For the last several years, dryland wheat growers in western Nebraska have dealt with a hidden problem that emerges in late spring and early summer: wheat stem sawfly.
The Nebraska Panhandle Research and Extension Center’s entomology lab, which has conducted a survey of stem sawfly infestation in the Panhandle since 2011, continues to see increasing population levels.
"It's a difficult insect to manage. It's primarily a dryland problem. There aren't any commercial pesticides available in Nebraska to control wheat stem sawfly," says Jeff Bradshaw, Nebraska Extension entomologist. "They emerge over a five-week time period, so you would have to apply multiple insecticide applications on dryland wheat, and input costs aren't low."
Wheat stem sawfly has been a problem for Northern Plains growers for years — starting with spring wheat in southern Canada and Montana, and gradually working its way south.
"When it all falls one way, you're basically stuck combining in one direction," Bradshaw says. "With more messy lodging, the harvest direction is less important. In both situations, you're harvesting slower. It becomes more than a yield loss. It's affecting profit in some unpredictable ways."
That's where solid-stemmed wheat comes in. Compared to the hollow stem of most varieties planted in Nebraska, a solid stem — a host plant resistance trait — doesn't give the sawflies room to grow after they hatch and, in some cases, crush the eggs before they can hatch.
Potential of solid stems
However, most solid stem varieties are bred and adapted for Northern Plains locations like Montana. This includes three varieties — Bearpaw, Warhorse and Judie — the three solid-stemmed varieties with the most potential in the Panhandle. All three were developed by the Montana Agricultural Experiment Station.
"When you're using Montana wheat in Nebraska, those varieties are a long way from home. It tends to be easier for wheats from the south to go north than go from north to south," explains Steve Baenziger, University of Nebraska-Lincoln agronomy professor and Wheat Growers presidential chair. "Even if you're in Montana, where the lines are bred for, there's a yield drag to the solid stem. There's a partitioning issue because more energy goes to the stem as opposed to the head."
Research at the High Plains Ag Lab near Sidney, Neb., has shown some years where varieties like Warhorse — typically the highest-yielding of those three solid stem varieties — will yield well, and other years where it's at the bottom of the pack compared to Nebraska varieties.
Last year, the average yield for Warhorse was 42 bushels. Meanwhile, two Nebraska varieties yielded 63 bushels per acre.
However, Cody Creech, Extension dryland cropping systems specialist, notes that doesn't mean solid stem wheat shouldn't be grown in western Nebraska. "That's the challenge we're facing: Where is that solid stem best used?" Creech says. "With low to moderate sawfly populations, we typically see most lodging around the borders of the field. One possible strategy [with lower populations] is planting a solid stem variety around the border of the field to take the brunt of the hit and plant something higher yielding on the inside."
Last year, Creech and Bradshaw compared sawfly pressure on different varieties on a sawfly-infested field. While an average of about 2% of the sawfly population could complete its development on Warhorse, some hollow stem varieties saw 100% survival of sawflies, although they were typically 50%.
Solid stem wheat may have an advantage outside of sawfly control. Creech is evaluating solid stem wheat residue, how long it takes to break down and how well it holds moisture compared to hollow stem varieties. And additional moisture retention may bring a yield boost to the following corn crop.
"If we can show residue from solid stem wheat like Warhorse lasts longer than residue from hollow stem wheat, and can boost the following corn crop yield, maybe we can capture that small hit in wheat yields back in corn yields," Creech says. "So far, after all the snow this winter, we've seen the solid-stemmed wheat, Warhorse, is still standing. A lot of other lines have laid over and are flat on the ground. Being up off the ground, it might keep it from breaking down as fast because it's not as wet."
Breeding for Nebraska
Baenziger notes solid stem wheat is something UNL's wheat breeding program is constantly working on. Typically, when breeders evaluate varieties for solid stems, they assess them visually at the end of the season for pith expression.
"Every time I see something new coming out of Montana, I'm crossing it with Nebraska varieties. We're using Warhorse quite a bit in our crossing block," he says. " Last year, we had about 15 populations that we planted in a sawfly-infested area. We let the sawfly do its damage, cut them, and took them back and did single seed descent. We're just getting started on the F5 generation."
Meanwhile, there may be a way to breed for solid stem varieties that don't take a yield hit. Some wheat varieties identified in Montana develop pith at different times. Also, it may be possible to breed for varieties that develop pith early on, and absorb nutrients and energy back into the plant to put more into head development later in the season, resulting in higher yields.
"Montana breeders have also found nonsolid varieties which are also resistant to wheat stem sawfly," Baenziger adds. "We think there may be different mechanisms involved, whether solid stem or other forms of sawfly resistance. We're making crosses to those as we speak."