Friday, July 27, 2012

Finding mites or something else in soybean?

This month, I've really stressed the importance of scouting for spider mites in corn and soybean. Some farmers have starting treating both crops for field-wide infestations. Many people don't have a lot of practice looking for this tiny pest (does 1988 ring a bell?). But with a little practice, most people can easily recognize these little specks moving on leaves, especially if accompanied by webbing and leaf discoloration. A hand lens may be required to see the eggs, nymphs and adults if your vision isn't great.

As more people are actually looking for tiny pests, they are noticing other things moving around at the same time. I've had LOTS of questions about these "long, fast moving insects" on soybean leaves in combination with spider mites. I am fairly certain people are seeing thrips [side note: thrips is singular and plural, kinda like moose]. They are fairly common to see every year, with soybean thrips the main species in Iowa. We have predatory thrips and other plant-feeding species here, too. I can try to point out body descriptions to distinguish the two pests.

Mites would be smaller (less than a millimeter or 1/20 inch) than thrips and round. They are light brown in color with two dark spots on the body. Mites are arachnids and are wingless as adults. Mites live in colonies and will produce a dirty webbing on the underside of leaves. In hot temperatures, spider mites can produce >15 overlapping generations each summer. See a previous ICM News article for management recommendations.

  Adult twospotted spider mites have 8 legs and two dark spots on the body. Photo by Frank Peairs, Colorado State University.

Thrips are cigar-shaped insects, ranging from 1-5 millimeters long (up to 1/5 inch). They are varible in color, including dark or light brown, orange and translucent. Adults can have small, fringed wings. Thrips' have unique mouthparts; they rasp (stab) and puncture plant cells then suck up the sap. Thrips can produce 3-4 generations per year. They can be found anywhere on the plant but don't usually form colonies, and are very fast-moving insects. Thrips rarely cause economic damage in Iowa, but their feeding damage can be more noticeable in drought-stressed fields. Consider a foliar insecticide if 75% of leaves show discoloration and there is an average of 8 thrips per leaf. See Purdue University's website for more info.

Soybean thrips adult and nymphs. Photo by John Obermeyer, Purdue University.

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