July 29, 2014

Wheat stem sawfly has been a significant pest of wheat in the northern wheat-producing regions of our country 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 significant harvest losses 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 noted in Banner County near the Wyoming border. The population has continued to increase and now is becoming a significant issue. Integrated pest management will be needed to attack the problem with multiple tactics, including:

• crop rotation,
• resistant varieties with solid stem characteristics,
• tillage,
• field width, and
• trap crops.

2014 Infestation

Following the heavy infestation of wheat stem sawfly (WSS) in winter wheat in the 2013 growing season, we had concern for ongoing heavy infestations in 2014. During May and early June 2014, 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 that we're seeing in the 2014 winter wheat harvest. Dryland wheat yields are looking good, ranging from 40 to 90 bu/ac in spite of sawfly activity. Cool wet spring and early summer conditions produced a thick stand of wheat in most places and because of this, the plants aren't lodging as seriously from sawfly cutting. This is helping reduce harvest loss this year.

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 50-70% of the stems are cut for the first 50 to 100 feet of the field edge (Figure 1). Cutting tapers off further into the field but may be as high as 15% across an entire field. 

Sawfly larvae overwinter in the stubble (Figure 2) of the previous year's crop and emerge in May and June to attack the developing crop during stem elongation. Females (Figure 3) 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 to girdle and weaken the stem, causing lodging. The larvae live in a pupal chamber inside the stub at the very base of the stem (Figures 4 and 5) after harvest and through the winter.

John Thomas
Extension Educator, Box Butte County
Jeff Bradshaw
Extension Entomologist, Panhandle REC