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Bean fly is a significant pest of common bean in semi-arid areas of East Africa. Apart from inadequate moisture in the dry land, bean fly simultaneously contributes negatively thereby adversely affecting bean productivity. The objectives of this study were to (1) identify sources of resistance to bean fly available in landraces, (2) confirm stability of host plant resistance in drought stress and (3) determine the effect of drought stress and seasonal variation on common bean genotypes in relation to bean fly attack for adaptability to the semi-arid areas of East Africa. Sixty four genotypes including landraces, bean fly resistant lines and local checks were evaluated for seed yield, 100-seed weight, days to maturity, plant mortality and pupae in stem in an alpha lattice design with two replications. This was under drought stressed (DS) and non-stressed (NS) environments and two treatments (insecticide sprayed and natural infestation) for three cropping seasons between 2008 and 2009. Genotypes differed in their reaction to natural bean fly attack under drought stressed (DS) and non-stressed (NS) environments over different cropping seasons. However, the effect of bean fly appeared to vary between the long rains (LR) and short rains (SR). It was observed that an increase in number of pupae per stem resulted in a higher plant mortality. The range of seed yield was from 345 to 1704 kg ha(-1) under natural infestation and from 591 to 2659 kg ha-1 under insecticide protection. Seed yield loss ranged from 3 to 69%. The resistance of most of the bean fly resistant lines seemed to break down in presence of DS owing to their dismal performance. Screening of genetic resources in common bean to breed for host plant resistance to bean fly offers high potential of success if researchers take full advantage of the diversity available within the landraces. (C) 2010 Elsevier B.V. All rights reserved.