Montana State University Extension Agent Tom Allen in Liberty County said the mites are causing significant damage to winter wheat, to the point that there are producers that are considering tearing their wheat out. Extension Agent Judee Wargo in Chouteau County has heard from farmers that the wheat does not appear to be growing.
Because brown wheat mites are such a sporadic problem, there are many aspects of controlling this pest that have not been researched, leaving chemical control as the only effective management practice, says MSU Extension Entomology Specialist Sue Blodgett.
Even the threshold of when the number of mites is high enough for it to be cost effective to treat them has not been well defined, said Blodgett. However, the "High Plains IPM Guide" states "at least several hundred mites per row-foot of crop in the early spring" are necessary for it to be cost effective to treat, she said. The best "insecticide" is a rainfall of at least one-third of an inch, which would reduce brown wheat mite numbers.
The choice of whether it is economical to treat brown wheat mite is complicated by the fact that the mites tend to become a problem during drought conditions, suggesting that the grain value even if saved may be in question. A plant weakened by cold night temperatures that Montana recently experienced, also may be more susceptible to mite damage, Blodgett added.
The mite also can be a disease carrier, potentially bringing barley yellow streak mosaic virus to the crop. During 1983-1987 the disease reached epidemic proportions in the Golden Triangle. Since 1988 the disease has spread through north central Montana and has been reported in Big Horn, Yellowstone, Gallatin counties and in southeast Idaho.
Early disease symptoms may be confounded with mite feeding damage as leaves take on a yellowed appearance. As the disease progresses, the more characteristic symptoms, yellow to gray-white streaks on the leaves parallel to the leaf veins, begin to appear. Streaks may be localized on one side of the midrib. In severe cases brown streaks of necrotic tissue appear on leaves and plants may die. Moderate to severe stunting accompanies the viral infection.
Research conducted during the 1980s suggested that early-planted barley was impacted less by barley yellow streak mosaic virus than later emerging barley during drought years. Barley following barley increases disease development. Though it is common to try to reduce transmission by burning crop residue, the effectiveness of burning is variable, said Blodgett.
The mites are tiny, spider-like creatures that are about as big as the period at the end of this sentence. They are oval and dark red-brown with lighter red-orange legs. The front legs are about twice as long as the other three pairs of legs. Brown wheat mites can be seen by eye but accurately counting can be aided by tapping plants over white paper and counting those that become dislodged. Volunteer wheat is an important reservoir for brown wheat mites and should be examined for their presence.
Female brown wheat mites are known to lay two types of eggs, both attached to soil particles near the base of the grain plant. In spring and early summer, mites lay red eggs that hatch after a brief incubation period, producing multiple, overlapping generations. As summer progresses with warm, dry weather, the brown wheat mite begins to produce white eggs. The white eggs are a resting stage and do not hatch through the summer. The presence of white eggs indicates that mite activity is declining. Cooler, fall weather triggers the white eggs to hatch.
It is believed that the brown wheat mite can reproduce without mating, because male mites have not been found, said Blodgett.
Sue Blodgett (406) 994-2402