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Cutworm Vigilance

Friday, April 1, 2016
filed under: Insects

Early Season Sunflower Pest Merits Close Attention
By Janet Knodel*
       The expansion of sunflower acreage grown under a minimum- or no-till management system has, along with its benefits, brought along at least one downside for some producers: a higher risk of cutworm damage. Plant residue from the previous crop can serve as an inviting host for cutworm. If not controlled, this insect can sharply reduce early season plant stands, contributing to poorer weed control and, eventually, lower seed yields come harvest.
       There are, in North Dakota alone, about 150 different species of cutworm — 26 of which are considered “economic insect pests.” The good news is that very few of these species constitute a significant concern for sunflower producers — and, those few can be effectively managed through timely scouting and prompt treatment.
       Here is a summary of the description, life cycle, damage and management for the three most prominent species of cutworm found in sunflower fields: darksided, redbacked and dingy.

Description — Adult forewings of the darksided cutworm are

Larvae of the dingy cutworm

light and grayish brown with indistinct markings. Larvae comprise the destructive life stage that sunflower growers deal with in the spring. For darksided cutworm, the larvae are pale brown dorsally and white on their ventral areas. At maturity, the larvae are 1.25 to 1.5” in length and 0.25” wide. The adult redbacked cutworm forewings are reddish brown with bean-shaped markings. The redbacked larvae are dull gray to brown in color and measure 1.0 to 1.25” long when mature. These larvae can be distinguished from other cutworm species by the two dull reddish stripes along their back. Adult dingy cutworm forewings are dark brown with bean-shaped markings similar to those of the adult redbacked cutworm. Larvae have a dull, dingy brown body mottled with cream color. There is a thin light line running down the middle of the back, with a series of diagonal markings on either side.
Life Cycle — Female darksided and redbacked cutworm moths deposit eggs in the soil in late July and early August. The eggs remain dormant until the onset of warm weather the following spring. Larvae of both species emerge from late May to early June, continuing to feed and grow until around the end of June. When mature, the larvae pupate in earthen cells near the soil surface. This event lasts for about three weeks. The adult dingy cutworms emerge in August and remain active until mid-October. Their eggs are deposited on plants of the Asteraceae family (which includes sunflower) during the fall. After developing to the second or third instar stage, the larvae overwinter in the soil. Larval feeding resumes in the spring, with pupation occurring in spring to early summer.
Damage — Cutworm damage results from larval feeding. It typically consists of seedlings being cut off from 1.0” below the soil surface to as high as 1.0 to 2.0” above the surface. Young leaves also may be severely chewed by cutworms climbing upward on foliage to feed. Most cutworms feed at night. During daytime, they usually are found just beneath the soil surface near the base of recently damaged plants. Wilted or dead plants often indicate the presence of cutworms, and bare field patches are the results of cutworm damage.
Management — Sampling for cutworm should begin as soon as sunflower plants emerge, with fields checked at least twice weekly until late June. This is particularly important in areas where cutworms have caused problems in previous years. A trowel or similar tool can be used to dig around damaged plants to determine whether cutworms are present (since gaps within a row may be due to another cause).

       The size of cutworm larvae also should be estimated. Small larvae (< 0.5”) pose the greatest potential for damage since they still have to feed and grow.
       The economic threshold in sunflower is one larva per square foot, or a 25-30% stand reduction. If using a “Z” scouting pattern, examine five one-square-foot soil samples per site (within the row), for a total of 25.
       Several different insecticides are registered for cutworm control. Postemergent foliar insecticide treatment provides quick control of surface feeding cutworms. Optimal control is achieved when applications are made at night since that is when the cutworms are active. Wet soil conditions also will improve insecticide efficacy, as cutworms feed near the soil surface in these conditions.
For a listing for registered products for cutworm control, consult the 2016 North Dakota Field Crop Insect Management Guide. Click here to access this publication online.

     Much of this information on cutworms is excerpted from North Dakota State University Extension Bulletin E-1457, “Integrated Pest Management of Sunflower Insect Pests in the Northern Great Plains.” The bulletin was authored by Jan Knodel; Larry Charlet, research entomologist (retired) with USDA-ARS, Fargo, N.D.; and John Gavloski, extension entomologist with Manitoba Agriculture, Carman. It can be viewed here.
       For High Plains producers, a valuable publication is Kansas State University’s “Sunflower Insect Management,” found here.
* Janet Knodel is extension entomologist with North Dakota State University.