Mature bertha armyworm larvae were found feeding on the pods of canola in northcentral McHenry County (Source: Kristine Keller, Farmers Union Oil of Velva, Butte, Drake and Anamoose). Older larvae reach a length of ¾ to 1½ inch and are velvety brown to black with a yellowish band along each side of the body. As leaves dry, these larvae begin feeding on pods or flowers. The greatest risk of crop injury occurs in August as the worms are mature. Larvae chew holes in the pods, eat the seeds and cause premature shattering. Mature larvae eat approximately 85% of the plant materials consumed during their larval development. Larvae feed at night and often hide underneath leaf litter and clumps of soil during the day, which makes them difficult to see when scouting.
The Economic Thresholds is an average of 20 to 32 larvae per square yard with insecticide + application costs of $6.50 to $10 per acre, respectively. However, thresholds may need to be lowered if larvae are feeding on maturing pods at high population densities.
Fields above the economic threshold level should ideally be sprayed once the hatch is complete and when larvae are small about ½ inch. Apply a well-timed insecticide in early morning or late evening when larvae are actively feeding. High volumes of water should be used for good coverage of the dense canola canopy. Insecticides that are registered to control bertha armyworm on canola are listed in 2018 North Dakota Field Crop Insect Management Guide E-1143. When larvae are mature, 1½ inch long, they are close to the pupal stage, which is a non-feeding, resting stage. So, no insecticides are necessary this late in the insect’s development and the feeding damage is already done.
Please see the NDSU Extension publication on Bertha armyworm in Canola: Biology and Integrated Pest Management E1347 (revised) for more information.

Posted in: NCGA News.
Last Modified: August 3, 2018