Agronomy •  2023-03-27

Managing Western bean cutworm

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A well-known corn pest in western Ontario, Western bean cutworm (WBC) has quickly made its way across the province, becoming a persistent problem in eastern Ontario and setting its sights on Quebec and the Maritimes.

Historically a secondary pest of corn, in favourable weather conditions, WBC can cause significant yield loss and reduce grain quality. For each larva per plant on average, yield loss estimates have been reported from 4 bu/acre to as much as 15 bu/acre.1 In addition to yield loss, a major consideration for areas with higher ear rot pressure is the risk of reduced grain quality resulting from a WBC infestation.

Monitoring and scouting are the first line of defense against WBC. Collecting as much information about WBC populations can help you take action against this pest.

Adult WBC emergence is linked to growing degree days, resulting in different flight seasons that depend on the weather and vary from year to year. The best way to determine when WBC season has begun is through trapping.

Monitoring Pheromone trapping is the most common and economical adult monitoring method, and provides a highly valuable resource for tracking moth flight. Pheromone traps can effectively detect the presence of adult male emergence and general flight activity.

Moths are mostly active at night, searching for mates and nectar to feed. When peak moth flight appears to have started, compare this timing to the approaching stage of your corn crop to determine if your fields are likely to be attractive to egg-laying females.

High-risk fields include those in late whorl to early tassel stage close to peak moth flight (mid-late July in Ontario). Fields with variable crop stages and plant heights are ideal for prolonged egg laying within the same field and may be attractive to moths over a long time period.2

Scouting for eggs and larvae in the crop is the best method to determine the necessity and the timing of a treatment. Female moths prefer to lay eggs in pre-tassel corn in order to synchronize egg hatch and preferred larval feeding on pollen, followed by ear feeding.

Tips for scouting WBC: Check 20 plants in at least five areas of each field. Inspect each plant from the ear leaf on up, looking for egg masses on the top and bottom sides of leaves. Be sure to look for larvae in the axils, tassels and ears. Continue scouting throughout the season. Corn fields will remain attractive to egg-laying moths throughout the growing season, with the risk of economic damage decreasing in the mid-milk stage as kernels are soon to harden as they mature, becoming less susceptible to damage from smaller larvae.

Action thresholds can range from approximately 4%- 8% of sampled plants containing one egg mass. If WBC is observed year after year, or the risk ear rot is high, farmers may consider even more proactive thresholds of (~2%).

Taking action against WBC means making the right treatment and timing decisions. Farmers can rely on these trusted detection methods to time treatments and rotate insecticide modes of action to limit the risk of resistance. Looking for a solution against WBC? Check out Delegate™ insecticide.

A farmer’s best defense against WBC is to follow a robust monitoring and scouting program, and use the thresholds collected to make informed decisions to take action and make any necessary management decisions. For more information or product recommendations contact your trusted agronomic advisor, retailer or local Corteva Agriscience™ Territory Manager.