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Fall armyworms of different sizes collected from an alfalfa field in Luzerne County. The pest has been reported in several counties this summer, causing damage to alfalfa and hay fields.

Perhaps you recall that fall armyworm was quite active and abundant in late August into September, feeding in alfalfa, timothy, and other types of hay crops; in some cases, stripping fields completely.

At the time, it was not clear if the species could squeeze in another generation, or whether the moths from those caterpillars would fly back down south.

Extension entomologist John Tooker reports that at least some of the moths must have stuck around to lay eggs.

In the last week Tooker has heard of new infestations of fall armyworms in small grains (e.g., barley and oats), grass hay, and cover crops.

The best solution for these infestations will be getting a frost, which should kill the caterpillars, but that may not be happening any time soon in some parts of Pennsylvania.

As we wait for cold weather to arrive, the best approach is to scout fields, particularly grass crops, for infestations.

An abundance of caterpillars in one place may be enough to identify fall armyworm; you should not be worried about finding just a few caterpillars.

In small grains, if you find more than six caterpillars per square foot, it will likely be economical to treat that population. Remember that caterpillars longer than three-quarters of an inch are difficult to control and tend to require the higher rates listed on the insecticide labels.

Grass crops tend to have fewer insecticides labeled than alfalfa, so be sure to check labels to be sure that the pest and crop species are listed.

Popular pyrethroids (active ingredients like lambda-cyhalothrin and zeta-cypermethrin) tend to be broadly labeled and should be effective on caterpillars.

Beyond the typical pyrethroids, products containing Bt (Bacillus thuringiensis), spinosad, or growth regulator insecticides (active ingredients like diflubenzuron and methoxyfenozide) are options that should stop fall armyworms.

For details on insecticide options, consult the Penn State Agronomy Guide.

Leon Ressler is a Penn State Extension educator based in Lancaster County, Pennsylvania.

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