The Oregon State University Extension Service has issued a pest alert regarding the presence of true (common) armyworms in Willamette Valley grass seed crops.
The service wrote that large numbers of the pest have been spotted in tall fescue and orchardgrass seed fields in the past two weeks in both the south and north Willamette Valley.
The pest, Mythimna unipuncta, also has been seen on sudan grass planted as a cover crop between nursery stock rows.
“Extensive damage may result if the population is not treated,” the alert states. “Large numbers of larvae feed so voraciously that mass migrations of larvae can occur within a field and to adjacent fields very quickly.”
Armyworm outbreaks occur suddenly, the alert states, and in large numbers. In the alert, extension personnel Amy Dreves, Nicole Anderson and Clare Sullivan compared the pest to the new winter cutworm, which erupted in grass seed fields last summer.
According to an extension publication issued in February, cutworm damage is less uniform than armyworm damage, but both pests move en masse, potentially inflicting widespread damage to new growth in late summer and early fall.
The armyworm, like the cutworm, also inflicts damage sporadically. The last time an armyworm outbreak occurred in the Willamette Valley was 2004-2006, according to the alert. In that outbreak, the pest also was found in Southwest Oregon near Myrtle Point, and damaged grass pasture and corn in that area, according to Dreves.
In grass seed crops, the pest damages new growth by feeding on leaves and stems, leaving notched leaves and jagged leaf edges, according to the alert. Armyworms, like cutworms, can cause extension defoliation of plants over broad areas.
The alert advises growers to scout for the pest in and around crowns where birds are feeding and to dig around in the thatch of a plant and at its base. The pest feeds at night and curls into a C shape in the day in areas where it can avoid daylight.
The pest is not well adapted to light and needs moisture, according to the alert.
“We expect larval activity may slow down for a short period of time,” the Aug. 29 alert states. “However, activity will likely pick back up.”
The pest looks similar to winter cutworm, but the true armyworm is more smooth-bodied, tan-to-brown in color, about 0.5 to 1.5 inches long, with several alternating dark and light stripes and yellow-orange bands.
The alert states that several pesticide products are labeled for armyworm control, and that insecticides are most effective when applied while larvae are small.
“There is little benefit to spraying when the (larval) pest is full grown,” the alert states. “We recommend spraying at night, and rotating chemistries if more than one application is needed.”