Along with dry conditions, wheat farmers in the Nebraska Panhandle have been fighting another resurgence of the wheat stem sawfly (Cephus cinctus).
According to University of Nebraska-Lincoln’s CropWatch, during May and early June, adult sawflies were observed in fields. This developed into another year of significant sawfly infestation and cutting during the 2020 winter wheat harvest.
Jeff Bradshaw, a UNL Extension entomologist, said records show that the sawfly was first observed in Colorado in the 1800s. The pest has spread prodigiously since then.
At the outset, it followed the spring wheat crops north. Montana suffered major crop destruction in the 1930s to 1940s, Bradshaw said. The sawfly continued to spread. Now, it can be found in Canada and as far south as Louisiana.
In the 1990s, the pest began shifting from spring to winter wheat, Bradshaw said. It is still mainly unknown why, he said.
That’s also about the time the sawfly became a major pest in Scotts Bluff and Banner counties in Nebraska. Then, it “kind of went away” for a while, Bradshaw said. It reappeared during the 2011-12 season.
“That’s about the time I moved here,” he said. “I’m sure it’s just a coincidence.”
The reemergence occurred in Box Butte County — a serious infestation. Bradshaw and his colleagues watched the geographic spread and the intensity increase for the next seven years. Now, it can be found in most counties of Nebraska.
Initially, farmers thought there was something wrong with the straw strength in wheat. After examining the plants, Bradshaw said he noticed no crimp in the stems.
“These were like razor blade cuts,” he said. “Nothing else does that to wheat.”
Bradshaw explained that the sawfly lay their larvae in the hollow stems of the wheat plant. The larvae cut and weaken the stems of maturing wheat causing the wheat to lodge or fall over. This means it would be difficult for a harvester to pick up the wheat.
“I’ve seen it in Nebraska and Colorado,” he said. “Whole fields lying flat.”
Stem sawfly infestations most generally start at the edge of the field. The sawfly will over-winter in crop residue. Adults will emerge and hang out on the edge of a field.
“Then, they will crash into the edge of a wheat field,” Bradshaw said. “Depending on population of sawflies, they can stay on the edge or move a couple of feet into the field.”
The female sawfly will drill a hole in the wheat steam and inject an egg. The larvae hatches and moves to the base of the hollow stem. It eats a circle around the inside at the bottom of the stem. Eventually something causes the stem to move and it snaps off at the weakened area.
Wheat farmers have to be vigilant and diligent to manage the sawfly. Walking the edge of their fields is one way farmers can notice if their crop has been impacted. There are two main indicators, he said.
“Wheat tillers will collect on your pant legs,” Bradshaw said. “If you give your plants a slight tug, they will break off easy.”
The closer to harvest the more checks should be made. It can literally happen in a week, he said.
As far as managing the stem sawfly, Bradshaw and other entomologists are working on that. The insect does have a natural predator. The Bracon cephi — a family of parasitoid wasps — will inject their eggs into sawfly-infested wheat. The wasp larvae will eat the sawfly larvae.
Scientists have noticed that large sawfly infestations can cause a similar surge in Bracon cephi. Bradshaw said this results in the sawfly being essentially wiped out in that area.
The three main ways farmers have been using to manage the steam sawfly are crop rotation, tillage and planting resistant varieties of wheat.
“Crop rotation may help,” Bradshaw said. “It depends on the cropping system.”
Farmers who conserve wheat residue or have neighbors that have wheat residue in their fields will most likely contribute to sawfly contamination, he said.
Tillage may be more beneficial, Bradshaw said. By tillage he means any modifying of the soil condition with an implement.
“We haven’t tested every possible soil impingement tool,” he said. “But, using a tandem disc is more likely to reduce emergence.”
Resistant wheat varieties would be those wheat varieties with solid stem characteristics. The solid stem prevents the sawfly from implanting its larvae.
“Solid stem is an older trait,” Bradshaw said. “Wheat’s natural condition is to have a solid stem.”
Unfortunately, at this time, there are only a handful of varieties that fit have that feature. None of them are suitable for growing in Nebraska, he said.
PlainsGold is working on the most promising variety. According to their nomenclature, Fortify SF is one-quarter Bearpaw by parentage. That is a solid-stemmed variety developed by Montana Ag Experiment Station. It is three-quarters Byrd by parentage. Fortify SF will be marketed as a Certified Seed Only variety.
“We are still looking at Fortify semi-solid stem wheat,” Bradshaw said. “We should have good info available this winter.”