The role of parasitoids in regulation of beanfly ophiomyia spp. Complex, at Kakamega, Kenya.

Abstract

Investigations were carried out on the phenology of beanflies (Ophiomyia spp.) and their parasi toids under different cultural practices (cropping system and different fertilizer levels) ln the field in Kakamega, Western Kenya. Three species of beanflies Ophiomyia spencerella (Greathead) , Ophiomyia phaseoli (Tryon) and Ophiomyia centrosemantis (de Meij) were recorded. Of these, O. spencerella was the dominant species constituting over 94% of the beanfly complex in Kakamega while O. phaseoli and O. centrosemantis constituted 4% and 2% respectively. Beanflies appeared in the field 1-2 weeks after crop emergence and their population rose si qn i f i cant.Ly to a peak 3-4 weeks after crop emergence only to stabilize thereafter. This trend was observed on the April, June and September 1996 crops. Three species of parasitoids emerged from samples of the beanfly pupae collected from the field. These comprised of a braconid Opius phaseoli which emerged from Ophiomyia phaseoli and O. spencerella; a cynipid Eucoilidea sp. and a Pteromalid Mesopolobus sp which emerged from O. spencerella. Of these, Opius phaseoli was the most dominant species parasitizing Ophiomyia phaseoli, while Eucoilidea sp. was the most dominant species parasitizing o. spencerella. Mesopolobus sp. appeared in very small numbers. The percentage parasitism of the beanfly increased progressively to a peak during the 7-8 week after emergence suggesting a lack of synchrony in the phenology of these parasitoids with that of their beanfly host. Apparently, the parasite populations build up too late well after the beans had been attacked suggesting that parasites alone cannot keep the pest population under check. Intercropping and fertilizer application appeared not to have an effect on the abundance of beanflies and parasitoids and may not therefore be useful control options against the stem maggot as previously thought.