Influence Of The Beanfly (diptera: Agromyzidae) On The Perfomance Of French Beans (phaseolus Vulgaris L.) and Some Aspects Of Its Chemical Control In Kenya

Abstract

Ophiomyia phaseoli Tryon and Ophiomyia spencerella Greathead were the two Beanfly species that infested Canadian Wonder plants at Kabete Kenya. The latter, was the most abundant species 'I throughout the experimental period. Beanfly females first attacked bean seedlings immediately after emergence and reinfest at ion and increased colonisation continued for the following two weeks. The attacked plants were recognised by punctures made in their leaves by ovipositors of the female Beanflies. Such plants when two to four weeks old, were characterised by lea! yellowing, co110used stem bases and wilting. Increases in the Beanfly infestation rates were estimated by the percentage o! infested plants per 9m2 plot per week, female Beanf1y punctures per plant per week and pest larvae plus pupae per plant per week. The highest infestation rate was recorded in the December sown crop where all the plants were invested within the first 7 days after emergence. Initial Beanfly population density and oviposition behaviour of their females were probably the two main factors that influenced the rates .• of infesting bean fields. Peak infestation intensities of bean fields were recorded during the third week of each crop stand. Infestation intensities also varied from one bean crop season to the next. The highest infestation intensities were recorded in the crops sown during the short rain (October to December, 1976) season. In the short rains and long rains (February to July, 1977) seasons, the early planted bean crops were less infested than the late planted ones. Infested plants had their potential leaf development reatarded and had overall stunted growth. The potential seed yield performance of infested plants were lowered. The threshold infestation levels of the beanfly were found to lie between 85 and 120 larvae per m of crop stand. The low seed yields were caused by few pods per plant and few seeds per pod from such plants. The lowest seed yields were recorded from infested plants of the December crop. The recorded low seed yields were mainly caused by small photosynthetic capacity and stunted growth infested plants. Multiple linnear regression analysis showed that, changes in the rainfall intensities considerably influenced the effects of the Beanfly infestation densities on the growth rate and seed yields of infested bean plants. Birlane 2G was the least effective at controlling the Bean flies when compared to aldrin 40%, Birlane, Miral and Furadan. None of the four chemical sUbstances tested completely protected bean fields against Beanfly invasion. But, Furadan was the most effective at controlling Beanfly infestations. All these chemical substances protected beanfields against the Beanflies for only two to three weeks.