Wheat stem sawfly has been a significant pest in the northern wheat-producing regions of the country, including Montana and North Dakota and well into Canada. Larvae cut and weaken the stems of maturing wheat, causing the wheat to lodge and creating significant harvest losses in many situations.
WSS damage in winter wheat was first noted in Nebraska in the early to mid-1990s. The first infestations were noted in Banner County near the Wyoming border. It has continued to increase since and now is a significant problem.
Integrated pest management will be needed to attack the problem with multiple tactics. Crop rotation, resistant varieties with solid stem characteristics, tillage, field width and trap crops are some of the tools that can be used.
After the moderately heavy infestation of WSS in winter wheat in the 2019 growing season, Nebraska Extension and wheat growers were concerned for ongoing infestations in 2020. During May and early June, adult sawflies were observed in fields, but observers were unsure of the level of infestation.
Those emerging numbers of sawflies developed into another year of significant sawfly infestation and cutting during the 2020 winter wheat harvest. Infestations this season are moderate in the northern Panhandle and heavier in the southern Panhandle.
Note the lodged tillers remaining after harvest in a Deuel County field (photo above). Dryland wheat yields are looking good, averaging from 40 to 50 bushels per acre despite sawfly activity in the northern Panhandle.
Some yields in the southern Panhandle are as low as 25 to 30 bushels per acre because of the sawfly. The wheat harvest this year is ahead of last year because of warmer, drier weather.
Dryland wheat is most seriously affected, but some level of infestation also occurs in irrigated wheat. Dryland wheat adjacent to undisturbed stubble from last year appears to have the worst infestations. In some fields, 50% to 70% of the stems are cut for the first 50 to 100 feet of the field edge (top photo). Cutting tapers off farther into the field but may be as high as 15% across an entire field.
Sawfly larvae overwinter in the stubble of the previous year’s crop and emerge in May and June to attack the developing crop during stem elongation. Females emerge from the stubble, mate and lay an egg in the newly elongating wheat stem.
The egg hatches, and the larvae feed and tunnel through the nodes of the developing wheat finally to girdle and weaken the stem, causing lodging for its exit from the remaining stub the following spring. The larvae live in a pupal chamber inside the stub at the very base of the stem after harvest and through the winter.
Thomas is a Nebraska Extension crops educator; Bradshaw is a Nebraska Extension entomologist; and DeBoer is a Nebraska Extension educator.
Source: University of Nebraska CropWatch