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

The soybean root miner fly belongs to the order Diptera, family Agromyzidae. Also known as the soybean root maggot, soybean root miner fly, etc., this insect is a monophagous pest that only damages soybean and wild soybean. It is distributed in Heilongjiang, Liaoning, Inner Mongolia, Shandong and Hebei, among other places. Damaged plants can suffer yield losses of more than 40% per plant compared with healthy plants.

1. Morphological characteristics

(1) The adult is about 3 mm long, glossy black, with wings that are light purple with a metallic sheen. The compound eyes are bright red.

(2) The egg is about 0.015 mm long and about 0.04 mm wide, kidney-shaped, white and translucent.

(3) The larva is about 3.2-4.2 mm long, pale yellow, semi-transparent, cylindrical, and slightly tapered at the tail end.

(4) The pupa is about 2.1-3.0 mm long, elongate-oval, black.

2. Damage characteristics The adults pierce and lap up the cotyledons and true leaves of soybean seedlings, and the feeding sites appear as blighted spots. After hatching, the larvae bore into the seedling roots to feed, damaging the cortex and xylem of the main root and creating wounds. Root damage in affected plants appears as poorly developed root systems, thickened and browned roots, cracked cortex or abnormal proliferation, or root galls. Above ground, affected plants show weak seedling growth, stunted plants and pale yellow leaves. In lightly affected plants, after the larvae pupate, the root wounds heal and the plants resume growth, but the root nodules are smaller and fewer, the top leaves turn yellow and there are fewer pods, resulting in reduced quality and yield. In severely affected plants, the plants gradually wither and die. In addition, the wounds caused by the larvae can also lead to infection by root diseases, increasing the damage and losses to soybean.

3. Occurrence pattern In the northern soybean region, 1 generation occurs per year, and it overwinters as pupae in the soil in soybean stubble or near damaged roots. The following year, the overwintering pupae emerge from late May to early June, and adults peak in early June. Mid-June enters the peak egg period, and the peak hatching period is from mid-June to late June, which is also the peak larval damage period, lasting 20-40 days. Mature larvae begin to pupate and overwinter in late June. The larvae damage the cortex of the main root of soybean seedlings, while adults feed on the juice of soybean seedling cotyledons or leaves. After emergence, the adults of the soybean root miner fly move among the soybean plants, and after mating they lay eggs in the cortex of the roots or stems of soybean seedlings near the soil surface. The eggs are mostly laid singly, 1 egg per plant, with an egg-laying period of 4-7 days, and each adult lays 20-40 eggs. After hatching, the larvae move along the epidermis of the hypocotyl toward the roots and bore into them to feed and cause damage. The final-instar larvae pupate and overwinter in the space 5-10 cm below the soil surface.

The occurrence of the soybean root miner fly is closely related to climatic factors and cultivation systems.

(1) Climatic factors: Temperature and humidity both affect the emergence, mating and egg-laying of the adult soybean root miner fly. Its optimum temperature is 20-25°C, and moist soil after rainfall is favorable for emergence and adult occurrence.

(2) Cultivation system ① Previous crop: Different previous crops result in different degrees of occurrence of the soybean root miner fly. Continuous cropping land has a larger number of overwintering insect sources and suffers severe damage; alternate cropping and proper rotation land have lighter occurrence. According to surveys, the damage rate was 30% in continuous cropping land, 15% in alternate cropping land and 7.5% in proper rotation land. ② Soil fertility: The level of soil fertility affects the growth vigor of soybean seedlings, and thereby affects the recovery ability of damaged plants. Surveys found that when soil organic matter content is below 5%, with insufficient fertilizer, the seedlings grow thin and weak, and the damage worsens. When soil organic matter content is above 5%, with sufficient water and fertilizer, the seedlings start growing early and develop robustly, the damage is reduced, and even if damaged, they recover quickly. ③ Sowing time: Different sowing times result in different degrees of occurrence. Sowing too late, with fields sown after May 20, results in severe occurrence. This is because in late-sown fields, when the larvae of the soybean root miner fly are at their peak, the root and stem epidermis of soybean seedlings is tender, which favors its occurrence and damage. ④ Tillage method: Fields with autumn deep plowing or autumn harrowing of stubble have lighter occurrence. Autumn deep plowing of more than 20 cm can bury the fly pupae deeper in the soil layer and reduce the emergence rate; autumn harrowing can bring overwintering pupae from below the surface of soybean stubble land to the surface, and under the influence of long-term low temperature and dry climate in winter, the mortality of overwintering pupae also increases.

4. Control methods

(1) Agricultural control ① Reasonable crop rotation: The soybean root miner fly has a single food source and only damages soybean and wild soybean, so reasonable rotation with gramineous crops can reduce the number of insect sources in the soil. Generally, rotation with gramineous crops for more than 2 years can reduce the degree of damage. ② Clean the field: Removing weeds in the field and along field edges can reduce insect sources. ③ Autumn flooding or autumn deep plowing of soybean stubble: In autumn, deeply plow the soybean field; if pupae are turned under to 30 cm, they cannot emerge; autumn flooding can bring overwintering pupae to the surface, and through the long-term low temperature and dry climate of winter, the mortality of overwintering pupae increases, thereby reducing the number of insect sources and decreasing the number of adults. ④ Early sowing at the proper time: Applying sufficient base fertilizer and sowing early at the proper time can promote early development of soybean seedlings. When the larvae are at their peak, the main root has a relatively high degree of lignification and can tolerate larval boring, so the damage is lighter; conversely, if the seedling root and stem are tender, the damage is severe.

(2) Seed treatment: Chemical agents or seed coating agents can be used to treat seeds. Seed treatment with seed coating agents: 8% carbendazim-thiram seed coating agent or 35% thiram-carboxin- thiophanate-methyl suspension seed coating agent, coating seeds at 1.2%-2.0% of seed weight. Soybean seed coating agent 26 is used for seed coating at a pesticide-to-seed ratio of 1:75. Treatment of seeds with emulsifiable concentrate agents: 5% mephosfolan or 35% ethoprophos, used as a seed dressing at 0.5% of seed weight; 40% dimethoate EC (emulsifiable concentrate) 700 mL added to 4-5 kg of water to dress 100 kg of soybean seeds, then aired dry and sown immediately.

(3) Field control: Field control mainly targets adults. During the peak adult period, that is, before the first compound leaf of soybean, when yellow spots appear on the cotyledon surface and adults (small flies) are seen in the field, spray to control the adults. If the insect population is relatively large, spray again after 7-10 days. Use pyrethroid pesticides at about 450 mL per hectare diluted with water for spraying, 40% dimethoate or 80% dichlorvos emulsifiable concentrate at 1000-fold dilution for spraying, or 90% trichlorfon crystal at 700-fold dilution.