Seven selected cultivare of common beans Phaseolus vulgaris L. were evaluated for resistance at two sites under different weather and beanfly population pressures in the field. The indices of resistance were the number of leaf punctures, eggs, larvae and puparia and per cent dead plants. Beanfly species, Ophiomyia spencerella (Greathead) and Ophiomyia phaseoli (Tryon) infested the bean plants in the field. There were seasonal differences in the response of the cultivare to beanfly infestation attributed to differences in the level of attack of the bean crop, being highest in the non-cropping season of 1985 and lowest in the long rains of 1985. Despite the variation in results, Gip 1004 and Gip x-92 showed some resistance on the basis of their least plant mortalities and low larvae/puparia surviving on them. When the beanfly population was high enough to cause death of the bean plants, percentage plant mortality was correlated to the numbers of larvae and puparia of beanflies and it may be the best parameter for resistance.

The population patterns of eggs, larvae and puparia in the bean plants and leaf punctures made by adults were investigated during cropping (March–July and October–January) and noncropping (July–October) seasons at two sites in Kenya. The beans planted in noncropping seasons had more leaf punctures, eggs, larvae and puparia than beans planted in cropping seasons. Beans planted in noncropping season attracted unusually high population from surrounding weeds as well as previous crop which cause severe damage. Under field conditions Ophiomyia spencerella Greathead and Ophiomyia phaseoli Tryon were the species of bean flies infesting the bean plants in all seasons. Both O. spencerella and O. phaseoli normally oviposited in punctures on the leaves but O. spencerella also oviposited in the stems of bean seedlings.

The effect of planting dates on beanfly infestation was investigated in successively planted bean plants during cropping seasons. The population tended to build up in the course of the season following the initial invasion by egg-laying females. Ophiomyia spencerella (Greathead) and Ophiomyia phaseoli (Tryon) were the species of beanflies infesting the bean plants in all seasons. The number of beanfly leaf punctures, eggs, larvae and puparia were more in late planted than early planted bean crop. The bean crop planted late in the season was severely damaged due to the generally higher population levels which had been building up in the course of the season.