Areas of Oklahoma with dry conditions have begun to see to some visible wheat injury due to brown wheat mite and winter grain mite. Late last week, Darrell McBee, Harper County Ag Extension Educator, indicated to me that he had been fielding quite a few phone calls in regards to mites in wheat. Other areas, including the panhandle and areas closer to the north central part of the state, have also reported some mite injury.
The remainder of this article was written by Dr. Tom Royer, Extension Entomologist. He has provided comments below on how to identify these two different mite species, what symptoms to look for, and how to control them:
There are two common mites that can injure wheat, the brown wheat mite and the winter grain mite. Producers need to remain alert so that they don’t mistake damaged wheat from small grains mites for drought or virus disease.
Brown wheat mite with oversummering egg
Brown wheat mite is small (about the size of this period.) with a metallic brown to black body and four pair of yellowish legs. The forelegs are distinctly longer that the other three pair. Brown wheat mites can complete a cycle in as little as 10-14 days. Oklahoma experiences multiple generations of brown wheat mite that usually peak in spring and the last generation occurs in April. At that time, females produce a whitish egg that will over summer.
Winter grain mite
Winter grain mite egg
Winter grain mite is small (about the 1 mm long) with a dark blue to black body and four pair of orange-red legs and a small reddish spot on the top of its abdomen that can be seen under magnification. WGM eggs are kidney-shaped, and change from clear, to yellow to reddish-orange after several days. They are laid on leaf blades and stems or the roots near the crown. Besides wheat, many grasses serve as host plants, including barley, oats, ryegrass, and fescue. We typically experience two generations each year a fall generation and a winter generation that cycles out in March.
Leaf stippling from brown wheat mite feeding
Field infested with winter grain mite
Both mites feed by piercing plant cells in the leaf, which results in “stippling”. The leaves take on a characteristic brown-grayish or cast and could be mistaken for injury due to herbicide. These mites are more likely to cause injury in wheat that is stressed from lack of moisture or nutrients.
Winter grain mite hiding in residue
Brown wheat mites are not light sensitive, but are vulnerable to driving rains of more than 0.25 inches, which tend to reduce populations. Winter grain mites are more tolerant of rainfall, but are very light sensitive and tend to avoid bright, sunny days and windy days, so adjust your scouting accordingly. It is best to scout for winter grain mite on still, cloudy days or early morning/late evening. On sunny or windy days, they hide under the soil surface (up to a couple of inches) or congregate under dirt clods. Both mites are associated with continuous wheat production. Research suggests that brown wheat mite can be economically treated when there are 25-50 mites per leaf in wheat that is 6-9 inches tall. An alternative estimation is “several hundred” per foot of row. The best recommendation for winter grain mite is to treat when plants show visible injury and there are still mites present.
Only a few insecticides include either mite species on their label. Work conducted by Dr. Gerald Wilde at Kansas State evaluated several insecticides for control of winter grain mites. Of those actually registered for winter grain mite, the insecticides dimethoate (Dimethoate and other generics) and chlorpyrifos (Lorsban and other generics) were effective. Other insecticides, lambda cyhalothrin (Karate and its generics) and beta cyfluthrin (Baythroid and its generics) were also effective at the high registered rate, even if they are not specifically listed on the label.
For more information on these mites consult fact sheet EPP-7093 Mites in Small Grains by clicking here. If you find active mite infestations in your field, consult fact sheet CR-7194 Management of Insect and Mite Pests in Small Grains for registered insecticides, application rates, and grazing/harvest waiting periods by clicking here. Both fact sheets can also be obtained from any County Extension Office, or found at the OSU Extra Website at http://pods.dasnr.okstate.edu.