Thursday, June 24, 2010

Wheat Stem Sawfly Spreading its Wings in Idaho

Wheat stem sawfly (Cephus cinctus Norton) is widely distributed across the northern Great Plains. There are no insecticides registered for sawfly control and no cost effective insecticides have been identified in research trials.

Production losses total $100’s of millions in the Northern Plains states. Losses in Montana alone are estimated at $75 million in each of the last two years, making it the most destructive agricultural pest in the state. As shown on the map, the Sawfly threat continues to spread causing production losses in other areas, including SE Idaho. Increasing awareness of this pest is important in control.

“I’ve dealt with sawfly for about ten years now,” says Gordon Gallup, wheat grower in Swan Valley. “Some years are worse than others. So far it hasn’t caused a significant crop disaster like in other states, but the potential is always there.”

It’s that potential that keeps Juliet Marshall-Windes, U of I Cereals Agronomist and Pathologist watchful. “Sawfly outbreaks can be devastating,” says Marshall-Windes. “They leave many calling cards such as reduced yield, stunted heads with fewer kernels and lower kernel weight, reduced protein content and lodging. When you see lodged grain in the fall, you will also see clean cut stems and stem stubs with darkened plugs often in multiple stems per plants.”

Spring wheat used to be its main target however it now infests winter wheat as well. That is because sawflies are emerging nearly a month earlier than previously. Now they are present when the winter wheat is susceptible. Late planted spring wheat fields frequently avoid attack.

Marshall-Windes suggests keeping an eye out for sawflies while checking fields for other pests. The adults emerge from May to July and will usually be found resting on wheat stems with their head directed downward.

Integrated Pest Management
If a crop rotation includes non-host crops, such as canola, and there are no wheat or barley fields nearby, crop rotation helps to reduce infestations. Tillage usually has little effect on sawfly populations because adults can dig through the soil. Even moldboard plowing did not reduce the number of adults in experimental trials. Swathing does not kill sawflies. Most larvae are in the lower stems, or stubs, when the wheat is dry enough to swath. The adoption of conservation tillage practices that leave stubble containing sawfly larvae intact may also favor increased sawfly populations.

Resistance of wheat to this pest has been limited to physical traits like solid stems and or stronger stem tissues. A problem often associated with solid stem wheat is that plant resources that would otherwise be allocated to kernel production are used in stem growth, resulting in an overall smaller yield compared with hollow stemmed wheat varieties.

“The solid stem trait is only partially effective,” says Marshall-Windes. “Sawflies can survive and reproduce in solid stemmed wheat, although not nearly as well as in hollow stemmed wheat. “Choteau, is a hard red spring wheat that has a solid-stem and does well in the SE area, but improving genetic resistance is critical for control in the long run.”

Reducing the Threat
Field level studies across Wyoming, North Dakota, Montana and Idaho are ongoing.
Juan Manuel Alvarez and Juliet Marshall-Windes are participating in an investigation of IPM practices influencing the interaction of wheat stem sawfly, Fusarium crown rot, and grassy weed impacts in wheat. Recommendations will be developed from three years of research on the integrated management of these dryland production pests. Information on crop variety, stand density and herbicide management will be combined to manage some of the most important pests of the Northern Great Plains.

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