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Army Cutworms Marching Through Wheat, Canola and Alfalfa Fields

Bill Spiegel 04/07/2014 @ 2:20pm I grew up in north-central Kansas, and am the Fourth Generation to maintain and manage our farm; we grow wheat, soybeans and grain sorghum. I'm a 1993 graduate of Kansas State University in ag communications.

Adding insult to the injury that is dryness and spotty winterkill for crops that are beginning to green up this spring, army cutworms are marching throughout wheat, alfalfa and canola fields in the Central Plains.

Army cutworms -- so called because they feed on green tissue and move en masse across affected fields like a troop of soldiers - feed on the green tissue of wheat, canola and alfalfa just coming out of winter dormancy. Most of the time the plants will outgrow army cutworm damage; this spring, however, many crops are slow to grow because of cool temperatures, giving the army cutworms a greater chance to feast on young plants and cause long-term damage.

Areas of concern are eastern Colorado, western and central Kansas, southwest Nebraska and northwest Oklahoma. That’s because army cutworms over-summer in the mountains of Colorado, then migrate back to these regions in August and September. They pupate in May and will fly back to Colorado for the summer. Most fields with a high degree of army cutworm damage are adjacent to areas with high residue, such as CRP, pastures or creeks, says Kansas State University Extension Agronomist Jeanne Falk-Jones, who works in northwest Kansas.

“Every year we find army cutworms, but this year we seem to have an abnormally high level of them,” she says. “They are hungry and looking for food.”

Jeff Whitworth, extension agronomist at K-State, says army cutworms will feed on these crops for another four to six weeks. “If leaf tissue continues to be removed,” he warns, “farmers may want to consider a spray treatment.”  

Wheat

In wheat, army cutworms devour leaf tissue, causing a ragged appearance across the field. As few as two 1-inch or longer larvae per square foot may affect thinner stands, especially if larvae graze plants down to the root crowns. In these situations, a pesticide application will often yield an economic return.

Average stands can withstand as many as four to five larvae per square foot before requiring treatment, whereas good stands can withstand as many as 8 to 9 larvae per square foot without a measurable yield impact or an economic return on an insecticide application. Wheat plants in more advanced growth stages can withstand a great deal more defoliation than those in earlier stages.

“Wheat can recover if there is moisture,” says Jeanne Falk-Jones, extension agronomist in northwest Kansas. “The problem is there is not enough moisture to continue to put on new leaf tissue.”

Larvae are easier to kill while they are still small, and early control will provide better damage prevention, as 70 percent of larval consumption occurs in the final instar.

Labeled treatment options for army cutworms on wheat include beta-cyfluthrin (e.g. Baythroid XL, and others), lambda-cyhalothrin (e.g. Warrior with Zeon Technology, and others), gamma-cyhalothrin (Proaxis), and zeta-cypermethrin (Mustang MAX).

Alfalfa

Seedling alfalfa is most susceptible to army cutworms, Falk-Jones says, as heavy feeding can kill many seedlings before stand establishment.  Established stands can usually compensate for army cutworm feeding using stored nutrient reserves. Otherwise, army cutworms may delay first cuttings or reduce yields. Newly planted alfalfa is at greater risk because heavy feeding can kill many seedlings before stand establishment. Early detection of an army cutworm problem is critical, and control (if required) is best accomplished with a late afternoon/early evening or early morning insecticide application when most larvae are above ground.

In seedling alfalfa, the threshold for control is 2 per square foot. In established alfalfa the threshold is 4 or more per square foot.

Most of the same insecticides listed above for wheat are also registered for alfalfa.

Canola

It is important to scout canola fields for these insects. Canola should be treated when there is an average of two or more larvae per foot of row. Army cutworms find canola very palatable, and 4 to 5 per square foot cause severe damage to stands. Stands can be completely lost if left untreated. Look for foliar tissue damage and severed green leaves lying on the ground as evidence of feeding. Damage may initially be more visible in areas of the field where stands are thin.

Labeled treatment options for army cutworms on canola include bifenthin (e.g. Capture, and others), lambda-cyhalothrin (e.g. Warrior with Zeon Technology, and others), and gamma-cyhalothrin (Proaxis).

General

When making an insecticide application for army cutworms, be sure that temperatures will be above 50 degrees for three to four days after the application is made. Better control is accomplished with the proper amount of carrier as these worms will be in the soil. You have to get contact insecticides down to where the worms are, and if there is much foliage, you will need enough carrier to do that.

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