04/30/2026
Mites in Winter Wheat Fields
There have been reports of mite damage in winter wheat fields in Montana this spring. The Schutter Diagnostic Lab has received samples of winter grain mites from a field in Cascade County, and brown wheat mites from a field in Gallatin County. These cool-season mites are more active during the spring and fall. They puncture the leaves of wheat and other grasses as they feed, sucking juice from the plant tissues. Feeding damage can cause the leaf tips to die, and extensive feeding may cause stunting, yellowing of the leaves, or death of the wheat plant. Damage may be mistaken as winterkill.
Mite Biology/Life Cycles
Winter grain mites (Penthaleus major) haven't been documented in winter wheat in Montana until this year. However, their presence in Montana wouldn't be surprising. This February they were documented in wheat fields in Eastern Washington and they are known to be distributed throughout the Unites States where continuous small grain production occurs. These mites are more common after short, mild winters.
Winter grain mites complete two generations per year. The first generation emerges in the fall -when temperatures are cooler and soil moisture increases- from dormant summer eggs. Eggs are laid on leaf tissue, stems, or in the soil near the base of plants. Adult populations peak during the winter. Eggs of the second generation develop during winter and early spring. A second generation emerges in early spring. Females from this generation lay the over-summering eggs that remain dormant through hot, dry conditions until favorable conditions return in the fall. These summer eggs are adapted to survive heat and drought. Emerged nymphs are bright orange. As mites molt into adults, their bodies become dark brown to black, while their legs maintain a bright orange coloration. Adult mites are approximately 1 mm long.
Winter grain mites feed on leaves at night or on cloudy, overcast days. During warm, dry weather, the mites move down the vegetation and seek protection in the cool soil where there's more moisture. Fields with lighter textured soils (sandy or loamy) may favor mite activity due to improved mobility and microclimate conditions.
Brown wheat mites (Petrobia latens) are common on drought-stressed winter wheat. Their populations tend to increase in Montana during prolonged drought cycles that extend for many years. Other plant hosts include alfalfa, barley, and a wide range of vegetable crops. These mites are vectors of Barley Yellow Streak Mosaic Virus in barley; this virus can cause chlorotic streaks/stripes on barley leaves and may stunt plants.
Brown wheat mites lay red eggs in spring and early summer, producing multiple generations. As the summer warms up and the soil dries out, they produce white eggs, which don't hatch until cooler fall weather arrives. The bodies of nymphs and adults are reddish/brown with light orange legs. The front legs are about twice as long as the other legs.
Scouting for Mites
Winter grain mites move down to the soil on windy or sunny days. The best time to scout for nymphs/adults is on cloudy, cool days, early in the morning, or in the evening when they're most active on the leaves. Using a magnifying glass, look for kidney-shaped eggs clustered on stems or on roots near the crown. The eggs change color after they're laid, changing from clear to yellow to reddish-orange after several days.
Brown wheat mites feed on leaves during the day, then move down into the soil at night. The best time to scout for nymphs and adults is mid-afternoon on warm, calm days. Using a magnifying glass, look for red eggs attached to soil particles near the base of the plant (look for white eggs in mid to late summer).
Mite Management
Mites can survive freezing temperatures during a mild winter, but heavy spring rains or irrigation can wipe out populations.
These mites are strongly associated with fields that are in continuous grain production with minimal tillage. Crop rotation with non-host, non-grain crops is an important long-term management tool to break the cycle. Also, mites can survive on volunteer wheat in fallow fields, and they may migrate into cropland from grass in ditches or pastureland.
Economic thresholds haven't been established for winter grain mites. For brown wheat mites, the economic threshold is not well defined either, but treatment can be considered if populations reach several hundred mites per row-foot and plants are showing active stress.
Chemical control: Dimethoate, a systemic insecticide with miticidal activity, can be effective when applications coincide with active mite feeding. Remember, winter grain mites are most active under cool, moist, and overcast conditions, but brown wheat mites are active on warm, calm days. It's important to know what mite pest is in your field so pesticide applications are applied at a time that will maximize their efficacy.
In contrast to brown wheat mite, the winter grain mite is not listed on the dimethoate label. Under FIFRA Section 2(ee), a pesticide can be used to control a pest not listed on the label if the crop is listed on the label. If the crop is not listed, the product cannot be used on that crop. Applications must not exceed labeled rates and must follow all other label directions. When used for an unlabeled pest, the applicator assumes responsibility for effectiveness and any potential risks.
Additional Resources
Mites in Wheat Fields
Brown Wheat Mite (MSU Extension MontGuide)
Winter Grain Mite
Wheat Mites
Please don't hesitate to email or call us if you have any questions!
Marni Rolston, Arthropod Diagnostician, Montana State University Schutter Diagnostic Lab: [email protected]; 406-994-2029
Tiziana Oppedisano, Agronomic Entomologist, Montana State University: [email protected]; 406-994-3560
From top:
Figure 1. Brown wheat mite damage in a winter wheat field (left) and feeding damage (right) in Gallatin County, April 2026. (Photo credit: Dylan Grubb)
Figure 2. Winter grain mite damage in winter wheat collected in Cascade County, April 2026. (Photo credit: M. Rolston & T. Oppedisano)
Figure 3. Mite adults. Winter grain mite (left) and brown wheat mite (right). (Photo credit: Modified from Oklahoma State University)
Figure 4. Winter grain mite eggs clustered around the crown and roots of winter wheat. Left: egg masses (red arrows) and newly hatched bright orange nymph (yellow arrow). Right: close-up of egg masses. (Photo Credit: M. Clark & T. Oppedisano)
