The wheat stem sawfly continued to spread eastward in 2009, giving producers another headache, says Jan Knodel, North Dakota State University entomologist.
Knodel says the sawfly's larvae tunnel into the stem where they feed, reducing grain yields by 10% to 25% or higher. The larvae later move to the base of the stem as the wheat matures in the fields, weakening the stems and causing them to break and fall over to cause lodging.
“It's still spreading," Knodel says. "We've received reports of it in Emmons County and as far away as Valley City and Carrington in 2009."
However, these were not economic losses, and NDSU researchers hope to find answers before the insect can invade into more wheat fields in eastern North Dakota and Minnesota. Economic losses are already being felt in the western and central parts of North Dakota.
Knodel said the main areas of infestation in North Dakota in 2009 were in the southwestern and west central regions, and in selected areas in the northwestern region around Williston.
Knodel says she recommends producers swath fields when they find heavy pressure from the sawfly. Swathing should be done before significant lodging occurs and the stems fall over and lay in the field, she says. Once that happens, it's difficult to harvest, and the field could end up with yield loss.
Lodging losses include lost grain; volunteer wheat, which uses water and herbicides; decreased snow retention; slower harvest using more fuel and more time; and equipment damage, Knodel says.
She says producers should survey their fields for infested stems that have a reddish-brown spot below the second or third node. NDSU recommends producers look at 50 consecutive stems in a row from a site near the field margin and one in the center of the field.
When more than 15% of the field is infested, producers can swath to save their yields.
“Producers should try to set the combine high up, as close to the head as possible — the higher up you can swath, the better it is for the natural predators,” she says.
Swathing is a biological control which preserves parasitoids, the natural sawfly predators. The taller the stubble left in the field is, the more parasitoids can thrive, Knodel explains.
Infested plants send out chemicals that attract parasitoids, she says. The parasitoids need to be able to find the infested plants and locate where the sawfly larvae are hidden within the stems to be effective, she adds.
In addition, research is showing that no-till is a better environment for parasitoids to survive in than tilling, she says. Another research study showed wheat on wheat without a break in the rotation cycle is more conducive to higher sawfly infestation.
Following a year of high infestation, Knodel recommends producers go with a solid-stem variety.
Eric Eriksmoen, NDSU agronomist at Hettinger Research Extension Center, says it's best for producers to not use all one hard red spring wheat variety in their fields. If there are several varieties, the sawfly are often attracted more to certain varieties and the economic hit won't be as severe, he says.
Using a trap crop is another production method producers have been using with success to prevent the sawfly from getting into their main crop, Knodel says.
She said producers choose an attractive HRSW variety for the trap crop, such as Reeder, and spray it, then hay it.
“That should trap the sawfly before they emerge in the middle of the season,” she adds.
In 2009, cool weather caused the insect to delay its usual flight. The peak outbreak for the sawfly flight was not until the end of June and the first week in July, she says.
That is why research into insecticides is not showing much yield affect.
“It's hard to determine when the insect will emerge from the wheat plant. If it's cold, they will quit coming out, and then will come out later than normal,” she says. “A lot of producers tried insecticides and weren't happy with the results.”
Insecticide seed treatment and foliar insecticide applications trials were evaluated for wheat stem sawfly (and maggot) control at several sites.
Eriksmoen said that at Hettinger, where 40% of the stems were infested with sawfly, Cruiser seed treatment at a low rate combined with Dividend and early applied Warrior produced slightly higher yields than untreated or other treatments.
“There was quite a range of infestation throughout the state. Some fields in southwestern North Dakota ended up 100% lodged because of the sawfly, while others were about 10% lodged,” Eriksmoen says.
To find out the range of infestation, Knodel takes sticky trap and sweep net samples at various western locations during June and July. In 2008, combined wheat stem sawfly counts at these three southwestern locations were 886 at Hettinger (off-site), 993 at Regent and 1,040 at Scranton.