Adult Corn Rootworms Emerging (07/29/21)
The first emergence of adult northern corn rootworm (NCR) was observed last week in corn fields near Casselton, Cass County, and Shenford, Ransom County. The first emergence of western corn rootworm (WCR) was observed near Colfax, Richland County. Other counties scouted last week that had no corn rootworms were Barnes, Griggs, Steele, Trail, and Grand Forks. All monitored fields were below the economic threshold (E.T.). From total counts, 83% of rootworms observed were NCR adults and 17% were WCR adults.
Adult corn rootworm beetles can easily be identified in the field. Northern corn rootworm adults are tan to pale green, ¼ inch long beetles. Western corn rootworm adults are from 3/16 to 5/16 inch long and yellowish-green with three black longitudinal stripes usually on the female’s forewings and nearly solid black marking, usually on the male’s forewings (see pictures).
For monitoring corn rootworm adults, position four unbaited Pherocon AM® yellow sticky traps into a linear transect at each field. Traps should be set at least 100 feet from field edges and/or waterways. The distance between sticky traps should be at least 165 feet. Attach the sticky trap on the corn stalk at ear height. Replace traps once per week for four to six weeks, or until the threshold is reached. Count and record the number of beetles caught on each trap. Lastly, calculate the average number of beetles per trap per day (add total number of beetles captured from all traps and divide by total number of sticky traps, then divide this result by the total number of days that traps were deployed in the field).
Scouting maps are posted weekly on the NDSU Extension IPM website. For more information on corn rootworms, please see the NDSU Extension publication Integrated Pest Management of Corn Rootworms in North Dakota E1852.