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Armyworms Return for 2010

Posted Oct. 1, 2010

Over the last several weeks of summer 2010, we have had numerous reports from northern Texas and southern Oklahoma of armyworm outbreaks. Armyworms have caused significant damage or stand loss in early planted small grains, alfalfa, bermudagrass and even tall fescue. The key to keeping these pests below the economic threshold is scouting at least every couple of days to locate worms when they are small and easier to control, as well as identifying what species is causing the damage. If left untreated, armyworms can destroy a newly established winter pasture in a very short time.

It is critical to identify and, if necessary, treat armyworms by their fourth larval stage. As indicated in the following table, they will consume approximately 75 percent of their total dry matter intake during the final two larval stages. Additionally, once they become greater than ½-inch long, they become more difficult to control with insecticides.

Scout along field borders as well as uniformly across the field. Armyworms will often move into fields from weeds along the field edges and fencerows. Treatment can sometimes be limited to the field margins, saving considerable chemical cost. Damage will start as feeding on the epidermal layers of the leaves, causing a windowpane effect, and progress to entire chewed leaves. Look on the plants for actively feeding caterpillars, but also look underneath dirt clods and litter for caterpillars that are hiding to escape daytime temperatures. They are usually easier to find in the early morning hours.

In addition to scouting for armyworm numbers, it is important to identify the species. The most common damaging species is the fall armyworm, Spodoptera frugiperda, but the beet armyworm, Spodoptera exigua, has also caused significant damage the last few years. Fall armyworm larvae are green, brown or black, and have a very distinctive white line between the eyes that forms an inverted "Y" on the head capsule. Beet armyworm larvae are light to dark green, with dark and light stripes running the length of the body and a somewhat less distinctive inverted "Y" on the head capsule. There is usually a very distinctive dark spot on the second segment behind the head and above these lines. Identification is important because there may be differences in control strategies depending on the species present. Continue scouting every few days until after a killing frost.

The economic threshold for control varies depending on the cost of chemical treatment and the value of the crop, but, in general, two worms per row foot in small grains and three worms per square foot in bermudagrass is a good starting place. There are a wide variety of insecticide options for control of armyworms. Which to choose will depend on the crop grown, the armyworm species present, their size or growth stage, proximity to sensitive crops or neighborhoods, etc. A few of the products available include: diflubenzuron (Dimilin®), spinosad (Tracer®), malathion (many trade names), carbaryl (Sevin®, many trade names), chlorpyrifos (many trade names), methomyl (Lannate®), methyl parathion (many trade names), zeta-cypermethrin (Mustang Max®), gamma-cyhalothrin (Proaxis®), lambda-cyhalothrin (Warrior®) and beta-cyfluthrin (Baythoid®). Always refer to the specific insecticide label to ensure it is labeled for your crop and in your state, for the correct rate to control the species present and for any grazing or harvest restrictions. Remember that the label is the law.

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