Wheat curl mites causing problems

Wheat curl mites have shown up sporadically in upper Southeast wheat fields in recent years, but seemed to be a bigger problem for growers in 2009.

The microscopic insects vector wheat streak mosaic virus, which can lower yields by 30 percent or more in severe cases.

Wheat planted into wheat stubble or volunteer wheat in soybeans creates an ideal environment for wheat curl mites. “Once you get these microscopic critters you have to live with them because there is no feasible insecticide for control,” says Virginia Tech Entomologist Ames Herbert.

Infestations can build up quickly and are virtually invisible without the use of a microscope. Under ideal conditions, one adult theoretically could produce 3 million offspring in 60 days.

Wheat curl mites can survive winters in North Carolina, Virginia and into Kentucky, but survival is best during mild winters. They can become active and reproduce during warm days in mid-winter, though reproduction is slower during warm weather in fall and winter.

The most critical time for the mite is the period from wheat ripening until the emergence of next year's crop. Wheat curl mites must constantly be in contact with a growing wheat plant or another host.

Though these insect-like organisms — not true insects — can cause damage to the wheat plant, the biggest threat is in vectoring wheat streak mosaic virus.

North Carolina State University Small Grain Specialist Randy Weisz says this virus often occurs in tandem with spindle streak mosaic virus. Leaf symptoms for both diseases first appear in early spring, persist until warmer weather, and then diminish at temperatures above 75º F.

The diseases look similar, and sometimes plants are infected with both viruses. Some varieties are susceptible to one disease but not the other, which can help with identification, Weisz says.

Both diseases appear as irregular patches of yellow or pale green wheat, often in low, wet areas or drainage paths and areas around old building sites.

One of the biggest problems related to wheat curl mite and wheat streak virus is what we call dirty fields, says Herbert. The fields we see in southeast Virginia with the biggest problem is when growers plant wheat into wheat and there is a strong green bridge.

If there is a lot of grass in the field or you are planting into a field with volunteer wheat from soybeans, this is a potential problem. You can’t see the mites and rarely see mite damage until it’s too late. We don’t have insecticides to control these pests — the best treatment is to clean up your act before planting wheat, Herbert says.

Management practices suggested include:

• Volunteer wheat and grasses should be destroyed at least 10 days prior to planting in the fall to limit vector survival.

• Early planting of wheat is discouraged due to increase probability of fall infection of WSMV and many other disease-causing organisms of wheat.

• Select cultivars with low severity ratings where WSMV is perceived a threat to production.

• Control grassy weeds in stubble fields, in conservation reserve acres and in set-aside acres to reduce virus and mite populations.

• The use of acaricides or insecticides against the leaf curl mite has not resulted in effective control of WSMV. Cultural practices and resistant wheat varieties are the major tools in mite and disease management.

The best strategy for managing wheat curl mites and wheat streak mosaic virus is to plant wheat varieties resistant to the disease.

Weisz notes that varieties that are resistant to WSMV may not be resistant to spindle streak virus and vice-versa. He says a serological test may be necessary to determine which virus is dominant in a field. Once which virus is dominant is decided, a grower can determine which resistant varieties to plant.

Southern States 8302 is among the most consistently high yielding variety with some resistance to wheat streak mosaic virus over the past few years. USG 3209 is a high yielding, but less consistently high yielding variety, with good resistance to the virus.

In the case of wheat curl mite and wheat streak mosaic virus an ounce of prevention can turn into a big savings in the fall.

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