Soybean Thrips

Sericothrips variablilis Beach

Appearance and Life History

Adult and nymphs greatly magnified Adult and nymphs greatly magnified
Photo by J. Obermeyer

There are many species of thrips, some are serious pests of fruit, vegetables, flowers, and field crops. Thrips' mouthparts make them unique in that they rasp and puncture plant cells then suck up the exuding sap.

Thrips are minute, slender-bodied insects ranging from 1/32 to 1/5 inch (0.8 to 5 mm) in length. Their wings, when present, are fringed with close-set long hairs. Although rarely noticed, thrips are probably the most numerous insects in soybean.

The adult female inserts oblong eggs singly into the leaves of the plant upon which she feeds. Young thrips go through 4 wingless stages between hatching and adulthood. An entire life cycle requires only 2 to 4 weeks. Thrips are present throughout the summer. Many generations occur each year.

Soybean Thrips Life Cycle

Damage

Trifoliolate with thrips damage Trifoliolate with thrips damage
Photo by Purdue University

Thrips make tiny, linear, pale-colored scars on soybean leaves where they penetrate individual leaf cells and feed on the contents. They usually feed on the undersides of leaves, much of which occurs along the veins. When leaves are heavily infested, the feeding scars may be so numerous that a mottling look appears along and between the leaf veins and leaves may become crinkled in appearance.

Soybean is particularly susceptible to thrips damage early in the growing season from growth stages VE to V6. Dry, hot weather increases the threat of damage.

Sampling Method

If one finds or suspect thrips' damage during early field visits, sample in 5 areas of the field to ascertain the extent of the population and damage. In each area, randomly select the first plant to be sampled and remove the fifth trifoliolate down from the uppermost node from 10 consecutive plants. On younger plants, which have not reached at least the V5 stage, remove the lowest trifoliolate for inspection.

As you pick the last trifoliolate in each sample area, use a hand lens to carefully examine the underside of the leaves and count the number of thrips present. Repeat this sampling pattern for each plant to be examined. Determine the average number of thrips per trifoliolate.

Also estimate the percentage of foliar discoloration exhibited by each plant. Consider that drought-stressed plants may be exhibiting symptoms from other factors, eg., chemical injury, etc.

Management Guidelines

Soybean Insect Control Recommendations: E-series 77-W (PDF)

Thrips rarely cause economic damage. However, yields may be significantly reduced if soybean is under moisture stress early in the growing season and the thrips population is high. If over 75% of the sampled trifoliolates are damaged and there is an average of 8 thrips per leaf, treatment may be advisable.

If control is necessary, contact your state Cooperative Extension Service or click here for control materials and rates.