Melanagromyza sojae (Zehntner, 1900)
The species is related to Melanagromyza cunctans (Meigen, 1830) which only occur in Europe on non-cultivated plants and Melanagromyza dolichostigma (Spencer, 1973). Identification of sojae can be achieved by the shape of the male genitalia and the posterior spiracles and feeding habit of the larva (mining in the stem pith of fabacean crops).
IMPORTANT CHARACTERS
Adults
Wing length: 1.7 - 2.1 mm. Ocellar triangle rather large, thorax with bluish and abdomen with greenish sheen on the cuticular surface. Squamae with white borders.
Male terminalia
Hypandrial triangle short, angulate below and with very thick frame without hypandrial apodeme.
Immature stages
Larval posterior spiracles with blunt, somewhat atrophied central horn (Mel sojae larva Sp.pct). This character can be used for distinguishing from other bean flies.
BIONOMICS
There is some conflicting information in the literature indicating either cryptic species within Melanagromyza sojae or taxonomical confusion.
In India oviposition takes place in the soft portion of the stem, the larva bores into the cortical region of the root and the base of the stem (Singh, 1982; Ipe and Bhati, 1977). The duration of the larval stage in India was about 7 days under laboratory conditions. Puparial stage lasts about 8 days (Singh, 1982).
In other countries the larvae appear to lay eggs exclusively in unifoliate and trifoliate leaves mostly near the base. Whereas punctures of the females are found on both sides, eggs are laid only on the underside of the leaf (van der Goot, 1930). After entering the stem through the petiole, the larva feeds on the pith of the stem, where also pupariation takes place. The fully grown larva mines through the xylem and phloem tissue in order to make a hole to the outside. The larva closes the hole with debris. The emerging adult escapes the mine through this hole. The larval period lasted in Indonesia 9-11 days (van der Goot, 1930) and 8-9 days in Taiwan (Lee, 1976, Wang, 1979). The puparial stage ranges from 6-12 days (Wang, 1979, van der Goot, 1930).
Sometimes the larvae suffer from remarkable natural mortality. Wang, 1979 found that of 100 eggs only 1.9 adults emerge.
Invisibility of the feeding traces from outside.
HOST PLANTS
Aeschynomene indica, Astragalus sinicus, Cajanus cajan, Crotalaria juncea, Flemingia sp. Glycine max (soybean), Glycine soja, Indigofera suffruticosa, Indigofera sumatrana, Medicago denticulata, Medicago sativa, Melilotus sp., Phaseolus vulgaris, Phaseolus sublobatus, Pisum sativum, Stizolobium sp., Swainsonia galegifolia, Vigna aconitifolia, Vigna angularis, Vigna mungo, Vigna radiata, Vigna umbellata, Vigna unguiculata.
The main host is considered to be the soybean (Glycine max) but nevertheless damage occur also on other cultivated legume crops (e.g. Verma et al., 1989).
In Java the common native plants Aeschynomene indica, Flemingia sp. and Phaseolus sublobatus were invariably infested and thus provide a permanent reservoir, from which populations are available for attacking any suitable leguminous hosts which may be cultivated in the vicinity (Spencer, 1973).
DISTRIBUTION
Australia, China, Egypt, India, Indonesia, Israel, Japan, Laos, Malaysia, Micronesia, Philippines, Saudi Arabia, Solomon Islands, South Africa, South Korea, Taiwan, Vietnam.
ECONOMIC IMPORTANCE
Soy beans belong to the most important legume crop worldwide. Because Melanagromyza sojae usually achieves high infestation levels on fields not treated with pesticides the stem miner receives much attention by applied biologists.
However, since Melanagromyza sojae normally attacks well-established plants rather than seedlings, the larvae hardly cause plant mortality and total yield loss. Furthermore, Spencer, 1973 mentioned that the internal stem miner feeding in the pith may generally cause less extensive damage because they normally do not affect the vessels. In contrast, the results of several other authors suggest, the overall yield reduction due to larval feeding can be of high economic significance (e.g. Talekar, 1990, Singh and Singh, 1992, Venkatesan and Kundu, 1994). Some damage may be often not recognized because the feeding traces are well concealed under the stem surface (but see van den Berg et al., 1998). Much research is done about breeding resistant or tolerant cultivars and pest control methods. An overview was given by Talekar, 1990.