What are the effective control methods for the soybean root miner fly

The soybean root miner fly Ophiomyia shibatsuji (kato), also known as the bean root snake miner fly, belongs to the order Diptera, family Agromyzidae. It is commonly called the soybean root maggot, bean root snake fly, root miner fly, etc.

I. Distribution and damage. The soybean root miner fly is mainly distributed in the main soybean producing areas; in China it is found in the three northeastern provinces, Inner Mongolia, Hebei, Shandong and Shanxi, among which Heilongjiang, Jilin and Inner Mongolia are the most prominent. The soybean root miner fly mainly causes damage during the soybean seedling stage; its feeding habit is monophagous, attacking only soybean and wild soybean. The larvae bore and cause damage in the cortex and xylem of the soybean seedling roots and excrete frass, causing the root cortex to rot and forming strip-shaped scars. The damaged roots become thickened, turn brown, and the cortex cracks or develops malformed growths; the larvae's frass and feeding stimulate the phloem tissue to become suberized, forming tumors, which causes the soybean root system to be damaged and unable to grow normally and absorb various nutrients from the soil. The adults pierce and lap at the cotyledons and true leaves of soybean seedlings, and at the feeding sites small white spots, even transparent small holes or blighted spots, are formed.

II. Morphological characteristics. (1) Adult: body length about 3 mm, wingspan 1.5 mm, shiny black, relatively stout in shape. Compound eyes large, dark red. Antennal flagellum flattened and short, with a bluntly rounded tip. Wings light purple, with metallic luster. Legs blackish brown. (2) Egg: length about 0.4 mm, olive-shaped, white and transparent. (3) Larva: body length about 4 mm, cylindrical, a milky-white maggot, later the whole body appears light yellow, semi-transparent; head retracted into the prothoracic cavity, mouth hooks black, bent at a right angle, with the tip slightly curved inward. One pair of anterior spiracles and one pair of posterior spiracles, relatively large, protruding from the tail end, perpendicular to the tail axis, parallel to each other, with the spiracle openings cauliflower-like. The surface has 28-41 spiracular pores. (4) Pupa: length 2.5-3 mm, long oval, black, with the anterior and posterior spiracles distinctly protruding, boot-shaped, and with two needle-like bristles at the tail end (posterior spiracles).

III. Life history and occurrence pattern

1. Life history: The soybean root miner fly has 1 generation per year, overwintering as pupae in the soybean root (inside soybean root nodules) or in the soil near the damaged roots, with the pupal stage lasting as long as 10-11 months. Overwintering pupae begin development in late April of the following year when the soil temperature gradually rises, the soil temperature at a depth of 10 cm reaches above 8°C, and soil moisture is about 35%; they emerge as adults from late May to early June, with the peak emergence period in mid-June. The deeper the pupae are in the soil layer, the lower the emergence rate. When the temperature is high in May, adults appear 5-6 days earlier and cause more severe damage. Adults can mate 2-3 days after emergence and can lay eggs the same day. Egg laying generally begins in early June, with the peak egg-laying period in mid-June. Eggs begin to hatch after 3-4 days of development, with the peak hatching period in mid-to-late June. The hatched larvae move downward along the stem. They enter the roots to bore and cause damage. The larval stage is about 20 days; mature larvae begin to pupate and overwinter in late June. Late June to early July is the peak pupation period, and pupae survive in the soil for as long as 10 months.

2. Occurrence pattern: The occurrence of the soybean root miner fly is closely related to environmental conditions, and its suitable development temperature is 20-25°C. When the last frost period is late, the temperature is relatively high, and the soil is dry, occurrence is lighter than in years with low temperature and high soil moisture. Fertile plots suffer more severe damage than poor plots. Hilly land suffers more severe damage than low-lying land, sloping land is heavier than flat fertile land, and sandy soil is heavier than brown earth soil. Damage is severe near wasteland with weeds and in continuous cropping land. With different previous crops, the degree of occurrence of the soybean root miner fly differs. Continuous cropping land has a larger number of overwintering insect sources, and occurrence and damage are severe; alternate cropping and normal rotation land have lighter occurrence. Plots with autumn deep plowing or autumn harrowing of stubble have lighter occurrence. Autumn deep plowing of more than 20 cm can bury pupae deep in the soil and reduce the emergence rate; autumn harrowing can bring overwintering pupae from below the surface of that year's soybean stubble land to the surface, and after the long-term low temperature and dryness of winter, mortality increases.

IV. Forecasting methods. According to the research of Chen Shenkuan et al. (2004), occurrence amount forecasting can be carried out using the following methods.

1. Major outbreak index: overwintering pupae amount per square meter above 20 individuals, fly emergence rate above 70%; during the adult peak period, more than 50 flies caught in 5 net sweeps, plant damage rate over 76%; when this occurrence level is reached, loss can reach more than 20%.

2. Moderate outbreak index: overwintering pupae amount per square meter 10-20 individuals, fly emergence rate 60-70%; during the adult peak period, 20-50 flies caught in 5 net sweeps, plant damage rate 50-75%, yield loss 10-20%.

3. Light occurrence index: overwintering pupae amount per square meter below 10 individuals, fly emergence rate below 60%; during the adult peak period, fewer than 20 flies caught in 5 net sweeps, plant damage rate below 50%, loss below 10%.

V. Control measures

1. Agricultural control methods: implement crop rotation of more than 3 years to reduce insect source density; adopt autumn deep plowing to reduce the emergence rate the following year; sow at the right time, sowing when the soil temperature stably exceeds 8°C, and never later than May 20. Press the soil after sowing, with a sowing depth of 3-4 cm. In addition, appropriately increase the application of phosphorus and potassium fertilizers and increase the application of decomposed organic fertilizer to promote seedling growth and root bark suberization, which can enhance the resistance of soybean plants to damage.

2. Chemical control methods: (1) Soil treatment. Treat the soil with 3% carbofuran granules at 15-100 kg per hectare, mix with fine moist soil and broadcast into the sowing holes or furrows, then sow soybean seeds. (2) Use seed coating agents. Coat seeds with a seed coating agent containing carbofuran (carbaryl), 1 kg of seed coating agent for 90-100 kg of seed; 8% carbendazim-thiram seed coating agent or 35% duokeFu suspension seed coating agent can be used, coating seeds at 1.2-2% of seed weight. (3) Chemical seed dressing. Use 50% phoxim plus 0.3% duofu mixture, spraying while stirring the seeds with a sprayer on an indoor floor until evenly mixed. However, avoid direct sunlight to prevent photolysis of phoxim, and sow after drying in the shade. (4) Use pesticides to control larvae. Spray or drench the roots with 80% dichlorvos emulsifiable concentrate at 800-1000 times dilution, or 90% crystallized trichlorfon at 700-1000 times dilution. When drenching roots, the pesticide solution must penetrate to the roots. (5) Spray pesticide to control adults. During the adult peak period, that is, before soybeans grow the first compound leaf, when yellow spots appear on the cotyledon surface and adults are visually observed in the field, spray with 40% dimethoate emulsifiable concentrate or 80% dichlorvos emulsifiable concentrate at 1000 times dilution, or 90% trichlorfon at 700 times dilution, using about 750 liters of pesticide solution per hectare. (6) During the adult occurrence period. Use 80% dichlorvos slow-release cards for fumigation to control adults.