What to Do When Soybean Aphids Strike? Detailed Control Measures

Soybean aphid is one of the major pests of soybean, with adults and nymphs piercing and sucking sap on the top leaves, tender leaves and tender stems of soybean plants. In severe cases they cover the stems and leaves, and can also attack tender pods, causing stunted plants, reduced fruiting branches and pod numbers, and lower 1000-grain weight; severe infestation at the seedling stage can kill the entire plant. In outbreak years, if not controlled in time, yield losses range from 20-30% in mild cases to more than 50% in severe cases. It is distributed in all major soybean-producing areas of China, with heavier damage in parts of the three northeastern provinces, Henan, Shandong and other provinces.

The soybean aphid uses its piercing-sucking mouthparts to feed on sap, causing chlorophyll to disappear at the damaged spots on soybean leaves, forming bright yellow irregular yellow patches, which then gradually expand and turn brown. Severely affected plants have curled and yellowed stems and leaves, stunted growth, and reduced branching and pod set, thus affecting soybean yield.

1. Occurrence pattern. The soybean aphid includes the winged viviparous female (body length 0.9-1.6 mm) and the wingless viviparous female (body length 1.52-1.95 mm). Winged aphids can migrate, expanding the range of damage. The peak damage period of the soybean aphid is at the end of June and the beginning of July. It generally begins to appear in soybean fields in mid-to-late June; sustained high temperatures and drought with little rain easily lead to massive aphid outbreaks, and the drier the conditions, the more severe the damage.

The soybean aphid produces 18 generations a year. In the north it overwinters as eggs in aphid axils or crevices of branches; in the following spring, around April, when the average temperature reaches about 10°C and the buckthorn buds first show green, the overwintering eggs begin to hatch into fundatrices, which reproduce by parthenogenesis and viviparity for 1-2 generations. In mid-to-late May, around the time the buckthorn blooms and after the soybean seedlings emerge, winged viviparous females appear, migrating to soybean fields, where they reproduce by parthenogenesis and viviparity for more than 10 generations. From late June to early July is the early stage of peak outbreak of the soybean aphid in soybean fields, and mid-to-late July is the peak outbreak period, which can cause devastating damage to soybean. From late July onward, climate and nutritional conditions gradually become unfavorable to the soybean aphid, and pale yellow, small-bodied aphids appear on soybean plants, which is a sign of aphid decline. From late August to early September, in the late reproduction period of the soybean aphid, winged sexuparae are produced and fly back to the overwintering host buckthorn, where they give live birth to wingless oviparous females; another portion on soybean give live birth to winged male aphids, which fly back to the overwintering host, where males and females mate and lay eggs to overwinter.

In the course of a year, the soybean aphid generally has 4 migrations, correspondingly with several population rise-and-fall stages on soybean. For example, in the northeast, the first migration occurs during the emergence period of soybean seedlings, when aphids migrate from the winter host to soybean fields, in a stage of sporadic occurrence in the field. The second migration occurs in late June, which is the stage of spread and expansion of the soybean aphid within the field. Aphid distribution has gone from point to area, but the population density is not yet large; this is the early stage of peak outbreak in the field, and if aphids are controlled before the second migration, damage can be effectively controlled. The third migration occurs in mid-July, exactly during the soybean flowering period, when aphids rapidly spread throughout the field and the number of aphids per plant surges, which is the stage of major outbreak of the soybean aphid. The fourth migration occurs in September, because from early August climate and nutritional conditions gradually become unfavorable to aphids, and aphid numbers decline accordingly. By early September, winged females are successively produced and fly back to the overwintering host.

Climatic conditions are the key factor affecting fluctuations in soybean aphid population size. According to surveys in the northeast, it can be divided into two stages: first, from late April to mid-May, the period of overwintering hatching, nymph survival and adult reproduction; if rainfall is abundant and buckthorn grows vigorously, aphid survival rate is high and reproduction is great. Second, from late June to early July, the early stage of peak outbreak of the soybean aphid; during this period, if the ten-day average temperature reaches 20-24°C and the ten-day average relative humidity is below 78%, it is extremely favorable to soybean aphid reproduction, leading to severe damage during the flowering period.

2. Morphological characteristics. (1) Adult: winged viviparous female, body elongate-oval, yellow or yellow-green, body length 1-1.5 mm. The sides of the body have prominent mammiform protuberances, frontal tubercles inconspicuous, compound eyes dark red. Antennae 6-segmented, about as long as the body, gray-black, with 6-7 secondary sensoria on segment 3, arranged in a row. Siphunculi (abdominal tubes) cylindrical, black, base about twice as thick as the tip, with tile-like transverse ridges. Cauda black, conical, slightly constricted in the middle, bearing 2-4 pairs of long hairs. (2) Wingless viviparous female: body elongate-elliptical, yellow or yellow-green, body length 1-1.3 mm. Antennae shorter than the body, without secondary sensoria. Cauda with 3-4 pairs of long hairs; other characteristics basically the same as the winged viviparous female.

3. Control techniques. The soybean aphid has many natural enemies, including lacewings, predatory lady beetles, parasitic wasps and hoverflies; when natural enemy numbers are high, they can suppress the growth of aphid numbers.

Attention should be paid to seedling-stage prevention. Seed treatment with 75% 3911 emulsion or 40% isofenphos-methyl emulsion at 0.2-0.3% of seed weight can control aphids at the seedling stage while also controlling certain other seedling-stage pests. In addition, seed coating technology can be adopted to both control aphids and achieve increased yield.

1. At the seedling stage, spraying 125 g/mu of 35% phosalone A gives a significant control effect on the soybean aphid without killing natural enemies. To avoid resistance in the soybean aphid, it can be used alternately with 13 g/mu of fenvalerate A. Spray 20% dimethoate emulsion, 50% mieyajing emulsion or 50% phoxim emulsion diluted 1500-2000 times. Dilute 90% dimethoate emulsion or 88-90% acephate 10 times, use 350 g of diluted solution per mu, for ultra-low-volume spraying. Dust with 1.5% dimethoate powder, 5% carbaryl powder or 2% aldicarb powder, 1.5-2 kg per mu.

2. When patchy damage appears in soybean fields, use 40% dimethoate emulsion at 800 times dilution, 40% omethoate emulsion at 1000 times dilution, or choose 2.5% deltamethrin emulsion, 5% fluvalinate emulsion, or 10% deltamethrin emulsion at 225-300 mL per hectare, mixed with 600-750 L of water for spraying.