The Pest Post

Back in Black

Scott Williams
By  Scott Williams , DTN Entomologist
If the winds blow just right, you may have this black cutworm visitor in your fields before you know it. (Photo courtesy of the University of Wisconsin)

A drawn-out harvest prevented many farmers from completing fall fieldwork. That may increase the susceptibility of many fields to one of our perennial pests, the black cutworm (Agrotis ipsilon). And, if the winds blow just right, they'll be paying your fields a visit before you know it.

WHAT TO LOOK FOR

A drawn-out harvest prevented many farmers from completing fall fieldwork. That may increase the susceptibility of many fields to one of our perennial pests, the corn pest black cutworm (Agrotis ipsilon). And, if the winds blow just right, they'll be paying your fields a visit before you know it.

-- Egg: Eggs are laid on non-crop foliage or other vegetation, with common hosts including chickweed, shepherd's purse and wild mustards. The eggs appear as whitish domes with around 35 to 40 ridges that run from apex to base. As the eggs mature, they change in color to a brown hue.

-- Larva: The larvae hatch five to 10 days after eggs are laid, depending on the temperature. The larvae are gray to black in color, giving the species its name. Other distinctive features include a "greasy" skin appearance, lighter-colored ventral area, pairs of unevenly sized black dots on each body segment and a brown head capsule. If disturbed, the caterpillar may curl up into a "C" shape to protect itself. As they grow in size, the larvae move off the plant on which they're feeding and into the soil nearby, only coming out at night.

-- Pupa: Larvae complete their growth roughly a month after hatching and pupate about 1 to 4 inches below the soil surface. The pupae are therefore rarely seen but appear as brown spindles less than 1 inch in length.

DAMAGE

Younger black cutworms feed on foliage and will leave small, irregular holes behind. Older caterpillars feed closer to the surface, notching the stem or cutting it off entirely to drag the plant below the surface and feed in peace. A single cutworm can kill up to five plants during its life span. Severe infestations can lead to losses of 80% but, more typically, damage is localized to areas of the field. Late planting, reduced tillage, wet soil and nearby weeds increase the odds of significant damage.

MANAGEMENT

Start by removing alternative hosts. Eliminating early-season weeds in and around fields makes them less attractive to incoming moths. Set the biofix (refers to when to start counting degree days for a specific pest) when pheromone traps record nine or more moths over a two-night period, and scout fields for signs of damage soon after plant emergence. If cutworms are present, leaf damage should appear 90 to 300 degree-days after the biofix. If damage is present, check five areas of the field (20 plants each) and note the growth stage of the plants and the number with feeding damage. If cutworms are present, collect 10 larvae to determine the average instar of the population. If damage exceeds the economic threshold for the average instar (see references below), treatment may be necessary.

Bt hybrids containing the Cry1F protein (Herculex/HX1) or Vip3a protein (Viptera), alone or in stacks, are labeled as controlling black cutworm. While they reduce risk, corn might still be damaged under heavy pressure. Seed treatments may not always provide satisfactory control, especially at lower rates. Post-rescue treatments can be effective when combined with careful scouting.

Black cutworm is among many insects targeted by DTN products and services, including the Agronomic Insights, AP and DTN Smart Trap.

For more information:

Purdue University: tinyurl.com/ja3lexw

University of Minnesota: tinyurl.com/y3oxxa4o

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Editor's Note: Write Scott Williams at Pest Watch, 2204 Lakeshore Dr., Suite 415, Birmingham, AL 35209, or e-mail scott.williams@dtn.com.

(ES/SK)

Scott Williams