Many species of thrips (in the insect order Thysanoptera) infest corn, soybean and cotton crops. The distinguishing feature setting them apart from other insect pests is that thrips thrive on dozens of plant types; most crops, including vegetables, flowers, forages, field and orchard crops, are attacked by at least one species of thrips.
Identification and Lifecycle
Thrips are tiny, slender, agile insects. They rarely exceed 0.4 to 0.6 inches in length, depending on the species and the crop they infest. Typically, thrips vary in color from black, yellowish-brown, tan and sometimes orange. Their uneven mouthparts are cone-shaped, and wings are narrow and fringed with hairs. When thrips target host plants, they feed by gnawing (rasping) on leaves and plant tissues to release sap that they consume.
Thrips appear on growing plants primarily in spring and summer in most geographies. Identification and lifecycle characteristics of thrips vary somewhat by the host crop or plant they attack.
Grass thrips are the most common thrips species infesting corn. Grass thrips feed on leaf cells by penetrating leaves and sucking out the cell contents. Both immature thrips and adults will feed on leaves. Adult corn thrips appear black in color, while immature thrips are lighter in color and smaller in size.
Thrips often overwinter on wheat and maturing weeds. Adult thrips migrate into field corn in the spring and early summer. Adults mate, and females inject eggs into young corn seedling tissue. In about five days under warm weather conditions, eggs hatch as larvae and feed on the leaf surface by gnawing cells and sucking sap. Adults may produce several generations each year.
Thrips in soybeans are not often noticed, yet they are among the most numerous soybean insect pests. They are tiny insects with thin bodies. Adult soybean thrips differ from immature thrips because they fold two pairs of wings behind their backs unless in flight. Adults range in color from black to yellow.
Typically producing several generations per year, soybean thrips overwinter as larvae or adults on hosts such as weeds and neighboring crop residue. They move onto soybean seedlings immediately after emergence. Most soybean thrips develop from egg to adults in 10 to 30 days, depending upon air temperatures. It’s common for a female adult to hatch 30 to 300 eggs.
Soybean thrips also act as a vector for transmission of soybean vein necrosis virus (SVNV) from plant to plant. In recent years, the disease has been documented in at least 16 states, so it’s important to monitor for high thrips populations that can spread SVNV. The disease is a relatively new soybean threat; it was first detected in 2008 in Tennessee.
Thrips may be found in cotton plants throughout the growing season, but early-season seedlings are most susceptible during cool, wet weather. Common thrips species in cotton vary by region. Farmers should check regional guides and management recommendations for the thrips that dominate in their fields. Similar to thrips in other field crops, thrips in cotton use their rasping and sucking mouthparts to invade young seedlings and feed on the liquids in plant tissues.
Eggs hatch in about six days after the female thrips’ sharp laying tube inserts them into cotton plants. This is followed by two larval stages, which take six days. Another four days are needed for the pupating process. The lifecycle involves about 16 days from egg to mature adult. Each female produces 50 eggs or more, and their lifespan averages 35 days.
The tiny, long, white scars in plant tissue caused by thrips can make leaves turn gray and destroy photosynthesis. When corn plants are stressed for water during long stretches of hot, dry weather, the most damage from corn thrips can occur. Corn during the seedling and ear development stage are most vulnerable to thrips injury. Although corn thrips are considered an occasional pest, large populations of corn thrips can cause leaves to turn yellow and stunt seedling growth. Thrips can be found in corn throughout the growing season. Typically, thrips themselves do little damage, but they create penetration into corn kernels that allows Fusarium spp. infection and disease, resulting in ear rot.
The soybean seedling stage (VE to V6) presents the most risk from economic damage caused by thrips. White to yellow spots on soybean seedling leaves may appear when thrips are feeding. Poor plant vigor and stunting may result. Although rare, seedlings may die under high soybean thrips populations, which can cause leaf crinkling. The threat of soybean thrips damage increases during hot, dry weather.
The early, slow growth of the terminal bud makes cotton more susceptible to thrips yield damage than corn or soybeans. Cotton plants are most likely to be harmed by thrips from emergence to the third or fourth leaf stage. They may develop high populations in cotton grown near corn and small grains such as wheat. After the third or fourth leaf stage, cotton plants should be in a vigorous growth mode and relatively secure from thrips yield damage.
Feeding thrips strike cotton leaves and terminal buds. Stunted plants can result from thrips feeding, which destroys plant cells and causes crinkled leaves that curl upward. When populations are severe, terminal buds may be destroyed and excessive branching may slow cotton growth. Long-term research indicates thrips are best controlled early with a pre-applied seed treatment, an insecticide applied in-furrow at planting or a foliar insecticide applied as needed after seedlings emerge.