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.

Pioneer Field Day

Today I'm at FEEL talking about insects and IPM. Should be a great day for extension!

Is your "spider sense" tingling for mites this year?

Do things seem a little too quite in your fields this summer? Maybe you've been out looking for plant pests, but haven't seen any major activity? I would strongly encourage everyone to get out and start looking for twospotted spider mites in soybean and corn, especially if your fields are under drought stress. Finding spider mites takes a little extra time, but you don't want to wait until it is too late to make a treatment decision.

Twospotted spider mites are often surrounded in webbing as they build colonies. Heavily infested plants look dirty and covered in webs. Look on lower leaves for initial infestations. Photo by David Cappaert, Michigan State University.

Leaves with heavy stippling is an indication spider mites have been feeding for a long time. Eventually the leaves will turn yellow, then die and fall off the plant prematurely. Photo by Whitney Cranshaw, Colorado State University.

Spider mites have many overlapping generations of adults, nymphs and eggs. Photo by Brian Lang, ISU Field Agronomist.

This photo was taken in Richardson County, Nebraska on 6 July 2012 by Tracy Cameron. He has been monitoring this field for over a month. Spider mites have spread throughout this soybean field and caused discoloration and leaf drop. 

Some of you may remember the dreadful summer of 1988, when hot and dry weather ravaged the midwest. Our field crops suffered from a horrible spider mite outbreak and farmers lost a lot of yield. I hope spider mite infestations don't get that bad this year. But it is important to be proactive with this pest and not wait for a field-wide disaster. Read this ICM News article for management information.