Winter grain mites in wheat
There have recently been some reports of winter grain mites in wheat fields in Kansas. Winter grain mites are most prevalent on fields that have been dry. For now, growers should be scouting their fields and note any locations that have significant populations of winter grain mites so that they can monitor them in spring, especially if adequate moisture isn’t received in the next few months.
For more information on control options, see K-State Research and Extension publication Wheat Insect Management 2017, MF-745 at: https://www.bookstore.ksre.ksu.edu/pubs/MF745.pdf
The following information about winter gain mites is from a K-State Entomology fact sheet: http://entomology.k-state.edu/extension/insect-information/crop-pests/wheat/winter-grain-mite.html
The winter grain mite is known to be a pest of small grains and grasses throughout the temperate regions of the world. Heavily infested fields take on a grayish or silvery cast as a result of the puncturing of plant cells as the mites feed. Many times the infested plants do not die, but become stunted and produce little forage or grain. Damage on young plants is more severe than on large, healthy plants.
Figure 1. Wheat plants stunted by winter grain mite feeding. Photo courtesy of Department of Entomology, K-State Research and Extension.
Figure 2. Wheat damage caused by winter grain mites. Photo courtesy of Department of Entomology, K-State Research and Extension.
Winter grain mites have a dark brown to almost black body with conspicuous reddish-orange legs (Figure 3). Their front legs are longer than the others, but not as pronounced as on the brown wheat mite.
Figure 3. Close-up photos of winter grain mite. Photo courtesy of Department of Entomology, K-State Research and Extension.
These mites generally have two generations per year. The first begins in September or October as weather conditions become favorable for the over-summering eggs to hatch. Populations reach a peak in December or January. The second generation develops from eggs laid by the first generation and reaches its maximum density in March or April. Populations then decrease as temperatures exceed the mite's range of tolerance. Females of the second generation lay over-summering eggs. Larvae become active soon after hatching and begin to feed on the leaf sheaths and tender shoots near the ground. The nymphs, as well as the adults, feed higher up on the plants at night and on cloudy or cool days.
As the sun rises, the mites descend the plants and seek protection on the moist soil surface under the foliage during the hot part of the day. Mites damage plants by puncturing individual cells, which cause the leaves to take on a silvery-gray appearance. Heavily damaged leaves will have brown leaf tips. If the soil is dry and there is little foliage cover, mites will crawl into the soil in search of moisture and cooler temperatures.
Temperature and moisture are the most important factors influencing mite development and abundance. Cool, rather than warm, temperatures favor their development. Activity of these mites is the greatest between 40 and 70 degrees F. Over-summering eggs do not hatch in the fall until rains provide adequate moisture. On hot, dry days it may be necessary to dig into the soil to a depth of 4 or 5 inches to find the mites. The mites do not seem to be harmed by high humidity, rainfall, short periods of sleet or ice cover, or by ground frozen to a depth of several inches in the fall. However, heavy spring rains may cause mite populations to disappear.
Fields with loose, sandy or loamy soils are more at risk than those with hard, clayey soils. Significant infestations are more common in central Kansas. Because fall populations develop from eggs laid the previous spring, problems are worse in continuous wheat. Crop rotation is helpful in reducing problems, although field borders may be affected when mites migrate from wild grasses. Control may be necessary if large portions of a field show symptoms and mites appear abundant relative to the amount of plant growth. Persistent dry conditions can lead to cumulative damage and plant recovery is often dependent on available moisture.
Jeff Whitworth, Extension Entomology
Holly Schwarting, Entomology Research Associate