Study of incidence and damage by bean fly and grain yield of common and climbing beans

Abstract

Bean fly is a major cause of low bean yields with losses of up to 100% recorded especially in dry seasons. Single control methods that have been used are ineffective and costly. Affordable integrated pest management strategies remain the best approach. Field studies were carried out in 2000/2001 at two sites, Njoioand Kitalc. The objectives were to investigate the effect oftime ofplanting and bean variety on bean fly population. damage and grain yield. At Njoro, three bean varieties (one climbing bean. Flora and 2 common beans. Red haricot and Roseeoco) were planted monthly" over I2 months in a randomised complete block design ( RCBD ) replicated six times. The plants were left to natural bean fly infestation in the field. Bean. fly population, species identification and damage were observed at 21 and 35 days after plant emergence. Results showed that Rosecoco had significantly higher (P<0.05) larvae and pupae than red haricot and flora across the l2 months. Up/riomyfu spencerella accounted for 92% of the total bean fly population. Bean mortality due to bean fly was highest between November 2000 and January 2001 when it wasrdry and lowest between February and May when there were heavy rains. Rosecoco suffered significantly higher (P<0.05) mortality than Flora and Red harieot. Ten bean varieties (five common bean varieties and five climbing bean types) were planted at onset of rains and two weeks later during the short and long rains (2000 and 2001) respectively at Kitale. Experimental design used was RCBD in a split plot arrangement, replicated three times with time of planting as main plot and varieties as subplots. Data collected included stand count at 14 DAE, larvae and pupae counts at 21 and 35 DAE. bean fly species in the two seasons, bean damage and grain yield. Results showed that incidence ofbcan fly were significantly higher in the short rains than in the long rains. In the long rains early planted crop had significantly lower bean fly incidence than late planted crop. ln general, the climbing beans had higher mean number of pods per plant, seeds per pod, weight of 100 grains and yields than the common beans. Significant interactions (P<0.05) between time of planting and variety influenced weight of 100 grains and seed number. (). .rpcm'e/'r'llu (Greathead) and 0 phaseoli (Tiyon) were identified but the former was more dominant in both early and late planted crop in both seasons. This study has shown that most of the climbing beans, if planted early. tolerated high bean fly pressure and gave high yields.