INFESTATION OF PHASEOLUS VULGARIS (L) BY THE BEANFLY O P HIOMYIA SPP. (DIPTERA: A G R O M Y Z I D A E ) AND ITS MANAGEMENT BY CULTURAL PRACTICES

ABSTRACT

Cultural practices as management strategy for beanfly

control were examined over four cropping seasons in 1991 and 1992

under f a r m e r’s developed field conditions at Oyugis, in Homa Bay

District of Western Kenya. In many parts of East and Central

Africa, the beanfly is a major constraint to the production of

the bean crop (Phaseolus vulgari s ), Its incidence causes yield

losses averaging 47-87%, Control methods used against the pest

are mostly insecticides based. Cultural control as a pest

management strategy is a less considered area of research which

needs to be studied, since it is the first line of defence

against pest populations and results in little or no added cost ^

For this reason studies on five cultural practices (soil

fertility, intercropping, weeding regimes, plant spacing and

planting time) on beanfly infestation were undertaken as possible

control methods,

Increase in nitrogen levels increased beanfly infestation by

12-66%. Phosphorus served as catalyst for nitrogen assimilation.

The fertilized plants were more succulent, tender and had more

nutrients and therefore offered better conditions for beanfly

penetration into bean stems, fecundity and development. However,

the infested plants in fertilized soils were able to compensate

for the damage caused to them and grew quickly to pass the

critical stages. Thus the beanfly infestation had little effect

on grain yield. The effect of beanfly infestation on yield when

no nitrogen and phosphorus were applied, was a 48% reduction in

yield, Therefore, the use of nitrogen and phosphorus fertilizers

reduced the effect of beanfly damage and increased grain yield.

Intercropping increased beanfly infestation compared to pure

stands of beans. The microc 1imatic conditions (light intensity,

temperature and relative humidity) created by intercropping of

beans with maize increased beanfly infestation compared to that

in the bean monocrop.

Weeding regimes had no effect on beanfly infestation,

However weed-free, and weeding three weeks after plant emergence,

provided better grain yield,

Plant spacing studies showed an increase of beanfly

infestation in wider spacings. The optimal plant density of

222,222/ha (30x15 cm) seemed to give the least beanfly

infestation and a high grain yield.

Early planting with the first rains reduced beanfly

infestation and gave better grain yield.

Two species of beanfly, Ophi omyi a phaseoli Tryon and 0,

spencerel1 a Greathead, were identified in Oyugis, where 0.

spencerel1 a was more predominant,

Parasitoids regulated beanfly population in nature. Six

species of parasitoids were identified, with Opi us phaseol i being

the dominant species. They were host density-dependent and

reduced beanfly population by 14-24%,

i v

The experimental results indicate that the use of nitrogen

and phosphorus fertilizers, monocropping system, optimal plant

density of 222,222 plants/ha and early planting time were good

cultural practices that can form a part of an integrated pest

management (IPM) strategy to reduce beanfly infestation with

consequent increase in bean production.