Bryan Jensen
UW Extension
Survey staff for DATCP’s Wisconsin Pest Bulletin have reported significant flights of true armyworm adults in several Wisconsin locations. What should we be doing?
First let’s review some basic information. True armyworm is the approved common name. However in Wisconsin we usually refer to them as “armyworms” because we rarely have other species of armyworms causing damage to field crops. Armyworm larvae may grow up to 1 ½ inch long. Coloration is variable, but commonly they have a lighter colored underside, orange stripes with white borders on each side and darker striping on their “backs”. Armyworm heads are tan w/ a network of veins that are easily recognizable. Especially on older instars.
Armyworm do not overwinter in Wisconsin. Instead, they migrate to the Midwest on spring weather fronts. Blacklight traps are an important monitoring tool because the timing and intensity of flights are unpredictable. High trap catches do not always correlate with field activity but they do give us enough forewarning to prepare for field scouting.
We usually have 2 generations/growing season. Larval damage resulting from the migrating adults will often be spotty and locally severe as compared to the summer generation which tends to be more wide spread. Understanding insect behavior can help fine-tune our field scouting activities. But please keep in mind we have a difficult time understanding human behavior let alone insect behavior. Adult females are attracted to grasses to deposit small white eggs which are usually laid is rows or clusters. Therefore concentrate your early scouting efforts in corn fields planted into grass cover crops or those with early season grassy weeds. Another helpful hint to focus on, are those corn fields no-tilled into alfalfa. Whether the alfalfa was fall or spring killed, both situations can have significant armyworm damage. Small grains are also attractive egg laying sites. Particularly those fields with high plant density or in areas that are lodged. Soybean and alfalfa, both broadleaf crops, generally escape damage, although larvae may feed on grassy weeds within a field.
Armyworm larvae do not like to feed during daylight hours. You commonly find them feeding nocturnally and/or in shaded areas like corn whorls. Look for frass (fecal material) in the whorls, ragged leaf feeding on the leaf margin or occasional ragged holes in the emerged leaf.
Consider treating wheat or other small grains when you find more than 3 larvae/square foot and you have significant leaf feeding. Be aware that larvae can also switch from leaf feeding to head clipping as plants mature. Feeding injury in wheat (and corn) may be variable, sometimes making spot treating and/or edge treatments practical.
In corn, treatment can be suggested when 25% of the plants have two or more larvae/plant or when 75% of the plants have one larvae/plant. Small larvae are much easier to kill than the later instars. Treatment may not be suggested if larvae are greater than 1 ¼ inch in length because of reduced mortality and the fact that the majority of defoliation has already occurred.
- Armyworm Larvae
- Armyworm Feeding Damage