I have received reputable reports from around the state that some field scouts and farmers are confusing soybean thrips with soybean aphids, and maybe some of these fields also are being sprayed. Soybean thrips almost never injure soybeans in Iowa and then probably only during dry weather. Correct identification of each insect is critical if the field is going to be sprayed with an insecticide because fields should not be sprayed for soybean thrips.
A thrips (and yes, one is called a thrips and two or more are called thrips) is elongated and shaped like a cigar with thin wings. An adult soybean thrips has a dark head and three dark bands on its body while larval thrips lack the band and are yellow or orange in color. Soybean aphids, in contrast, are green with no bands and are shaped like a pear.
|Soybean thrips have a dark head, three dark bands on the body and are cigar-shaped.|
|Two soybean leaflets with soybean aphids. A population of 115 soybean aphids is shown on the underside of the left leaflet.|
Adults of both of these insects are very small, each being about 1 millimeter in length. Soybean aphids can occur in colonies of hundreds on a plant while soybean thrips seldom exceed half a dozen on a leaflet.
Additionally, movement on a soybean leaf can help you identify these two insects. Soybean aphids are mostly sedentary--content to feed in one location and when they do walk, it is very slowly. Soybean thrips seem to be constantly on the move, running rapidly between the soybean pubescence on the underside of a leaf.
This article originally appeared on page 88 of the IC-492(15) -- July 12, 2004 issue.