Recent moth flight of armyworm (AW), Mythimna unipuncta (Figure 1), in western Kentucky has been much larger than is normal for this time of year. It is not terribly unusual to see problems with this insect on pasture and forage grasses in late June and July, especially in central Kentucky. This year’s much larger AW flight during late July and August in western Kentucky is truly unusual. It is difficult to know what to make of this because it has no precedence in our data set. Nevertheless, the fact is they are here.
Generally, I would expect to be looking for fall armyworm (FAW), Spodoptera fugiperda, (Figure 2) at this time of year, but our moth captures for this insect in both central and western Kentucky have been quite low, perhaps because the cold winter and spring pushed their overwintering locations farther south, and/or they were later beginning their annual northward migration. Even so we have seen some localized, but significant, populations in south-central Kentucky.
Armyworm Flight Events
The data we see on the armyworm moth flight of course, describes the movement of adults, which are not the damaging stages. It is the juvenile caterpillar stage (Figure 3) that will appear in September that might pose the threat. Moth flights in the earlier portion of the season followed a relatively normal pattern. Our most common problem with AW comes in May on small grains and that is what we saw this year. Also, there is often a small bump in flight in June and July that usually doesn’t amount to much. Those events were relatively normal this year. What is different this year is: following that relatively normal flight in June and July, we have another and much larger flight in July and August that will produce caterpillars in September.
Host Plants in Danger
AW has a very broad host range; it eats on many plants but really prefers grasses. On the whole, corn and grain sorghum should be too close to maturity for much damage. However, late-planted soybeans, forage crops, and especially newly seeded forage crops, could be in some danger.
Producers are advised to keep an eye on very late planted/late developing soybeans, along with forage crops (particularly grasses and especially newly planted grasses, and grass-alfalfa mixes). There is no established thresholds for this pest in these circumstances, but populations around 4 to 6 worms per square foot probably require treatment.
Doug Johnson, Extension Entomologist