Bean Stem Maggot

Pests of Field Crops in Southern Africa

BEAN STEM MAGGOT

(Diptera: Agromyzidae)

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Text Box: Ophiomyia phaseoli, the bean stem maggot, is well-known in gardens where young bean plants are often destroyed following the formation of gall-like swellings in the stems at ground level. The adults are very small, shiny black flies (about 2 mm in length) with a pair of clear wings of wingspan 4-5 mm. Their larvae are whitish, torpedo-shaped maggots that reach little more than 2 mm. They develop into smooth, light brown pupae, which are found in the stem swellings. TOP

Text Box: Bean stem maggot attacks a variety of legumes, the main ones being species of Phaseolus, such as French beans, Lima beans and haricot beans, as well as soyabeans and occasionally cowpeas and dolichos beans. No native hosts have been found. TOP

Text Box: To the gardener, the first indication of the presence of bean stem maggots is often the yellowing and subsequent stunting or dying of young plants. Closer examination reveals swellings in the stems at ground level, and if these are opened, the pupae will be seen readily. The stems may also be hollowed out to some extent. In older plants that have attained a fair size before becoming infested, as is usually the case with soyabeans, the maggots fail to reach the base of the main stem, and become lodged in the petiole bases instead. The swellings and distortion can be seen in this position, and the result is wilting leaves and broken petioles. TOP

Text Box: This pest does not arrive at the plant through the soil, as is sometimes believed. The female fly lays her eggs in the leaf tissues, about 100 eggs being laid over a period of two weeks. The punctures she makes in the leaves are readily recognisable as small yellow spots, although some of these are made to obtain nourishment rather than to lay eggs. In warm weather the eggs hatch in two to four days, and the young maggots take a couple of days to mine through the leaf tissue to the petiole. They then tunnel towards the stem and gradually make their way down towards the stem base. Here they complete their development and pupate. Larval development occupies about ten days in the summer and the pupal stage takes a further nine to ten days, so that the whole life cycle takes about three weeks. As the temperatures drop towards the end of the season, the life cycle takes longer, and by March can be up to six weeks. Gardeners seem to report infestations in garden beans from November and May, with the peak period apparently being between January and March. Infestations have been recorded throughout the year however, a smaller peak existing in September/October. TOP

Text Box: Control of this pest is by necessity preventative. One seldom sees the first signs of the infestation (the yellow spotting on the leaves), although this could become part of a specific scouting schedule early on in leguminous crops. By the time stem swellings or wilted young plants are noticed, it is too late for effective insecticidal control. The organophosphate, diazinon, has been used effectively for many years, but must be applied as a foliar spray three or four times from a very early stage (about three days after bean emergence). Otherwise some success has been achieved in other countries by using some of the more modern systemic carbamate seed dressings, and this method of treatment, although expensive, will have the advantage that interference with parasitoids is kept to a minimum. Since the pest tends to build up during the season, earlier plantings may escape it, while repeated plantings (overlapping) will cause the population to build up to serious levels, and require repeated use of insecticides. From about March onwards, it appears that bean stem maggot comes increasingly under the control of a small Braconid wasp, Opius liogaster, although this is too late to save most crops. It may be a significant factor in the survival of the pest from season to season. TOP

Adult fly (2-3mm) lays eggs on leaves. Maggots form the swollen bean stem.