Update of the er-ante impact assessment of biological control of Plutella xylostella in Kenya

摘要

An ex-ante impact assessment of the biological control of the diamondback moth(DBM)in Kenya predicted a benefit cost ratio of 24：1. This result wa$based on yield losses of 31％measured in farmers' fields due to DBM,an assumed 30％abatement of yield loss and an assumed reduction of pest control costs of 50％. The aim of this paper is to validate the assumptions with new survey data and update the findings from the ex-ante impact assessment. A farmer group meeting waLs held in Wundanyi Division,Coast Province of Kenya,where the first release was made in July 2002. The participants filled in self-administered questionnaires assessing the DBM problem before and after release of the parasitoid.Before release,every farmer considered DBM as the most economically damaging pest.Now,aphids are regarded as the most serious pest.Significant reductions (66％)were achieved in the farmers'pest control costs and number of sprays per crop(73％).Average increases of gross margias were at 81％.Area under cabbage increased significantly from 0.12 ha to 0.16 ha with yields increasing from 28.4 to 33.1 t/ha.Crediting the reduction of pest control costs to the biological control,still assuming 30％yield loss abatements and extrapolating for the country resulted in all economic surplus of US ＄32.1 million for 25 years with a benefit-cost ratio of 28：1. The ratio includes direct costs and benefits only. Indirect benefits,like farmer health and the benefits to other countries where the biological control has been successfully introduced,are not taken into consideration,making the benefit ratio a rather conservative estimate.