Detailed Explanation of the Occurrence Characteristics and Control Techniques of Spider Mites on Soybean

The soybean spider mite is the carmine spider mite (Tetranychus cinnabarinus), belonging to the class Arachnida, order Acariformes, family Tetranychidae. In 2007, affected by high temperatures and drought, soybean spider mites broke out over an area of 120 million mu in Heilongjiang Province, causing severe soybean yield reductions. According to surveys, in recent years the damage from soybean spider mites has risen to a primary position, generally reducing soybean yields by 20-30%, and in severely affected plots by 70-90%.

I. Morphological characteristics. The adult body length is 0.3-0.5 mm, reddish-brown, with 4 pairs of legs. The female mite body length is 0.5 mm, egg-shaped or pear-shaped, slightly broad and raised at the front end, slightly pointed at the tail, with fine long setae on the dorsal surface, and one black elongated patch on each side of the dorsum; overwintering females are vermilion red and glossy. The male body length is 0.3 mm, purplish-red to light yellow, spindle-shaped or pear-shaped. The egg diameter is 0.13 mm, spherical, colorless and transparent when first laid, gradually turning yellow with red. The larva has 3 pairs of legs, a round body, yellow-white, becoming egg-shaped and light green after feeding, with dark green elongated patches appearing on both sides of the dorsum. The nymph has 4 pairs of legs, light green to pale orange-yellow, with setae appearing on the dorsum.

II. Life habits. The soybean spider mite overwinters as fertilized female adults in soil crevices, at the roots of weeds, and on soybean plant residues. It begins activity in mid-to-late April of the following year, first reproducing and causing damage on weeds such as thistle, small bindweed, dandelion, and plantain, then moving to soybean to cause damage in June-July; from mid-to-late July to early August, as temperatures rise, reproduction accelerates and it spreads rapidly; after mid-August it gradually decreases, and by September, as temperatures drop, it begins to move to overwintering sites, starting to overwinter in October.

The soybean spider mite has 8-12 generations per year in Heilongjiang Province. The developmental threshold temperature is 10.5°C, the upper limit temperature is 42°C, and the effective accumulated temperature for completing one generation is 163.25°C. The time required from egg to adult development is shortest at a relative humidity of 35-55% and an average temperature of 22-28°C, requiring only 10-13 days. Therefore, when continuous drought lasts more than 14 days, reproduction is fastest and damage is most severe. When relative humidity exceeds 70%, it is unfavorable for spider mite occurrence; low temperature, rainy, and windy weather are unfavorable for spider mite reproduction.

III. Damage characteristics. The soybean spider mite can occur throughout the entire growth period of soybean, initially in localized patches, with adult and nymph mites gathering on the undersides of leaves, spinning silk webs, and piercing and sucking leaf sap. In the early stage of damage to soybean leaves, yellow-white spots appear on the upper surface of the leaves; after 3-5 days the spots expand and become denser, reddish-brown patches appear on the leaves, and parts or even the entire leaf curl up, scorch and turn yellow or reddish-brown, with leaves falling off or even bare stems, and in severe cases the entire plant dies. Plots with heavy nitrogen fertilizer application have more severe occurrence. When food is scarce, it has a habit of migration; July-August is the peak damage period, and plots with many weeds or sparse plants with poor growth have more severe occurrence.

IV. Control methods. ① Agricultural control: Apply sufficient base fertilizer, increase phosphorus and potassium fertilizer, ensure no fertilizer shortage in the later stage, remove weeds thoroughly in time, irrigate in time during drought, and where conditions permit carry out paddy-upland rotation, which can reduce the incidence. ② Chemical control: When localized patches occur and the rate of curled-leaf soybean plants reaches 10%, pesticide should be applied immediately for control; in combination with aphid control, choose 73% mie man jing at 3000 times dilution, or 40% dichlorodiphenyl dichloroethanol at 1000 times dilution, or 25% propargite emulsifiable concentrate at 3000 times dilution, or 20% sao man jing and mite-killing emulsifiable concentrate at 2000 times dilution for spraying, spraying 2-3 times consecutively. Under drought conditions, add 1% of the spray volume of plant-type spray adjuvants such as Yao Xiao Bao and Xin De Bao. ③ Biological pesticide control: For organic soybean, choose 1.8% avermectin emulsifiable concentrate, 0.3% azadirachtin emulsifiable concentrate at 1500-2000 times dilution, or 10% liuyangmycin emulsifiable concentrate at 1000-1500 times dilution, 2.5% huaguangmycin at 400-600 times dilution, or bionic pesticide 1.8% nong ke man emulsifiable concentrate at 2000 times dilution for spraying; under drought conditions, add 1% of the spray volume of plant-type spray adjuvants such as Yao Xiao Bao and Xin De Bao.