Fall armyworms in lawns and fall forage crops


By Robert Bourne - Guest columnist



The fall armyworm has been observed in several Bryan County farms over the past few days. In addition, fall armyworm infestations can be found in residential lawns where they can be very destructive. Since survival of fall armyworm eggs is highest following rainfall or supplemental irrigation, some serious problems could develop for seedling wheat fields, sod farms, golf courses, and residential lawns.

Fall armyworms are caterpillars that cause direct damage to green turf and forage. Female fall armyworm moths lay up to 1,000 eggs over several nights on grasses or other plants. Within a few days, the eggs hatch and the caterpillars begin feeding in groups. Caterpillars molt six times before maturing, increasing in size after each molt. They can complete a generation in 18-28 days depending on temperature. Newly hatched fall armyworms are white, yellow, or light green and darken as they mature. Mature fall armyworms measure 1½ inches long with a body color that ranges from green to brown or black. They can be distinguished by the presence of a prominent inverted white “y” on their head. Small larvae do not eat through the leaf tissue, but instead scrape off all of the green tissue and leave a clear membrane that gives the leaf a “window pane” appearance. Large larvae can quickly denude a turf or forage canopy.

Preventive insecticide treatments are not practical because outbreaks are sporadic and mortality due to natural enemies is usually high. Unnecessary insecticide applications can eliminate these natural enemies from the landscape, causing a worse armyworm problem following treatment. The key to controlling fall armyworm is early detection because Infestations of fully mature larvae feed voraciously and can completely consume a lawn overnight. We will not be out of the woods for a fall armyworm outbreak until we get a good killing frost, so don’t let your guard down.

To scout for fall armyworm, examine turf or seedling wheat plants from eight locations measuring 1 square foot each. Examine turf along the field margin as well as in the interior. Look for “window paned” leaves and count all sizes of larvae. Total the number of larvae in each size class and divide each number by 8 to calculate the average number per square foot. Thresholds are not well developed for fall armyworm in turfgrass, but treatment is suggested when average counts reach two or three ½-inch larvae per square foot.

It is crucial that you target smaller caterpillars (1/2 inch or less) for control for two reasons. First, the caterpillars don’t cause really severe damage until they reach a size of one inch long, and secondly, smaller caterpillars are much more susceptible to insecticide control than larger caterpillars. Any product labeled for caterpillar control in turf should be effective for fall armyworm control in sod fields, lawns and golf courses. Some of the insecticides include those with the active ingredient of Bt, Spinosad, Bifenthrin, Imidacloprid, Carbaryl, Cyfluthrin, Lambda Cyhalothrin and Permethrin to just name a few. Labeled products for wheat include Sevin, Tracer, Karate, Warrior, Mustang Max, Tombstone and others. Most of these products are restricted-use, and most have grazing restrictions, so read and follow the label carefully. Be sure to apply insecticides only when periods of dry weather are expected since insecticide can wash off target with moderate to heavy rain. Light irrigation following application of granular formulations may be prescribed on the label, but don’t overdo it. Be sure to follow all label directions carefully to enhance safety and minimize harmful environmental effects.

If you have any questions, or would like further information on this or other related management topics, visit us on the west end of the Clay Jones Community Building at 1901 S. 9th Avenue in Durant, or call (580) 924-5312.

The Oklahoma Cooperative Extension Service offers its programs to all eligible persons regardless of race, color, national origin, religion, sex, age, disability, or status as a veteran, and is an equal opportunity employer.

Robert Bourne is a Bryan County Extension Educator.

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By Robert Bourne

Guest columnist

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