Watch Out for Fall Armyworm Damage, Particularly in Hay Fields

Entomologists from around our region, ranging from Kentucky, Virginia, Ohio, Indiana, and Ontario, are reporting surprising infestations of fall armyworm in grass hay and alfalfa fields. Some of these states have also had reports of fall armyworm feeding extensively in turf. In Pennsylvania, just this week we started to hear of infestations, with reports coming from Franklin, Somerset, and York Counties. In York County, the infestation more of less removed leaves from grass hay fields (Figure 1). Given the regional scope of the challenge, it would be surprising if infestations in Pennsylvania were limited to those three counties. My best advice is to scout hay and alfalfa fields for infestations of fall armyworm caterpillars, noting the size of the caterpillars and what portions of the fields are infested. The abundance of caterpillars in one place may be enough to identify the species; we are not worried about just a few caterpillars. If you find more than 2-3 caterpillars per square foot, it will likely be economical to treat that population. If you find damage, but the caterpillars are absent, look at adjacent fields to see where they might have moved.  They can feed in many crop species, but are known to damage corn and soybeans. Their feeding in corn should be limited by aboveground Bt traits.

When considering how to control the infestations, a few inter-related details need to be kept in mind. First, the smaller the caterpillar, the easier they are to control; once caterpillars are three-quarters of an inch to a inch long, they become more challenging to control. Second, it will be easier to control the caterpillars when they are in hay than if they get to soy or corn fields. If you catch the infestation early enough, cutting the hay would be an option to protect the crop, but cutting may also prompt the caterpillars to march to adjacent fields or lawns, where they can find healthy green plants. A moving population may be vulnerable and treatable, particularly if the great majority of caterpillars head in one direction, but controlling them would require good timing and availability of a buffer area to treat before they get to an adjacent crop. To avoid driving caterpillar populations out of fields, spraying the hay, rather than cutting (or before cutting) is probably a good approach. Third, the choice of insecticide is important. Of course, the insecticide needs to be label for the crop species and armyworms; alfalfa has more options than grass hay. When caterpillars are small (less than three-quarters of an inch), popular pyrethroids (active ingredients like lambda-cyhalothrin and zeta-cypermethrin are broadly labeled) should be effective on caterpillars, but efficacy will decline when caterpillars are bigger. Beyond the typical pyrethroids, growth regulator insecticides (active ingredients like diflubenzuron and methoxyfenozide have broad labels) are options that are more specific and kill the caterpillars when they molt, but caterpillar death may take a few days to materialize. Growers should use higher rates the heavier the infestation and the larger the caterpillars. For details on insecticide options, see the Agronomy Guide and be sure to consult labels for specifics for each product.  For more detail on fall armyworm biology and life cycle, see our factsheet on fall armyworm in corn.