AROCK, Ore. — An infestation of army cutworms has caused significant damage to alfalfa fields and rangeland grasses in some areas of southwestern Idaho and eastern Oregon.
Bill Buhrig, an Oregon State University cropping systems extension agent who surveyed the damage, said the caterpillars also damaged crested wheat seedings on Bureau of Land Management allotments.
Buhrig said one alfalfa field that he and OSU livestock and rangeland extension agent Sergio Arispe viewed was almost entirely defoliated.
“They took it down to the crown,” he said. “It was kind of discouraging to see the damage that had been done.”
He said the cutworms, which go through one generation a year, appear to be nearing the end of their feeding pattern this year, but the damage they caused was significant.
Buhrig said some ranchers he spoke with said they “had not seen anything quite like this before and, quite frankly, I haven’t either.”
Stuart Reitz, a cropping systems agent at OSU’s Malheur County extension office, said army cutworm damage was reported by ranchers south of Homedale in Owyhee County on the Idaho side and down to Rome and Arock in Malheur County in Oregon.
“The damage is pretty extensive down in the Jordan Valley area and across the border in Owyhee County in Idaho,” he said. “We’ve heard reports that a lot of hay fields there have been pretty much mowed over.”
The caterpillars hatch in the fall and a warmer-than-normal winter in that area allowed large numbers of them to survive into the spring and start feeding earlier, Buhrig said.
“There was just a set of circumstances that allowed them to go crazy,” he said. “I’m hoping this was a 50-year event.”
Given the extent of the infestation, it’s likely that pockets of large cutworm populations had been building up in that area unnoticed for a while, Buhrig said.
“Every so often, we just have these major outbreaks of them. This year was especially bad,” Reitz said.
In large numbers, they can cause significant damage, he added.
“The name ‘army cutworm’ is appropriate,” he said. “There are just large masses of them that will move across a field and eat the plants down to the ground.”
The worst of the damage is likely done this year, Reitz said, but farmers and ranchers, along with land managers and researchers, need to be aware that the cutworm could be an issue again next year and scout closely for the insect this fall.