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Dictyostoma, a pest belonging to the family Diptera, is particularly harmful to leguminous crops. In the 1970s, it was effectively controlled in Dalian, and for about three decades, it was not reported. However, after the seed market was liberalized, the black maggots of bean stalks re-emerged, showing a rapid spread trend. Between 2005 and 2007, the infestation of Drosophila in soybean fields in Liangjiadian and Xingshutun Towns within Jinzhou District significantly reduced soybean yields by 30% to 50%.
Morphologically, the black maggots of Dictyostoma are small with limited mobility. The adult fly measures approximately 2.5 mm in length, has a dark body, a blue sheen on its abdomen, dark red compound eyes, three antennae, and a blunt third segment. It also features a central horn on the dorsal side, fine hairs, transparent wings, and a lavender coloration.
In terms of life habits, the species completes 2-3 generations per year in northern Liaoning, while in Dalian, it can have 3-5 generations annually. The larvae overwinter in the rhizomes and stalks of host plants. Eggs hatch in mid-June of the following year. Adults are weakly phototactic and prefer moderate temperatures (25-30°C), often gathering on the upper parts of bean leaves. They pierce the leaf surface to feed on sap, causing small, scattered holes. After hatching, the larvae feed on the leaves, creating translucent tunnels. As they grow, they move toward the main veins, then into the petiole and stem, feeding on the pith and xylem. When mature, they create a small hole in the stem wall and pupate nearby.
The damage caused by Dictyostoma is severe. It primarily affects green beans, kidney beans, soybeans, red beans, mung beans, and other leguminous vegetables. In soybean-producing areas, the loss rate can be as high as 50%, with even mild infestations reducing yield by 30%. The larvae hollow out the stems, disrupting water and nutrient transport, leading to plant death. Seedlings may show stunted growth, with hollow stems and premature leaf drop. Late-stage infestations cause flowers, pods, and leaves to fall off early, reducing grain weight and overall yield.
Several factors contribute to its resurgence. First, unregulated seed trade allowed eggs to be introduced from infected regions. Second, some seed distributors used low-quality edible soybeans as seeds, which were heavily infested. Third, warm winters between 2000 and 2007 led to higher survival rates of overwintering eggs. Lastly, fragmented land management and lack of proper field cleanup allowed pests to thrive and spread quickly.
To manage this pest, strict seed quarantine is essential to prevent the spread of infested seeds. Establishing clean seed fields and removing impurities and pests during seed production improves quality. Crop rotation and thorough post-harvest cleaning of straw and root pods help reduce overwintering eggs. Chemical control methods, such as spraying 50% phoxim EC at 1:1000 or 20% cypermethrin EC at 1:3000, are effective, with more than 90% control efficacy when applied early. These measures are crucial in preventing further outbreaks and protecting soybean production.