August 8, 2008

Wheat stem sawfly

Strong wheat prices have motivated some growers to seed winter wheat back into winter wheat stubble in order to increase their wheat acreage. While at first this may sound like a reasonable response to market signals, it is fraught with perils. Growers should think twice before succumbing to the siren call of seeding wheat directly into wheat stubble.

Continuous wheat can lead to a number of pest problems.

• Plant diseases such as root and crown rots, tan spot, cephalosporium stripe, scab (also known as Fusarium head blight), and wheat streak mosaic virus. (See Control of Volunteer Wheat Critical to Virus Management in this week's CropWatch.)

   

• Winter annual weeds such as downy brome, jointed goatgrass, feral rye, and mustards thrive in the second year of wheat following wheat. All of these pests have been problematic in winter wheat-fallow rotations and they will be more so in winter wheat seeded directly into wheat stubble. 

Insect Problems

The risk levels for several insect pests, for example cereal aphids and Hessian fly, may be increased in continuous wheat. The wheat stem sawfly, a growing insect threat in the Panhandle, is of greatest concern in wheat that is no-till seeded into winter wheat stubble. The larva of the sawfly overwinters in the base of the wheat stem. When it emerges next spring, it tends to infest plants at the edge of the adjacent winter wheat strips or fields; however, if wheat is seeded back into an infested field, the sawfly infestation can affect a much greater portion of the field because the fly does not need to move to an adjacent field to find wheat plants.

The sawfly larva cuts the stem at the base of the plant and these plants tend to fall over right before harvest, resulting in significant yield loss. While once limited to fields in western Banner and Scotts Bluff counties, in the last two years the wheat stem sawfly has begun to cause problems throughout Banner, west Box Butte, and Morrill counties. Problem fields in these counties have been limited to no-till wheat fields.

Although plowing can aid in the control of many of the aforementioned disease, weed, and insect pests, plowing has its own problems, including increased risk of soil erosion, increased loss of surface soil organic matter and stored soil water. It is far more advisable to avoid seeding winter wheat into wheat stubble and stick with a sound crop rotation that includes a summer crop, summer fallow, or both. While wheat prices are strong, so are prices for many other crops including proso millet, sunflower, and corn. All of these work well in a crop rotation of winter wheat-summer crop-fallow.

Drew Lyon, Extension Dryland Cropping Systems Specialist
Gary Hein, Extension Entomologist
Both at the Panhandle Research and Extension Center, Scottsbluff