Wheat stem sawfly has been a very significant pest of wheat in the northern wheat producing regions of the nation such as Montana and North Dakota and well into Canada. Larvae cut and weaken the stems of maturing wheat, causing the wheat to lodge and creating very significant harvest losses in many situations, say John Thomas, University of Nebraska-Lincoln Extension educator in Box Butte County and Jeff Bradshaw, UNL entomologist in Scottsbluff.

This photo shows significant damage after the wheat stem sawfly cut the stems in this wheat field.

Wheat stem sawfly damage, which was first noted in Nebraska in the early to mid-1990s near the Wyoming border, has continued to increase and is now becoming a very significant issue. 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 to combat this problem.

"Following the heavy infestation of wheat stem sawfly in winter wheat in the 2013 growing season, we had concern for ongoing heavy infestations in 2014," Thomas says. "During May and early June this year, large numbers of adult sawflies were observed in fields and caught in sweep net samples. Those emerging numbers of sawflies developed into another year of heavy sawfly infestation and cutting, as we saw during the 2014 winter wheat harvest."

Cool wet spring and early summer conditions produced a thick stand of wheat in most places in the Panhandle, and because of this the plants aren't lodging as seriously from sawfly cutting, they say. This is helping reduce harvest loss this year.

Dryland wheat is most seriously affected, but some level of infestation also occurs in irrigated wheat. "Dry land wheat adjacent to undisturbed stubble from last year appears to have the worst infestations," Thomas says. "On some fields 50 to 70% of the stems are cut for the first 50 to 100 feet of the field edge. Cutting tapers off further into the field, but may be as high as 15 percent 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 feeds and tunnels 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.