Bean Stem Maggots of Ethiopia: Their Species Composition, Geographical Distribution, Importance and Population Dynamics

Abstract

Results of ecological studies carried out in Ethiopia between 1990 and 1993 showed that three species of bean stem maggots (BSM): Ophiomyia phaseoli (Tryon), O. spencerella (Greathead) and O. centrosematis de Meijere occur on haricot bean (Phaseolus vulgaris) in various parts of Ethiopia. Ophiomyia phaseoli and O. spencerella were the most widely distributed and abundant of the three species; O. centrosematis occurred rarely and represented less than 2 percent of the total BSM population in most instances. BSM intensity and the resulting seedling mortality were more severe at altitudes ranging between approximately 1700-1900 m above sea level; most of these areas lie in the vicinities of Areka, Wolaita Sodo, Awassa and Arsi- Negele in southern and south-central Ethiopia. BSM did not appear to be important at altitudes below 1600 m and above 2000 m. Incidence o f BSM species was found to be influenced by one or a combination of environmental factors and cultural practices including altitude, sowing date, and
growth stage and type of the host plant. Ophiomyia phaseoli and O. centrosematis were more prevalent at'altitudes below 1800 m and warmer climatic conditions whereas O. spencerella was dominant at higher altitudes and cooler, wetter environments. Ophiomyia phaseoli was more abundant in early sown bean whereas O. spencerella became more common later in the season; within the same sowing date, proportions of O. centrosematis and O. spencerella declined and those of O. phaseoli increased with the progress in the growth stage of the host plant. The incidence of O. phaseoli was more frequent on thin-stemmed, narrow-leaf, smallseeded, pea bean types whereas O. spencerella and O. centrosematis showed preference for more succulent, broad-leaf, large-seeded, navy bean types.