Armyworms surprise grass seed growers

At night they come out en masse to notch the tops of post-harvest growth, leaving grass looking as though livestock had grazed.

By Brenna Wiegand

For the Capital Press

Published on March 13, 2018 3:35PM

Oregon State University Field Crops Agronomist Nicole Anderson informs Oregon grass seed growers on the latest research during a field day event. This year’s major problem was a surprise invasion by the
true armyworm on grass seed crops.

OSU Extension

Oregon State University Field Crops Agronomist Nicole Anderson informs Oregon grass seed growers on the latest research during a field day event. This year’s major problem was a surprise invasion by the true armyworm on grass seed crops.


Grass seed growers had to scramble last year when the true armyworm launched a surprise attack first noticed during harvest. Standing crops and those in windrows did not suffer, but the worms, hiding beneath windrows, showed up by the thousands in combine tanks.

“There was massive feeding to newly harvested fields; it was really an all-hands-on-deck type of situation,” said Nicole Anderson, Oregon State University Extension Field Crops Agronomist in Oregon’s North Willamette Valley. “I don’t know that anybody remembers something like the outbreak of the true armyworm this year.”

The feeding was so intensive and the population so high that growers did a lot of spraying in late August and early September in less-than-favorable conditions.

“They can move over a number of acres in a night,” Anderson said. “That’s why they call them armyworms: They march like an army.”

At night they come out en masse to notch the tops of post-harvest growth, leaving grass looking as though livestock had grazed.

By late October, most of the armyworms pupated, but instead of emerging as spring moths, Oregon growers hope the pupae won’t survive a Pacific Northwest winter, something they won’t know until they start making observations this spring.

“The moths can get caught up in wind and move over very large geographic distances,” Anderson said. “There were significant outbreaks from Northern California all the way to Southern British Columbia and Vancouver Island. … One theory is that they blew up from California.”

Anderson is communicating with entomologists from California, Washington and Canada to discuss a potential region-wide trapping and DNA analysis effort to better understand their origins and biology in the Pacific Northwest.

“I think we did a good job of reacting quickly; now we just need to hope that most of the fields have come back strong … and that we don’t have high numbers going into 2018.”

Since the Oregon Legislature’s 2010 ban on all field burning except 15,000 acres in the Silverton Hills it’s been harder to banish such loathsome pests.

“We’re far enough away from field burning now that we can definitely see an upward trend in pest pressure,” Anderson said. “Field burning is a very old and effective pest management tool that had a lot of pros. Its major con is air quality, but its pros are that we’re able to recycle nutrients back into the soil from the grass seed straw and are better able to manage things like insects, slugs, pathogens and weeds.”

Anderson’s ongoing research at OSU’s Hyslop Field Lab and in farmers’ fields across the Willamette Valley includes testing evidence from other parts of the world suggesting a synergism may be created by mixing different classes of plant growth regulators. This idea is being extensively tested in tall fescue.

Anderson is also evaluating new classes of fungicides for stem rust management, particularly in perennial ryegrass, and finding a few that may outshine current offerings.

Another major project is revising OSU’s nitrogen fertilizer recommendations for some of the smaller acreage grass seed crops like orchardgrass, written decades ago before the introduction of plant growth regulators. They are also working to better nail down the understanding of plant growth regulator timing and rate in this seed crop.

A new study further evaluating the interaction between grazing or mowing and plant growth regulators in annual ryegrass seed crops seeks to identify potential across common varieties produced in both Oregon and New Zealand.

Despite the armyworm outbreak, the coming year’s crop looks promising.

“The planting conditions in the fall of 2017 were ideal so we’re going into 2018 in a pretty good situation and so far, the slug pressure has been manageable,” Anderson said. “We’ve had a lot of good opportunities to apply herbicides this fall and early winter which should help with weed pressure throughout the season. I am hopeful that we’re off to a good start.”



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