Melanagromyza obtusa

Melanagromyza obtusa (Malloch, 1909)

M. obtusa is distinct because of a greatly enlarged ocellar triangle. This character is only shared by Melanagromyza chalcosoma from Africa, which can be only separated by the geographical range and slight behavioural differences: In contrast to chalcosoma, obtusa pupates always in the pods but outside the seeds.

Wing length: 2.4 - 2.8 mm. Arista long, with conspicuously long hairs. Frontorbital hairs numerous; apically some frontorbital hairs proclinate. Ocellar triangle considerably enlarged.
Male terminalia
Hypandrium and aedeagal apodeme long; hypandrium with very long hypandrial apodeme. Projection at tip of epandrium rounded with long hairs. Aedeagus hyaline, basiphallus with unusual long side arms, which are extending up to the distiphallus, dorsal bridge of basiphallus seems to be missing.
Immature stages
The egg is white, broad and rounded at the posterior end and narrowed anteriorly into a slightly curved, elongated process, which is an extension of the egg sheath. The shape of the larva and the puparium is rather short and broad; the posterior spiracles have several semicircular distributed bulbs without median projection.

On legume crops, Melanagromyza obtusa does not feed in the stems but exclusively in the seeds of their host plants.
The eggs are laid in the pods, where also feeding punctures are created. One or many eggs can be laid in one pod. Singh and Rai, 1984 found, one female lays up to 70 eggs in her life, However, the measured average number was much lower (33).
For oviposition the females prefer the "green stage" of the pod (this is defined as an intermediate stage between the developing stage and the mature stage). The unavailability of suitable pods can result in oviposition in older pods and, accordingly, in higher larval mortality.

The young larva first feeds just under the epidermis of the seed like a leaf miner. As the larva grows, the feeding continues deeper into the seed and even damages the embryo. One seed appears to supply enough resources for one larva, but sometimes a larva move from one seed to the next. The fully grown larva leaves the seed and pupariates in the pod. Prior to pupariation a partial hole into the pod wall is created to enable the future fly to escape. The hole is covered only with the unaffected epidermis layer. The life cycle was studied by Ahmad, 1938, Ipe, 1974, Singh, 1982, Singh and Rai, 1984, their results were similar:
At higher temperatures (29 - 34°C) the egg, larval and pupal stages last about 2 - 3; 5.3 and 8 days. Thus, one generation can be as short as 17 days. At lower temperatures, however, the development is delayed considerably and can take about two months at temperatures around 20°C.

Fabaceae: Atylosia cajanifolia, Atylosia platycarpa, Atylosia volubilis, Cajanus cajan (pigeon pea, red gram), Dubaria sp. Flemingia sp. Flemingia congesta, Moghania macrophylla, Rhynchosia aurea, Rhynchosia bracteata, Rhynchosia minima, Rhynchosia rothii, Rhynchosia suaveolens, Tephrosia purpurea, Tephrosia sp., Vigna radiata (mung bean, green gram), Vigna unguiculata (cowpea).
Usually, the pigeon pea is the main host and economically of greatest importance. Some fabacean weeds can serve as reservoir to maintain the population when pigeon pea is lacking. However, there are some reports indicating damage on economic level also on cowpea seeds (Mehrotram et al., 1990).

There are some further records of Melanagromyza obtusa infesting cultivated plants of other families, which should be handled with care:
Malvaceae: Several obviously independent authors reported Melanagromyza obtusa mining in the stem and petioles of Hibiscus esculentus (Okra), see Venugopal and Venkataramani, 1954, Patel and Verma, 1974, Sharma and Singh, 1984, Rajmohana, 1999.
The earliest record was rejected by Spencer, 1973 (p. 115) and explained by a confusion with Melanagromyza hibisci, a species that feeds on the stems or petioles of Okra. This might also be true for the later records. Although these two species are not closely related, confusions are possible because of the general difficulties in identifying species of the Ophiomyia-group. Therefore, such records need further confirmation.

Another unusual host plant found in the literature is Sesamum indicum (sesame) (Abraham et al., 1973). It is interesting that the only other agromyzid fly being known from sesame is the stem miner Melanagromyza azawii, which probably also primarily feeds on Fabaceae (Spencer, 1973). The record of obtusa feeding on sesame should be handled with care because feeding in pods and seeds must be considered as specialized. A switch to a host without pods and much smaller seeds seems to be not plausible. Possibly the record was due to misidentification of the agromyzid fly.

Mainly East palaearctic/Oriental region: Bangladesh, Burma, India, Indonesia, Japan, Malaysia, Papua New Guinea, Sri Lanka, Taiwan, Thailand. The record by Khamala and Okeyo-Owour, 1980 from Kenya may be due to confusion with the closely related podfly Melanagromyza chalcosoma, which is well known from Kenya and other African countries (Spencer, 1973).
The main country suffering from Melanagromyza obtusa is India because over 90% of the world pigeon pea production is located there (Talekar, 1990).

Pigeon pea is an important crop in semi-arid tropical and subtropical areas. Especially in Asia it is a major protein supply for humans (Shanower et al., 1999). Therefore the pigeon pea pest Melanagromyza obtusa deserves much attention.
By feeding on the seeds this podfly attacks just that part of the plant that is mainly desired by the farmers. A seed damaged by the pod fly is unfit for human consumption. Therefore yield loss can be measured directly and varies usually between 5-30% (Talekar, 1990), but sometimes even higher. Together with other pest species on pigeon pea, which cause additional damage, Melanagromyza obtusa is a serious threat for farmers.
The prevailing control measures against the pod flies are synthetic insecticides. Much effort in producing podfly resistant or tolerant cultivars was not very successful because resistant lines frequently show other disadvantages such as smaller seeds and lower nutritious value. Although several parasitoids are known to be present on pigeon pea fields they are apparently not effective enough as control agents for Melanagromyza obtusa. (Talekar, 1990, Shanower et al., 1999).