COMMON NAMESBean fly (English); Boontjiemaaier; Boontjievlieg (Afrikaans). No local names are recorded.PEST STATUSUntil more is known about the damage, bean fly must be considered a minor pest on cowpea (Vigna unguiculata) in northern Namibia. Bean fly is known to attack all beans and many other legumes. In Namibia it has especially been noticed on Phaseolus-beans, cowpea and lablab beans (Lablab niger).The common occurrence of bean fly on cowpea in north central Namibia was overlooked until 1998 when the extension pest team found symptoms in the mid-season. It was then found on nearly all cowpea plants in the fields and it is believed to be widespread and common in all four regions. The bean fly is a very small inconspicuous fly. Damage symptoms are the most reliable method to detect bean fly attacks: Bean flies cause a characteristic swelling of the stem at ground level where the maggots (fly larvae) burrow into the stem. The maggots pupate at the base of the plant and as the stem grows it often cracks open. Pupae can be found in the cracks and on the outside of the stem. Leaf symptoms are more difficult to spot in field crops. One or two leaves on the plant turn yellow while the other leaves remain green. The petiole often shows dark streaks where the maggots have moved through and damaged tissue. Damage on Phaseolus-bean is severe and often results in complete crop loss. The young plants wilt and die without setting pods. Attacks were also recorded on cowpea and lablab bean. Leaf symptoms are not easy to see, but the swelling of stems at ground level is always visible. Both of these crops continue production, however, and the plants are seldom killed. DESCRIPTIONThe tiny white eggs are laid individually in small holes in the leaf surface. The egg hatches on the leaf and the small white maggot bores down through the stem and feeds just above ground level. Here the stem will swell and crack. The small brown pupae can sometimes be found in the stem.The adult bean fly is 2 mm long and black. It is difficult to see, but in still weather one may see the small black flies move around and lay eggs on the cowpea leaves. CONTROLFor a rain-fed crop such as cowpea there is little to do apart from keeping the field free from legumes debris. For irrigated legumes, one can try and change the planting season. Little is known about the bean fly during the various seasons, but it has been noted that some plantings of host legumes get through without any attack at all.At present, chemical control is not recommended on communal farms. Spraying of the few scattered or intercropped cowpea plants will be difficult, and it is not yet certain that the damage from bean fly justifies chemical control.
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