Plantwise Factsheets for Farmers
Cultural control of bean fly in cowpea and bean
Recognize the problem
larva of the “bean fly” is a small, less than ¼ cm, white maggot which is
known as a “bean stem maggot”. The adult is a small, 2mm long, black fly. It
is difficult to see but in still weather one may see the small black flies
moving around on the beans. The maggots move inside the stem of cowpeas and
beans down to ground level. This causes swellings and cracks of the stem and
then leaves may wilt and turn yellow. Yield is low.
Bean flies prefer tender leaves of beans and cowpeas for laying eggs.
The newly hatched larvae feed in the leaf blades before entering the midrib
and then the stem. Adults feed on three general food sources: water droplets on
the leaves, natural secretions of plants, and plant sap exuding from feeding
fly numbers are very low during the onset of the rain season, but increase
quickly in numbers with time. There are some cultural methods which work very
well and prevent the flies from laying eggs and make the environment
unsuitable for the survival of the bean fly.
Plant early. This means no
later than 3 days after the first rains in the season. At that time the fly
numbers are very low and not many eggs will be laid.
- Cover roots
with a layer of soil by ridging (building up) the soil around the plants at
2-3 weeks after seedling emergence. This helps the roots to grow quickly and
strong. Therefore, the effect of the maggots is reduced.
- Remove and
destroy crop residues and all plant parts with symptoms of damage. You may
give them to pigs.
- In addition, remove wild
hosts plants especially legumes, around the crop area.
- If practical, rotate
cowpeas and beans with crops the bean fly does not like, such as maize or
groundnut. This breaks the life cycle of the bean flies.
The recommendations in this factsheet are relevant to:
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