Ophiomyia phaseoli (Tryon, 1888)
This important pest of tropical legumes is assumed to be closely related to Ophiomyia spencerella (Spencer, 1973).
IMPORTANT CHARACTERS
Adults
Wing length: 1.8 - 2.2 mm. In males the vibrissal fasciculus is missing. Body dark with fine pubescence, mesonotum and abdominal tergites clearly shining. Frons darkish. Alcohol material: Frons and genae light brown, covered with very fine dark denticles. In dry specimens this may appear as dark brown.
Male terminalia
Hypandrium short and of intermediate thickness, hypandrial apodeme short and broad. Ejaculatory apodeme long and unusually narrow. Epandrium short in side view; tip of epandrium (surstyli) rather broad, covered with long hairs. Aedeagus as in figure Oph phaseoli aedeagus1.pct.
Immature stages
Larva rather long and slender, facial mask with conspicuous frontal process (Kato, 1961). Mandibles strong, left one with one large and two much smaller mouth hooks. Right mandible much smaller than left one, hardly visible from the left side. Anterior spiracles rather short, rounded, posterior spiracles rather long, bifid with about 10 bulbs.
BIONOMICS
(mainly after Talekar, 1990)
The eggs are mainly laid into the leaves, in younger plants preferably into cotyledons, unifoliate or early trifoliate leaves. Reports about eggs found in stems are probably due to misidentification and confusion with Ophiomyia spencerella in Africa and Ophiomyia centrosematis in Asia (Talekar, 1990). Often, the eggs of Ophiomyia phaseoli are situated near the midrib close to the petiole. As in other mining flies, many feeding punctures are normally distributed all over the leaf. The first instar mines from the leaf blade to the midrib where the first moult takes place. The second instar proceeds from the leaf into the petiole and in most cases moults at the junction between petiole and stem. In older or slightly infested plants, eggs are also deposited in trifoliate leaves and the entire larval feeding of the larva can be restricted to the leaf, petiole and the junction between petiole and stem. In seedlings or higher infested plants, however, the third instar can enter the stem and mine towards the root.
A completely different larval behaviour Abate, 1991 found on the wild host plant Crotalaria laburnifolia in Ethiopia. He observed the larvae act as true leaf miner on this plant.
The whole egg and larval development requires normally 9-14 days but can be strongly delayed by low temperatures. That is also true for the puparial stage, which can vary from 3-20 days. The number of eggs, their fecundity and the female longevity varies considerably. There may be an average of 100-200 eggs per female. The eggs are laid from the third day of a female's life over a period of 10-15 days.
HOST PLANTS
Ophiomyia phaseoli has a wide host range amount cultivated legume crops. The main hosts appear to be Snapbean (Phaseolus vulgaris), Soybean (Glycine max), Cowpea (Vigna unguiculata), Mungbean (Vigna radiata), Blackgram (Vigna mungo), garden pea (Pisum sativum). The following list of all known host plants is mainly taken from Talekar, 1990.
Cajanus cajan, Crotalaria juncea, C. laburnifolia, C. mucronata, Cyamopsis psoraloides, Dolichos auxillaris, Glycine max, Lablab purpureus, Macrotyloma axillare (see Cameron, 1986), Medicago sativa, Macroptilium lathyroides, Mucuna derringiana, M. pruriensis, Phaseolus austropurpureus, Ph. coccineus, Ph. lathyroides, Ph. lunatus, Ph. panduratus, Ph. semierectus, Ph. vulgaris, Pisum arvense, P. phaseolus, P. sativum, Psophocarpus tetragonolobus, Solanum nigrum, Vigna aconitifolia, V. angularis, V. hosei, V. marina, V. mungo, V. radiata, V. luteola, V. trilobata, V. umbellata, V. unguiculata (ssp. sesquipedalis, ssp. unguiculata).
The host plant choice of Ophiomyia phaseoli can show considerable variability among cultivars (e.g. Talekar, 1990, Waterhouse, 1998). Some morphological properties of cultivars can be useful for breeding resistant crop (e.g. Chiang and Norris, 1983 a, b, c).
DISTRIBUTION (after Spencer, 1973 and Talekar, 1990)
India, China, Nepal, Hawaii (Spencer, 1983), Indonesia, Sri Lanka, Australia, Reunion, Africa: Malawi, Ethiopia, Kenya.
Not known from Europe and the Americas.
ECONOMIC IMPORTANCE
Ophiomyia phaseoli is doubtless one of the most dangerous agromyzid flies in the world. This is for two reasons: 1. The species' wide host spectrum includes the most important supplier of proteins for human nourishment in the Old World Tropics. 2. Larval feeding threatens especially seedlings and young plants and can cause high mortality among the plants and, therefore, high yield loss.
As mentioned above, in seedlings the larvae feed in the cortex towards the root. Larval mining damage vascular tissue and can cause severe disturbance in the transport of nutrients. Seedlings of snapbean and soybeans suffer from bean fly larvae especially in the dry season. The shoots are often swollen at the root junction. Highly infested snap beans can survive and recover only if they manage to produce adventitious roots above the destroyed shoot-root junction. The infested plants can maintain water and nutrients supply. Larval feeding on older plants may also reduce the yield but it does not cause severe damage and mortality.
The impact of the damage Ophiomyia phaseoli causes depends, however, on the region and on plant cultivars. Furthermore, in Africa a fair part of the yield loss sometimes attributed to Ophiomyia phaseoli is presumably due to the closely related species Ophiomyia spencerella, although both species occur in the field.
Research on control measures mainly focus on insecticide treatment, breeding of agromyzid-resistant cultivars and cultural methods (e.g. Chiang and Norris, 1983 a, b, Talekar, 1990). By ridging the soil and irrigation, the development of adventitious roots and, thus, the survival of young plants can be supported.
Although many hymenopterous parasitoids infesting O. phaseoli are usually present in the field, their effect normally seems to be limited.