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The most serious damage by adults occurs when plants are at the unifoliate stage. The unifoliate leaves show a large number of feeding and oviposition punctures on the upper side with corresponding light yellow spots, especially on the basal portion of the leaf. The first and second trifoliate leaves show some egg holes, but leaves situated above this are practically undamaged. Larvae feeding soon after hatching produce numerous larval mines which are better seen on the underside of the leaves just under the epidermis, and appear as silvery, curved stripes; on the upper side of the leaf only a few tunnels are visible. Later, both egg holes and larval mines turn dark brown and are clearly visible. In cases of severe attack, infested leaves become blotchy and later hang down. These leaves may dry out and may even be shed. When mature plants become infested, insect damage is confined to the leaf petioles, which become swollen and at times the leaves may wilt.
The developing larvae in second and third instar mine downward into the cortex just underneath the epidermis. The third instar continues to feed downwards into the tap root and returns to pupate still inside the stem, close to the soil surface. The feeding tunnels are clearly visible on the stems (Talekar, 1990). If the O. phaseoli larvae population is high, larval feeding leads to destruction of the cortex tissue around the root-shoot junction. This initially leads to yellowing of the leaves, stunting of plant growth and even plant mortality. If the damage is less severe, the root-shoot junction area appears swollen. In some cases the host plant produces adventitious roots above this swollen area on the stem.
In Indonesia, where a biotype of O. phaseoli attacks soybeans soon after emergence, larval tunnels in cotyledons are clearly visible (Talekar, 1990). Later, damaged cotyledons turn yellow and are shed. In most cases the plant is killed within 10-15 days of emergence.
Adequate control of O. phaseoli in tropical to subtropical Asia is necessary to get satisfactory yields in most economically important legumes, such as common bean, soybean (especially in Indonesia), mungbean, cowpea and peas. This is especially true when the crop is grown in the dry season, as is the case with most field legumes such as soybean and mungbean which are traditionally planted after the wet-season rice crop. As O. phaseoli infestation in the seedling stage causes economic yield loss, it is essential to control this pest during the first four to five weeks after germination. Since O. phaseoli adults are tiny and agile, and its major damage is hidden inside the plant stem, it is important to adopt control measures such as spraying of the chemicals immediately after germination, or protect the crop by applying chemicals in soil simultaneously with sowing of the crop. It is, therefore, important to know the seasonality of the pest in order to undertake appropriate prophylactic control measures.
At present use of insecticides is the only control measure in practice. This is mainly due to the absence of other reliable control practices. Considerable research has been done on developing alternate and safer control measures but so far none of this research has resulted in practical control measures.
Despite its versatility and potential, host-plant resistance has not yet been successfully utilized in O. phaseoli control. This is mainly because the insect is confined to tropical and subtropical areas where research on host-plant resistance is virtually non-existent. Most of the significant research on this aspect in recent years has been carried out at the Asian Vegetable Research and Development Center (AVRDC), Taiwan. AVRDC has identified sources of resistance to O. phaseoli in common bean (Talekar, 1990), soybean (Talekar and Tengkano, 1993), mungbean (Chiang and Talekar, 1980) and cowpea (Talekar, 1990). At present, breeding for resistance is actively pursued in common bean by the International Center for Tropical Agriculture (CIAT), Cali, Colombia, at its program in Africa. However, no active breeding is pursued in the other three crops. AVRDC has developed several mungbean breeding lines with moderate levels of resistance to O. phaseoli (AVRDC, 1990). However, the yield and level of resistance in these lines needs further improvement.
Amongst various cultural practices attempted so far, only the use of rice straw and other similar plant straw mulch gives some control of the biotype of O. phaseoli that attacks soybean in Indonesia (Van der Goot, 1930). This biotype lays eggs in soybean cotyledons and second- and third-instar larvae feed in the stems of newly germinated plants, which invariably results in plant mortality. The rice straw mulch covers the cotyledons making them inaccessible for oviposition. Insects then lays eggs in unifoliate leaves, but by then the plant has developed adequately and tolerates O. phaseoli damage, although the yield may be reduced.
Ridging of the crop reduces plant mortality caused by O. phaseoli both in common bean and soybean (Van der Goot, 1930). Ridging the crop 2-3 weeks after germination helps to cover the adventitious roots which are produced by O. phaseoli damaged plants. The soil support prevents lodging and improves the survival of the damaged plants.
Intercropping with 60 crop plants belonging to 14 botanical families failed to protect common bean, soybean and mungbean in tests carried out in Taiwan (AVRDC, 1981a, 1981b).
In Hawaii, where O. phaseoli was accidentally introduced in 1968, Opius phaseoli and Opius importatus were introduced from East Africa in 1969 (Davis, 1971) specifically to control the newly introduced pest. Studies on Kauai and Maui islands showed that the Opius spp. usually acjieved high levels of parasitism but had only weak density dependence. Thus, there was a useful degree of control in most months, but occasional minor outbreaks still occurred (Greathead, 1975). However, on Oahu island, Raros (1975) observed lower levels of parasitism during 1973-74, which may have been due to the weak density dependence combined with the possibility of greater use of pesticides on that island (Talekar, 1990). These two parasitoids were also introduced into Brunei, but their impact on O. phaseoli populations has not been assessed.
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:
In tropical to subtropical Asia, O. phaseoli remains a destructive pest of most food legumes, particularly common bean, cowpea, mungbean, blackgram, lima bean and soybean (at least in Indonesia). The nature of extent of O. phaseoli damage in different hosts varies from crop to crop and season to season. In general, however, plants are more seriously damaged in the seedling stage than later stages. The consequences of insect attack in the seedling stage, if the plant survives, are manifested in the older plants. In general, the yield during the rainy season is much less than in the dry season. In Java, Indonesia, in 30 observations at Bogor, Van der Goot (1930) found that up to 100% of common bean plants were damaged, with high plant mortality and yield loss. In Tanzania the yield loss ranges from 30-50% (Wallace, 1939; Walker, 1960; Swaine, 1968). In New South Wales, Australia, Morgan (1940) found it impossible to grow common bean, indicating thereby 100% plant damage and total yield loss if plants are not adequately protected. In Taiwan O. phaseoli causes 35% yield loss in common bean (Talekar, 1990).
In Indonesia, a biotype of O. phaseoli attacks soybean. Whereas in the rest of its range, O. phaseoli lays eggs in the leaves of host plants, in Indonesia the biotypes lays eggs in soybean cotyledons soon after these plant parts emerge above ground. Larvae, after initial feeding in cotyledons, enter the stems and in most cases kill the soybean plant. The extent of damage and subsequent yield loss varies from season to season. In dry seasons (June to October ) van der Goot (1930) found the plant mortality to be 80%, compared to 13% in the wet season (November to April). O. phaseoli causes very little if any loss in soybean crops in the rest of its range.
In cowpea, O. phaseoli damage varies from location to location. In Indonesia, although the damage can reach up to 100% of the plant population, plant mortality is rare (van der Goot, 1930). In the Philippines, however, O. phaseoli infestation is high throughout the year, especially in the dry season when plant mortality can reach 60%. Those plants that survive the attack remain stunted and produce few or no pods (Otanes, 1918). In Taiwan O. phaseoli damage reduces cowpea yield by 32% (AVRDC, 1985).
In mungbean, Van der Goot (1930) reported 100% plant mortality due to O. phaseoli infestation in South Sumatra in the dry season. Similar plant mortality was observed in Malaysia (Ooi, 1973). In Taiwan yield losses of about 20% occur in autumn planting (Talekar, 1990). In peas, Kooner et al. (1977) reported plant mortality of 40% and yield loss of roughly 50% in India.