Source: Janet J. Knodel, Extension Entomologist, North Dakota State University, Crop and Pest Report

Wheat curl mites were found in volunteer spring wheat and winter wheat samples collected last week near Berthold, N.D., in Ward County. Wheat curl mites vector wheat streak mosaic virus (WSMV) and High Plains Virus (HPV). The North Dakota Plant Diagnostic Laboratory confirmed the presence of WSMV in these samples, and some samples also tested positive for HPV. Additionally, a Hessian fly "flaxseed" pupa was collected from one ofthe samples. Fields with moderate to high WSMV symptoms have been reported in Renville, McHenry, McLean and Bottineau Counties (D. Waldstein, NCREC, Minot).

Wheat curl mites are white and carrot-shaped, but are microscopic (1/100th of inch) and not visible to the naked eye. These mites overwinter in all life stages, and have a short life cycle of only 8-10 days at 77 F. Hosts include wheat, corn, foxtail, millet and grassy weeds. Wheat curl mites feed deep in the curled leaves or whorls and are protected from pesticides. Injury from leaf feeding is minimal and secondary to the deadly vectored viruses. During heading, mites move up to the heads and can affect yields by 10 percent under very high mite populations. Winds readily disperse mites from an infested field to neighboring fields.

I've received many questions about how to control wheat curl mites with insecticides or acaricides in wheat. Unfortunately, we have NO insecticides or acaricides registered in wheat that will kill wheat curl mite. In the past, growers have applied Furadan (carbofuran) at winter wheat planting to control fall mite infestations and reduce the incidence of WSMV the following spring. The use of carbofuran on wheat was voluntarily cancelled by the registrant. Additionally, all tolerances for residues of carbofuran on food crops were revoked by the EPA, effective Dec. 31, 2009. Carbofuran CAN NOT be used on wheat. Neonicotinoid insecticides such as imidacloprid (Gaucho) and thiamethoxam (Cruiser) are systemic seed treatments commonly used for control of soil insects (like wireworms), but are NOT effective against wheat curl mites. In fact, field studies have shown that imadacloprid can increase wheat curl mite populations.

The bottom line is that there are no currently labeled insecticides or acaricides with proven efficacy against wheat curl mite. Please avoid ineffective and costly pesticide applications for wheat curl mite.

The best control strategy is breaking the "green bridge" by destroying living plant host material with herbicides or tillage for at least a two week period prior to planting winter wheat. Mites must have GREEN living host plant material to survive, and can only live for 48 hours off of a host. Herbicides that kill the mite's hosts quickly will lower the chances of mite survival. Herbicides are typically more effective in moist conditions, whereas tillage will be faster than herbicides
in dry conditions. Cool temperatures (40-50 F) will slow down mite reproduction whereas warmer temperatures (70-80 F) will speed up mite reproduction and increase the likelihood of dispersion to other fields.

Our observations have led us to believe that winter wheat planted prior to Sept 25, 2009, is at the highest risk for infection (D. Waldstein, NCREC, Minot). Early planting in fall does not break the "green bridge" and allows for a longer period for mites to develop and transmit viruses. It is best to avoid early planting of winter wheat in the fall. The warm September in 2009 and insulating snow cover this winter provided two additional factors that favored mite development and virus transmission.