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M. sojae overwhelmingly prefers soya bean. There are no clear external symptoms of infestation except for some minute oviposition/feeding punctures at the base of the leaf lamina. When the stem is cut open, feeding tunnels containing larvae and pupae are visible. In slightly older plants, two separate tunnels are often found. The one in the lower half is older and has developed a dark brown colour. It originates in the stem roughly at the junction of the unifoliate leaves, and extends downwards up to the soil surface, indicating that the infestation occurred earlier, from the eggs laid in the unifoliate leaf. The second tunnel starts just under the top of the plant and extends downwards up to the first tunnel. Presuming that the plant at the unifoliate leaf escaped infestation, this tunnel can extend up to the soil surface. This feeding results from the later infestation of trifoliate leaves. If the plant is damaged very early, at times the later infesting larvae do not have enough pith tissue to feed on. Under such circumstances, the larva moves upwards hollowing out the shoot which may lead to withering of the top.
In most legumes, the seasonality of occurrence of M. sojae is similar to a more predominant agromyzid, Ophiomyia phaseoli, which infests host plants at the same stage or even earlier. In most cases, the control measures adopted for combating O. phaseoli also control M. sojae. Since M. sojae prefers soya bean to other host plants, the control measures described here are for the control of this pest on soya bean.
Since infestation by M. sojae at the seedling stage causes economic yield loss, it is essential to control this pest during the first 4-5 weeks after seed germination. It is important to know the season when the pest is serious to undertake appropriate prophylactic control measures. In general, the pest is more serious in the dry season than in the rainy wet season and precautionary control measures are more important in the dry than in the wet season. Since adult flies are small and remain hidden in the plant canopy, and since larval damage is internal, it is important to resort to control measures either at sowing (soil treatment) or immediately after germination (foliar spraying).
At present farmers use only chemical control to combat M. sojae. This is mainly because of the absence of other reliable control practices. Some research on soya bean is in progress to devise other control measures but advancement in Asia, where this pest is more serious, is very slow.
Chiang and Talekar (1980) found four wild Glycine soja accessions highly resistant to M. sojae. The resistant accessions are plants with very thin stems. Breeding resistance into cultivated soya bean failed because in order to obtain high yield, it was essential to have plants with thick and strong stems to support a large number of pods without lodging. Any increase in stem size beyond the slender stem of the resistant parent increased the susceptibility of the progeny. Despite 100% plant infestation, yield loss is barely 30% and this pest rarely kills plants. It is therefore possible to develop soya bean cultivars tolerant to this pest in a hybridization programme using G. soja. Some soya bean accessions indeed show tolerance - no yield loss despite heavy infestation - to M. sojae (AVRDC, 1979).
Although several species of parasites are present in areas such as Indonesia, India and Taiwan where M. sojae is endemic on soya bean, their level of parasitism rarely exceeds 50%. Most of this parasitism comes rather late in the season after the insect has infested the plant and caused significant damage. During the dry season in areas endemic to M. sojae the pest population is so high that enough insects escape parasitism and continue to cause serious crop damage. Most of the reported parasites are native and there has been no introduction of any natural enemy specifically to control M. sojae.
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:
M. sojae is a pest mainly of soya bean and to some extent mung bean and black gram. In soya bean, infestation occurs in the unifoliate or early trifoliate leaf stage. By this stage the seedlings are well established and the insect infestation rarely results in plant mortality. Yield loss varies from location to location, and according to the plant growth stage when infestation occurs. Yield reduction occurs only when the plant is damaged at the seedling stage. The later the damage, the lower is the yield loss. In Taiwan, yield loss among 163 soya bean varieties was 31% (AVRDC, 1981). In Shandong Province of China, there are reports of M. sojae causing plant mortality and yield loss in soya bean (Anonymous, 1978). In India, Bhattarcharjee (1980) studied the relationship between M. sojae infestation, plant height and yield loss in soya bean. According to his calculations, this insect has the potential to cause up to 80% yield loss. This pest probably causes significant yield loss in soya bean in Indonesia. However, in most cases, if the crop is not protected, Ophiomyia phaseoli causes severe damage before M. sojae infestation begins. Hence no independent information is available on the extent of plant damage or yield loss by M. sojae in that country.