By John Thomas and Jeff Bradshaw

Wheat stem sawfly has been a major pest in northern wheat-producing regions of the U.S., such as Montana and North Dakota, as well as in Canada. Larvae cut and weaken the stems of maturing wheat, causing them to lodge and reducing yields in many situations.

In Nebraska, wheat stem sawfly damage in winter wheat was first noted in the early to mid-1990s. The first infestations were noticed in Banner County near the Wyoming border. It has continued to increase ever since, and now is becoming a significant issue. Integrated pest management with multiple tactics will be needed to attack the problem. Tactics include crop rotation, tillage and use of resistant varieties with solid-stem characteristics.

Dryland suffers most
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. On some fields 50% to 70% of the stems are cut for the first 50 to 200 feet of the field edge. Cutting tapers off further into the field, but it may be as high as 15% to 30% 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 girdling and weakening the stem and causing lodging. The larvae live in a pupal chamber inside the stub at the very base of the stem after harvest and through the winter and then exit the following spring.

SAWFLY STAGES: The stages from left are an adult female wheat stem sawfly, the cellophane-like larvae and pupal chamber, and the wheat stem sawfly larvae.

Research sheds light on control options
This year high populations of wheat stem sawfly were in some areas of the Nebraska Panhandle and northeastern Colorado. The Entomology Lab at the Panhandle Research and Extension Center in Scottsbluff, Neb., has investigated spring tillage in fallow as one option for sawfly control.

Data analysis is not final; however, data from 2017 indicates a threefold reduction in wheat stem sawfly emergence from fallow ground that has been disturbed by a single pass with a tandem disk in April.

Thomas is a crops educator and Bradshaw is an entomologist with Nebraska Extension. This report comes from UNL CropWatch.