We are continuing to see high fall armyworm moth catches in some of our Ohio traps [see the Lepidoptera Monitoring Report in this week’s newsletter]. It is likely that many of these moths may be “home grown” – meaning they are not individuals that got blown up from the Southern US [the way we get them in the first place], but offspring of those immigrants that hatched and grew to adulthood in Ohio. Those home-grown fall armyworm moths will still be laying eggs as long as warmer days without hard night frosts hold. Those eggs will hatch into caterpillars which do the feeding damage. Corn and soybean are likely beyond any danger as they are rapidly reaching maturation. However, in the 2021 outbreak we continued to see damage to forages and turf into the fall [which was an exceptionally warm and drawn-out autumn].
Therefore, we encourage farmers to be aware of feeding damage, especially in forage crops. For those concerned with turf, visit the recent turf newsletter article at https://buckeyeturf.osu.edu/news/fall-armyworm-alert
Fall armyworms are much easier to kill when they are smaller, and feeding accelerates rapidly as they grow, so early detection is important. Look for egg masses glued not only to vegetation but to structures like fence posts. Egg masses have a fluffy-looking cover (Figure 2). When the cover is peeled back, eggs are pearly and tan when new, and turn darker as they approach egg-hatch.
Fall armyworm caterpillars vary in color from greenish to tan to dark brown with stripes along the body. They can be easily confused with other species, but a good identifier is an inverted white “Y” shape behind the head. (Figure 3). Another species, true armyworm, feeds at night but fall armyworm will feed during the day.
Insecticides will not penetrate egg masses well; it’s best to spray caterpillars when they are less than ¾ inches long, at which point most armyworm-labeled pyrethroids will kill them reasonably well. For larger caterpillars, products containing chlorantraniliprole will provide longer residual which may help with control of the harder-to-kill caterpillars over ¾ inches.
In forages, a threshold that can be used is 2-3 fall armyworm larvae per sq foot. If larvae are smaller (less than ¾ inch), they can still do a lot of feeding and are worth treating with an insecticide application. An early cut can help limit damage to the alfalfa, but one must check the field for survivors. If survivors are abundant, an insecticide application may be warranted to protect nearby fields. Armyworms get their name from moving in large bodies (marching) to new feeding areas.
Hay fields with infestation that are near harvest should be harvested as early as practical, and then the regrowth closely monitored for fall armyworm activity. The last cutting before the fall rest period in Northern Ohio should be taken around September 7th and in Southern Ohio around September 15th. Cutting or fall armyworm damage during the fall rest period can decrease winter survivability. In Kentucky, the fall armyworms have been reported to be present in hayfields after harvesting the crop off. This and the fact that we could get another generation if the fall is long and warm are reason to continue monitoring closely.
Badly damaged alfalfa or grass hay fields should be cut and then rested the rest of this fall with no fall cutting. Fertilize according to soil test recommendations. Monitor the regrowth closely to catch any re-infestation that occurs. Established alfalfa should come back from fall armyworm damage. Recovery of the cool-season perennial grasses will depend on the relative severity of the damage, the overall health of the stand going into the infestation, and how many young tillers were not consumed.
Please visit the Forages chapter in the Michigan State/Ohio State Field Crops Insect Pest Management Guide for management notes and labeled insecticides in forages. https://aginsects.osu.edu/sites/aginsects/files/imce/MSU%20-%20OSU%20Insect%20IPM%20Guide.pdf