Which pesticides are effective for controlling onion thrips?

Onion thrips belongs to the order Thysanoptera, family Thripidae. It mainly damages Allium vegetables such as onions and Chinese chives, and can also damage crops in the Solanaceae family, cucurbits, cotton, tobacco and others. Its host plants number more than 30 species across 18 families, and it is distributed throughout the country. So, what pesticide kills onion thrips?

1. What pesticide kills onion thrips?

You can spray pesticides such as 99.1% dichlorvos emulsifiable concentrate, 5% fipronil suspension concentrate at 2500 times dilution, 40% chlorpyrifos emulsifiable concentrate at 1000 times dilution, 12% malathion, 6.3% abamectin·beta-cypermethrin wettable powder at 5000 times dilution, or 2.5% spinosad suspension concentrate at 1000 times dilution. Spray once about every ten days, and 2-3 applications are enough.

2. Occurrence pattern of onion thrips

In northern China, onion thrips produces 6-10 generations a year. Adults of onion thrips can begin activity when the daily average temperature reaches 4°C; above 10°C the adults feed actively, and when the ten-day average temperature rises above 12.5°C they lay eggs and reproduce. When the ten-day average temperature rises to 16.6-19.6°C, they reproduce rapidly and the population grows very quickly. When the temperature exceeds 38°C, nymphs cannot survive; near 38°C, although development is fast, the mortality rate is relatively high. When relative humidity reaches 100%, at 31°C, nymphs cannot survive at all; when humidity drops to 75%, they can complete development. The most suitable conditions for onion thrips occurrence are a temperature of 20-28°C and a relative humidity of 40%-70%. A long spring drought without rain is a sign of a major outbreak.

Adults and nymphs overwinter in the leaf sheaths of unharvested Welsh onions, onions and garlic, and a few overwinter as pupae among weeds and crop residues and in the soil of onion fields covered with grass and manure in winter or near the damage sites. In March-April they are active on early spring crops and weeds; from late April to early May they gradually migrate into onion fields or cotton fields to cause damage. The peak damage period is generally from mid-May to mid-June. July-August is the hot, rainy season, and because humidity is too high, the population density of onion thrips declines naturally. In addition, cotton fields near overwintering sites or with more nearby weeds, fields with loose soil, cotton fields intercropped with onion or continuously cropped, and early-sown cotton fields generally suffer more severe occurrence.

3. Damage caused by onion thrips to crops

Both adults and nymphs use their rasping-sucking mouthparts to suck plant sap. Among vegetables, Welsh onion suffers the most severe damage. After the leaves are damaged, many fine, long grayish-white spots or yellow spots form. In severe cases the leaves lose turgor and droop, and even become twisted and yellow. At moderate occurrence, Welsh onion can suffer 30% loss, and at severe occurrence it can cause 50% loss, affecting the yield and quality of Welsh onion. In cotton fields, it mainly damages the cotyledons, young true leaves and growing tips of cotton seedlings. After small leaves are damaged, silvery-white patches appear, and in severe cases the cotyledons wither, scorch and shrink. After true leaves are damaged, yellow patches appear, and in severe cases they scorch and split. Before true leaves emerge, after the growing tip is damaged it turns black and withers and falls off, and the cotyledons become enlarged, becoming a “male cotton plant” that cannot grow into a seedling (that is, a headless cotton plant), and it soon dies; if damage occurs after true leaves appear, a “multi-headed cotton plant” forms, with branches and leaves growing in clusters, affecting the later plant shape and leading to reduced yield; when flowers and buds are severely damaged, they can also fall off.

Onion thrips causes relatively great damage to crops such as Welsh onion, so prevention and control should be the main approach. In early spring, clear weeds and dead branches and fallen leaves in the field and around it, and take them out of the field for centralized destruction to reduce the insect population base. In dry weather, water and weed frequently, which can reduce the damage.