Matthew Schuster, Extension Educator, Crops
In late-May and early-June of 2003 a pest called the Brown Wheat Mite was confirmed in wheat and barley fields in Minidoka County. This is the first time that this pest has been seen in that area and since the first confirmation it has now been found in Cassia and parts of Jerome and Lincoln counties.
This mite is common in Montana; therefore, I am basing this article off of their information and what we have observed thus far.
The Brown Wheat Mite is oval and dark with a reddish stripe down the back. In addition, the front legs are about twice as long as the other three-pair. It typically shows itself after several years of drought conditions.
The Brown Wheat Mite prefers dryland or surface irrigated fields. Sprinkler irrigated fields do not show significant, if any, mite damage. And according to Montana State University, a ˝-inch of overhead water is just as effective as an insecticide application. Consequently, the mite has not been a problem in sprinkler-irrigated fields.
The resulting damage from this mite looks like drought stress. Plants in sections of the field turn brown and eventually dry up. To confirm that the pest is present in your field, locate areas that appear to be drought stressed. Carefully place a white sheet of paper at the base of the plant and tap the leaves. The mites will then fall onto the paper and can be observed using a magnifying glass.
The mites will immediately drop once the plant is disturbed so don’t expect to find any mites if you pull the plant from the ground.
The threshold for treatment is several hundred mites per row-foot in early spring. However, I only saw a few fields at that high of level and it wasn’t until early- to mid-June, and the damage was already significant. At that point an insecticide treatment may not be economical, or possible, if the insecticides pre-harvest interval has been reached.
Egg color is another indicator of how long the mite will continue to be a problem. The presence of red eggs indicates that another generation will feed on the crop. However, once white eggs are seen on and below the soil surface the mite is entering a resting stage and should not be a problem for the remainder of the season.
One of the main concerns related to this pest is that it is known to transmit the barley yellow streak mosaic virus (BaYSMV). Montana has documented a yield loss of up to 30% due to this disease. Characteristic symptoms of the disease include yellow to gray-white streaks between the leaf veins that may be localized on one side of the midrib.
Barley samples from Minidoka County in 2003 were sent to MSU for analysis and BaYSMV was not found in any of the samples.
As far as a recommended course of action, I wish I had one. It is not known whether or not destruction of residue, either by burning or sulfuric acid, will reduce the mite population. Montana has indicated that residue destruction by burning has had variable results in reducing BaYSMV, which has not been confirmed in our area.
In the spring carefully monitor wheat and barley fields that are grown under dryland or surface irrigation in order to prevent significant plant damage. Be on the look out for BaYSMV. If it is suspected call your local county extension office. And finally, do not follow barley after barley.
For more information check out Montana State Universities extension bulletin titled “Brown Wheat Mite” at their website