Howdy everyone!
We've been moving into cooler weather and with the moisture we've received, remember to keep an eye out for fall armyworms. Some reports of small (2-3 instar) worms have made their way to our office.
Photo by Sarah Marsh (2018)
Armyworm Injury
Fall armyworms can inhabit multiple different hosts but feed primarily on bermudagrass, ryegrass, sorghum, and wheat. Defoliation typically occurs from August to November, with injury appearing on the edges of the affected areas and traveling in as the worms "march" on once resources are depleted.
Injury from small larvae can appear as "windowpaning", but as larvae progress through the instars, defoliation rapidly appears. Fields may have a "frosted" appearance.
Photo by Holly Davis
Photo by Alabama Extension
Scouting
Armyworm scouting should occur every 2 weeks at minimum. For ideal scouting conditions, check areas in the early morning or evenings when the larvae are most active and feeding.
There are multiple methods to scout for armwyorms. The simplest is to simply get down to ground and look in the grass or soil. On hot days, the larvae may be on the undersides of leaves or even in the top layer of soil, hiding from the heat.
Sweep nets are very effective for collecting samples in tall grass. Another method is to run your hands across the grass in a 1-2 square foot area, knocking the worms to the soil, and count the larvae that are dislodged.
Regardless of the method, knowing the number AND size of the worms is important to determine the economic injury potential. Worms smaller than 1/2 inch are considered younger instars and are not as injurious as worms sized larger than 1/2 inch. The economic injury threshold for pastures and hay fields is 2-3 worms sized 1/2 inch or longer per square foot OR 2 worms per sweep. The threshold is not exact, but larger numbers of smaller worms also qualifies.
Control
(The information given herein is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement by Texas AgriLife Extension and Texas A&M University is implied. Always consult the label)
Control measures should always take into consideration several factors. If the field is being used for hay, harvesting as early as maturity allows is a option to try to prevent further defoliation. However, the larvae may still feed on the cut grass.
Chemical control is one of the best tools in the fight against armyworm infestation. In Texas, there are 47 products registered for use in bermudagrass against fall armyworm. The cost, efficacy, hay and grazing restrictions, and days residual varies amongst all products. Always consult the label; the label is the law. When making application choices, consider the economic injury threshold and incoming weather conditions; there are only a few truly rainfast options available, such as Prevathon, Vantacor, or Besiege, all of which contain chlorantraniliprole. These products are more expensive but do produce longer-lasting effects.
For additional questions about FAW control, please reach out to your local County Extension Agent, IPM Agent, or Extension Entomologist. Always use an insecticide according to the label.
Texas A&M AgriLife Extension Service is not responsible for insecticide applications, damages, or other issues encountered.
For more information about fall armyworms, there is an excellent distribution here written by Dalton C. Ludwick, Holly Davis, Sonja L. Swiger, and David L. Kerns, Extension Entomologists, Texas A&M AgriLife Extension Service
Have a good week,
Sarah Marsh
IPM Extension Specialist
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