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Fall armyworm Q&ABy Miles Dabovich Texas Agrilife Extension agent, agriculture, Wichita County Q: Fall armyworms seemed to destroy my pasture "overnight", where do they come from? A: Fall armyworm is a tropical insect and overwinters only in the warmest areas of the US. As populations build throughout the summer, they move northward on weather fronts, often arriving in Oklahoma in late summer. Because several generations develop during the summer, the generations overlap and effectively create a continuous supply of moths (eggs). Any fall-planted wheat field that is emerging out of the ground could become infested. Fall armyworms infestations often go unnoticed for a while because they don't cause obvious damage until they get bigger. The caterpillars shed their skin five times before they quit feeding. To indicate the stage of growth that a caterpillar is in, we refer to them as instars. The first instar is the caterpillar just after it hatches. A second instar is the caterpillar after it has shed its skin for the first time. A six instar has shed it skin five times and will feed, bury itself in the soil, and pupate. If you were to ration out a supply of food to feed each instar, you would need to reserve 70 percent of the total supply just to feed a sixth instar caterpillar. It is similar to trying to feed a hungry teenager. Like a teenager, a sixth instar fall armyworm can eat lots of food in a relatively short period of time, and leave little for anyone else. Q. Is my wheat susceptible to fall armyworm? A. YES. Producers should be monitoring any emerged wheat for signs of fall armyworm feeding. Q. How can I recognize a fall armyworm infestation before it cause major injury? A. During the first three instars, the caterpillar does not remove much plant tissue. It will scrape off the epidermis of the leaf, leaving a clear, papery membrane that you can see through. This type of feeding is called "windowpaning" or skeletonizing. As the caterpillar gets larger, it chews through the leaf and begins eating along the margin of the leaf blade. Fourth through sixth instars chew along leaf margins and eat the entire leaf blade, as well as stems. The key is to look for the "windowpaning" as an early sign that you have an infestations. Q. How many fall armyworms are too many, and how do I control them? A. In fall seeded wheat, treat if you find two to three armyworms per foot row. In pasture, not established treatment threshold has been determined, however a general guideline for fall armyworm control in the southeastern US suggests that if you find two or three large larvae per square food in grass pasture, consider treating. Several insecticides are registered for control of fall armyworm in wheat, including Lorsban SG, methomyl (Lannate), Warrior T, and parathion (methyl or ethyl). Remember to follow all label restriction. In pasture, Sevin, malathion, Lannate (for Bermuda pasture only). Confirm 2F, and methyl parathion are labeled for control of fall armyworms. Q. Once this brood of armyworms is gone, can we quit worrying about them? A. The short answer is no. Fall armyworms are likely to be with us until we have a killing frost. However, we are getting late enough in the year that this will probably be the lst generation we see. The bottom line is that producers should remain vigilant. Q. If I treat for fall armyworm, when can I put my cattle back out to graze? A. Grazing restrictions for wheat depend on the chemical you use and are from 7 to 30 days. Grazing restrictions for pasture are from 0 to 15 days. Q. I have a wheat field that has been chewed to the ground, will it come back if I control the worms, or should I consider replanting? A. The answer to this question involves several considerations. First the armyworms are not controlled, they will continue to feed and keep the leaves from getting above the ground. At some point, the plants will simply "wear out" and die. If the wheat was very small seedling stage and under stress, the plants may not have enough energy reserve to recover. However, if the wheat has some time to develop top growth before the armyworms chewed them down to the base and you have adequate soil moisture, you may see a nice recovery of the stand. Q. My lawn is infested, what should I do? A. There are products available for control, but for Bermuda turf, you might consider not doing anything. It is late in the year, and fall armyworms will not likely cause serious damage. If you decide that control is required, several products are registered, including Bacillus thuringiensis (Javelin), Orthene or Sevin. A couple of new Pyrethroid insecticides are available for homeowners as well. One line is the Bayer Advances Lawn and Garden Multi-Insect Killer which contains cyfluthrin. Another line is the Ortho Home Defense Indoor and Outdoor Insect Killer, which contains bifenthrin. Remember to follow all label directions before applying any pesticides. 11/3/08 Date: 10/21/08
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