There have been a few reports of Wheat streak mosaic virus (WSMV) and Triticum mosaic virus (TriMV) from winter and spring wheat throughout South Dakota. Both viruses are vectored by the wheat curl mite. Although wheat curl mites may be observed in developing wheat fields, there are currently no pesticides labeled for their management.
Wheat Streak Mosaic Virus and Triticum Mosaic Virus
Wheat streak mosaic virus is the most common virus detected in wheat. Triticum mosaic virus is occasionally detected, and it is usually found co-infecting wheat with WSMV. A few samples this spring tested positive for TriMV, with the majority also being positive for WSMV. Yield losses caused by these viruses depend on the time of infection.
Fall infections lead to greater yield losses in winter wheat and an entire field can be lost due to WSMV (Figure 1). The increased WSMV incidence this spring may be due to infections that took place in fall. A warm fall, like that of last year, can lead to higher WSMV incidence. Viral infections are systemic, and once plants are infected they remain infected throughout the season.
Common Mites Found in Wheat
The two mites most frequently observed in wheat are the brown wheat mite and the wheat curl mite. However, only wheat curl mite transmits WSMV and TriMV.
- Brown wheat mites are very small and are dark red-brown in color (Figure 2A). These mites have eight legs that are light yellow-orange. Brown wheat mites are distinguished by their front pair of legs, which are nearly twice as long as the other three pairs. Because of their small size, identification of this pest may require a hand lens. Feeding by the brown wheat mite produces stippling (small white spots) on the leaves. This feeding injury may cause the leaves to turn white or brown, and is sometimes confused with symptoms of drought stress.
- Wheat curl mites are very small and white in color (Figure 2B). They can be identified by their elongated cigar-shaped body which is approximately 1/100th of an inch long. Identification of this pest requires a hand lens.
Figure 2. (A) Brown wheat mite, and (B) Wheat curl mite. (Photos courtesy of Kansas State University and University of Nebraska)
When scouting a field for the presence of aphids or mites, start at one side of the field and walk in a W or zig-zag pattern and randomly choose plants to examine. Wheat curl mites are commonly found on the stems and undersides of leaves. Plants should be examined from several locations on the W or zig-zag pattern.
For the wheat curl mite, management recommendations focus on preventing populations from establishing in a field. It is important to reduce the presence of alternate hosts around the field and eradicate volunteer wheat within the field prior to planting (Figure 3).
These hosts can act as a ‘green-bridge’ between the maturing wheat and the newly emerged wheat. Practice crop rotation with broad-leaf crops to reduce wheat curl mite survival. Delaying fall planting in areas where WSMV is severe can reduce chances of infection in the fall. There is nothing that can be done in-season for plants that are infected with viruses.