Wheat Stem Sawflies Moving South Into Winter Wheat States
For decades, wheat stem sawflies have been a problem for spring wheat farmers in Canada, North Dakota, and Montana.
In recent years, however, the annual pest has moved south, wreaking havoc in winter wheat fields in Nebraska, Colorado, and Kansas, according to J.P. Michaud, Extension entomologist at Kansas State University’s Western Agricultural Research Center in Hays, Kansas.
This spring, researchers and crop consultants are finding large populations of wheat stem sawflies in the Nebraska panhandle.
“Insect populations are always changing, but what we are now seeing is directional selection,” Michaud says. “Peak insect population is about two weeks earlier now than it was 15 years ago, and the populations are moving south.”
The insect is actually a wasp that bores a small hole in the stem of wheat plants to lay its eggs. Usually one egg survives, and the larva tunnel to the base of the wheat stem. While the insects can reduce seed development 5% to 15% by causing shriveled and shrunken seeds, the biggest losses are attributed to plant lodging, Michaud says.
In Kansas, Nebraska, and Colorado, wheat stem sawfly adults emerge in mid-May. During stem elongation, adults choose large tillers in which to lay eggs; the larvae feed for about 30 days within the stem and move down the stem to the base of the plant. Larvae will remain in wheat stubble until the following spring, when pupation occurs. Some larvae may wait two years to pupate, Michaud says.
Montana and North Dakota farmers lose about $50 million each year due to damage caused by wheat stem sawfly. Populations are not large enough in Kansas, Nebraska, and Colorado to ascertain economic impact, but the pest is definitely on the rise in these states, he adds.
“In 2011, University of Nebraska entomologists found wheat stem sawflies in several counties along the Wyoming border and into Wyoming, and south along the Kansas border,” he reports. “In 2014, cold spring weather slowed down the insect, but there are reports of wheat stem sawflies in western Nebraska.”
Because wheat stem sawflies live within the wheat stem, there is no pesticide that can control the pest as it tunnels through the wheat stem.
Control is best obtained when producers till fields. The wheat stem sawflies overwinter close to the soil line, and aggressive tillage can dramatically reduce populations. However, most producers want to preserve as much residue as possible, so tillage may be out.
Solid-stem wheat varieties prove to be effective at keeping wheat stem sawflies at bay, but there are few varieties adapted to Colorado, Kansas, and Nebraska. Also, planting attractive hosts plants such as oats or rye along wheat fields can lure wheat stem sawflies away from wheat fields.
There are natural parasitoids that prey upon wheat stem sawfly larvae; elevating the header at harvest (or using a stripper-header if possible) can keep these natural predators intact. Or, swath the wheat, then harvest with a pickup reel, he advises.
“Our best means of management are crop rotation away from continuous wheat,” Michaud says. “The days of continuous wheat may be over.”