Use Of Neonicotinoid Insecticides And Varietal Resistance To Manage Bean Fly (ophiomyia Spp.) And Other Arthropod Pests Of Snap Bean (phaseolus Vulgaris L.)

Abstract

A study was undertaken to assess the efficacy of some neonicotinoid formulations used in snap bean production as seed dressings and soil drenches against snap bean pests and to evaluate some commercial varieties for resistance to the pests. The study was carried out in Mwea, Central Kenya where most export snap beans are grown by small scale farmers. The study consisted of two experiments carried out from December 2009. to January 2011. In the first experiment, four neonicotinoid seed dressing (Gaucho, Monceren, Cruiser, Apron Star) and two soil drench formulations (Actara and Confidor) were tested on Amy variety in a randomised complete block design replicated three times. Untreated plots were included as control. Seed dressing chemicals were applied on site just before planting whereas the soil drenching chemicals were applied twice on the 3rd and 13th day after emergence. In the second experiment, seven commercial snap bean varieties, Amy, Alexandra, Bravo, Serengeti, Paulista, Tana and Mara, were tested with and without a pesticide (Confidor) in a split plot design. Confidor pesticide formed the main plots and variety the sub-plots. Data was collected weekly on the number of plants per plot, bean fly maggots, whitefly nymphs, bean fly ovipuncture marks, number of thrips on leaves and flowers and finally on pod yield. The number of bean fly maggots was significantly (P<0.001) lower in plots treated with Confidor, Actara, Gaucho, Monceren and Cruiser than in Apron Star and non-treated control plots. Plots treated with Confidor and Actara had significantly (P<0.001) lower bean fly infestations and gave significantly (P<0.001) higher yields. After partial economic analysis, Cruiser, Apron Star and Monceren had the highest benefit-cost ratio of 1.82, 1.60 and 1.24 respectively while Actara and Gaucho had the lowest benefit-cost ratio of -0.11 and 0.24 respectively. Thus neonicotinoid treatments with Cruiser, Monceren or Confidor in snap bean production can effectively reduce bean fly infestation and increase yields, hence offering great benefits to snap bean growers. Therefore, growers and seed packers should be encouraged to seed dress with Monceren or Cruiser or drench with Confidor to protect the crop against bean fly as they are effective and give good financial returns. The snap bean varieties were significantly different (P=0.001) in the number of plants as well as in the marketable (P=0.001) and total pod yields (P=0.001). Alexandra and Serengeti varieties had higher plant stand and yields than the Amy variety, the commonly grown variety in Mwea. Treatment with Confidor further improved plant stand and increased pod yield by between 50 % and 490 %. Some degree of resistance was noticed in Alexandra and Serengeti varieties against bean fly which was enhanced by treatment with Confidor. Resistance to other pests was not apparent. Alexandra and Serengeti varieties should be recommended to snap bean growers in areas like Mwea which experiences high pest infestations. For effective protection against snap bean pests, use of safe insecticides such as Confidor should be integrated with resistant varieties in the production system