Brown wheat mites (BWM) have been confirmed in several sites in western South Dakota.
These spider mites belong to the family Tetranychidae, the same mite family as the two-spotted spider mites. They also do well in dry conditions, says Ada Szczepaniec
assistant professor, SDSU Extension entomologist.
"The lack of moisture likely contributed to outbreaks of these mites on wheat. BWM populations will decline rapidly if it rains; another good reason to hope for some moisture to aid the wheat crop this year," Szczepaniec said.
BWM are small (1/16 of an inch), round, dark-colored mites with long pair of front legs. Szczepaniec explains that BWM can attack several crops but they can be particularly troublesome on small grains planted in the fall.
"These mites feed on the plants during the day, and spend the nights in the soil at the base of wheat plants. This is also where the mites lay their eggs, on particles of soil at the base of the plants," she said.
These mites prefer arid conditions but do not do well in very hot temperatures, and they will enter diapause (i.e., inactive, 'resting' period) during the summer. The mites over-summer as white eggs in the soil; they will hatch when temperature and moisture levels are adequate, and females from that generation will lay red eggs.
"This is relevant because if most of the red eggs are already hatched and white eggs are present, this indicates that the population is in natural decline and chemical treatment may not be economically sound. If, on the other hand, most of the red eggs are not hatched, the mites are likely to keep increasing in numbers. A good 10X hand lens will be sufficient to see the eggs," Szczepaniec said.
Scouting for these pests is recommended during mid-afternoons on calm days. Because heavy BWM infestations are rare, Szczepaniec says there is not a well-defined threshold for management.
Chemical control is recommended when at least several hundred mites per row-foot in the early spring. Szczepaniec says this decision should be made cautiously because these mites are associated with drought-stressed plants.
"If it rains, the BWM populations will decline; if it does not, the crop yield may be already affected by the drought and managing BWM will not affect it," she said.
Taking note of the eggs may be helpful in management decisions as well: if white eggs are present and most of the red eggs have hatched, BWM populations are in decline and treatment is not recommended. If chemical control is deemed necessary, products containing dimethoate or chlorpyrifos are recommended.