The young leaves show a large number of punctures and light yellow spots on the upper side, especially on the bottom of the leaf. Larvae mine through leaf stalks and stems, which later appear as silvery, curved stripes. On the upper side of the leaf only a few tunnels are visible, which later turn dark brown and are clearly visible wilt. These leaves may dry out and may even be shed. In infested mature plants stalks becomes swollen and at times the leaves may wilt. The feeding tunnels are clearly visible on the stems. Intense larval feeding leads to destruction of the internal tissue around the root-shoot junction, leading to yellowing of the leaves, stunting of plant growth and even plant death. In most cases the plant is killed within 10-15 days of emergence.
Symptoms are caused by the larvae and adults of the bean fly, Ophiomyia phaseoli, one of the most destructive pests in the world. It is widespread in tropical and subtropical regions of Asia, Africa, Hawaii and Oceania. In some cases, it can lead to yield losses of 30-50%. The severity of the damage seems to be seasonal, resulting in far higher mortality in the dry season that in wet season (80% vs 13% respectively). Both the adults and the larvae cause damage, especially in seedlings. The adults produce holes in the young leafs and lay their white, oval eggs near the leaf stalk. The developing larvae mine downward through the stem into the tap root and returns to pupate at the base of the stem, close to the soil surface. Pupation lasts about 10-12 days depending on temperature.
There are several natural enemies of the bean fly. Several braconid wasp larva parasitoids of the Opius species are widely used in both Asia and Africa. Two species, Opius phaseoli and Opius importatus, were introduced into Hawaii in 1969 from East Africa, but occasional outbreaks of bean fly still occur. The pest mortality in some regions reaches up to 90%. Products based on fungal pathogens of the fly were also tested as possible pest management tool in East Africa.
Always consider an integrated approach with preventive measures together with biological treatments if available. Where infestations are severe, insecticides can be considered for the control of bean fly. However, the larvae which do the damage are well protected inside the plants. Spraying of chemical products containing imidacloprid into the soil simultaneously with sowing of the crop or immediately after germination is effective. Seedlings are treated about 3-4 days after emergence and, if bean fly infestations are severe, repeated at 7 days, and possibly at 14 days. Other commonly used active ingredients are dimethoate, which is systemic. All listed chemicals are classified as hazardous and should be handled with care.