Wednesday, July 18, 2012

Soybean Thrips on Soybean

Doug Johnson, Extension Entomologist

Several people have contacted me concerning the presence of thrips on soybeans. Thrips are a common pest of soybeans but rarely do they cause economic damage. Actually, given the drought stress that our production season has endured, I am surprised that this has not occurred earlier.

 Soybean thrips are very small slender insects. You will need a hand lens to see them. The immatures are yellow, while adults are black with yellow bands. These pests are common on flowers and weeds. They feed by rasping off the outer layer of the leaves and sucking the juices. This leaves a silvery appearance to the leaf surface. “Cupping” or other similar leaf distortion is often associated with thrips feeding. Another dead giveaway to their activity is the presence of black “tar spots” on the leaves. This is thrips excrement and the descriptive name is quite accurate. You will see lots of tar spots if there are many thrips around.

Thrips rarely cause economic damage. Nevertheless, yields may be reduced if plants are under moisture stress and thrips populations are high. If over 75% of leaflets are damaged and there are an average of 8 thrips for every leaflet sampled, treatment may be advised.

Following is a sampling and decision-making aid. Examine at least five locations in each field. At each location examine 10 individual plants. On each plant examine the fifth trifoliate below the the uppermost node. If the plants are not developed to R5, then examine the lowest trifoliate. Examine each leaflet using a hand lens. Count and record the number of damaged leaflets and the number of thrips on each leaflet. An insecticide application may be useful if:

1.) Plants are under drought stress

2.) 75% of all leaflets examined are damaged,

3.) on average each leaflet has 8 or more thrips present.

All three conditions should be met to consider an insecticidal application.

Insecticides for control of thrips are not found in our insecticide recommendations. However, many of our commonly used insecticides lists thrips control on their labels. These include but are not limited to products containing:

Acephate (Orthene & others), beta-cyfluthrin (Baythroid), bifenthrin (Brigade & others), lambda cyhalothrin (Warrior & others), carbaryl (Sevin & others), cypermethrin (Mustang & others), and many others.

Use of Trade name does not constitute a recommendation. These products are not ranked in preference or efficacy. The information is supplied for the readers convenience.

Photo Credit: Marlin E. Rice, Iowa State Univ.

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