Monitor Fields for Fall Armyworms

Joshua Salley  |  8/21/2010 1:41:51 AM

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From drought, to extreme heat, to insect infestation, a farmer’s problems never seem to cease. Fall armyworms can be a farmer’s worse nightmare as they feed on pasture forage, corn, cotton, beans, and numerous other plant species. Dry, hot summers typically create favorable conditions for armyworm infestations. Armyworms can wreak havoc on bermudagrass and bahiagrass fields. Intense infestations have been known to eat through large bales of hay, destroying them so severely that they fall apart.

Armyworms may be spotted on foliage any time during the day; however your best chances to observe these insects are early in the morning and late in the evening. With heavy infestations, droppings from the caterpillars may be spotted on the ground. They also tend to gather in the leaf litter.

Adult fall armyworms are 1.5 inches long. Fall Armyworms always have striped bodies, but they are not always the same color. Their background color ranges from light green to almost black in some cases. Fall armyworms can be identified by four black dots on the back of the tip of the abdomen. Larger caterpillars usually have a light-colored, upside-down Y-shape on the head and three white lines on top of the segment just behind the head.

Before any control measures are taken, producers should first scout their fields to determine the extent of their infestations. Scouting is a quick and easy process which consists of walking into your field from all four sides. Stop at different locations in the field and look closely for small caterpillars feeding on the grass and other foliage. If any are spotted, do your best and estimate the number of caterpillars per square foot in 8 different sites. Divide the total worm count by eight to find the average number of armyworms per square foot. Take samples in the interior of the field because armyworms tend to be the heaviest at field margins. A lot of times only the field margins are required to be treated. Thresholds for pastures and lawns vary with conditions but typically control measures should be taken when worm counts exceed three or more worms per square foot. Pay attention to the size of the pests, as this is important to the success level of treatment. Treatment should be considered while the worms are .5 to 1 inch in length. Once they have matured to over 1 inch long, the majority of the crop damage has already been done; therefore chemical control will not likely provide economic return.

According to the 2010 Louisiana Insect Pest Management Guide, insecticides for controlling armyworms and fall armyworms on pasture grasses and hay crops include;:carbaryl (Sevin), Lannate, Methyl parathion and Tracer.

For more information on this topic and others, please contact the LSU AgCenter Red River Parish Extension office at 318-932-4342.
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