Winter Grain Mites

Joanne Whalen, Extension IPM Specialist; jwhalen@udel.edu and Gordon Johnson, Extension Ag Agent, Kent Co.; gcjohn@udel.edu

A number of fields were sprayed for winter grain mites in Virginia in late December and early January. In early February, a number of fields were also sprayed on the eastern shore of Maryland. Although we are now past the first generation, a second generation in March and April could cause additional damage. The following is an overview of this pest including pest identification, biology/life history and management options written by Dr. Ames Herbert from Virginia Tech.

“Winter grain mites attack small grains, including wheat, barley, and oats. Other hosts include grasses, especially bluegrass, bentgrass, ryegrass, and fescue. The mite also infests and damages legumes, vegetables, ornamental flowers, cotton, peanuts, and various weeds. Adult mites are about 1 mm long, black, with red legs and are fast moving. They quickly run to ground cover when you approach plants. As the name implies, they are winter pests. There are two generations per year. The first develops from over-summering eggs. Development begins after the onset of favorable temperature and moisture conditions in late September and October with populations peaking in December and January. The second generation develops from eggs laid by the first generation reaching maximum infestation density in March and April. Populations then decrease as temperatures exceed the range of tolerance. The females of this generation lay aestivating or over-summering eggs.

“Temperature and moisture are the most important factors influencing mite development and abundance. Cool rather than warm temperatures favor their development. Egg-laying is heaviest between 50° and 60°F; the optimum conditions for hatching are between 44° and 55°F. When temperatures drop below or rise above these ranges, the mites stop feeding and descend to the ground or burrow into the soil. Mite activity in the spring drops rapidly and the eggs fail to hatch when the daily temperature exceeds 75°F.

“Aestivating (over-summering resting stage) 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 four or five inches to find mites. The mites are not harmed by short periods of sleet or ice cover or by ground frozen to a depth of several inches. The larvae become very active soon after hatching and begin to feed on the sheath leaves or tender shoots near the ground. The larvae as well as the adults feed higher up on the plants at night or on cloudy days. As the sun rises, the mites descend the plants and seek protection during the hot part of the day on the moist soil surface under foliage. If the soil is dry and there is little foliage cover, they dig into the soil in search of moisture and cooler temperatures. At sunset and thereafter the plants become covered with feeding mites where, with the aid of a searchlight, they can be observed feeding at all hours of the night.

“Dispersion from field to field may occur by transportation of aestivating eggs or mites on grain stubble or leaves, on soil adhering to implements that are moved about, or on forage or straw carried from infested fields in livestock feeding operations. Aestivating eggs may also be transported on debris by wind, and local distribution may occur by adult migration. Such migrations to grain fields may take place from fencerows or other uncultivated areas. Heavily infested fields appear grayish or silvery, a result of the removal of plant chlorophyll by mite feeding. When high infestations feed on the plants for several days, the tips of the leaves exhibit a scorched appearance and then turn brown, and the entire plant may die. These mites do not cause the yellowing characteristic of spider mite feeding.

“Many of the infested plants do not die, but become stunted and produce little forage or grain; damage on young plants, however, is more severe than on large, healthy ones. Damage may also be greater in plants stressed by nutrient deficiencies or drought conditions. There are two types of damage to the small grains, namely, reduced amount of forage throughout the winter and reduced yields of grain in the spring and summer. Cropping practices have a marked effect upon the occurrence and damage caused by the winter grain mite. Injury by this mite may be prevented by crop rotation, that is, by not planting small grains more than two years in succession.”

Although we have no experience with winter grain mite control, materials that have appeared to provide control in areas to our south include the pyrethroids (Warrior, Mustang MAX) and certain organophosphates (dimethoate). Be sure to follow the rates and usage restrictions on the labels.

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