reduced the effect of beanfly damage and increased grain yield. Intercropping increased beanfly infestation compared to pure s t a n d s of beans. Th e m i c r o climate condition (light density , temperature and relative humidity) created by intercropping of beans with maize increased beanfly infestation compared to that in the bean mono cr op. Weeding regimes had no effect on beanfly infestation, However weed-free, and weeding three weeks after plant emergence, provided better grain yield, Plant spacing studies showed an increase of beanfly infestation in wider spacings. The optimal plant density of 222,222/ha (30x15 cm) seemed to give the least beanfly i n f e s t a t i o n and a h ig high grain yield. Early planting with the first rains reduced beanfly infestation and gave better grain yield. Two species of b e an fly , Ophi omyi a phase o l i Try o n and 0, spencere l 1 a Greathead , were identified in Oyugis , where 0. s p e n c e r e l 1 a was more predominant , Parasitoids regulated bean fly population in nature. Six species of parasitoids were identified , with Opius phase being the dominant species. They were host density-dependent and reduced beanfly population by 14-24% The experimental results indicate that the use of nitrogen and phosphorus fertilizers, monocropping system, optimal plant density of 222,222 plants/ha and early planting time were good cultural practices that can form a part of an integrated pest management (IPM) strategy to reduce beanfly infestation with consequent increase in bean production.