True Armyworms Migrate With Assistance of Strong Winds

HOUSTON – True armyworms are unpredictable, and their distribution has no set pattern from year to year. That is why, according to University of Missouri Extension Agronomy Specialist Sarah Kenyon, it is important to scout for this pest periodically.
“True armyworms migrate with the assistance of strong winds and storm systems, which carry them from southwestern portions of the country into more northeastern regions. The threat of true armyworm infestation increase when storm systems follow this pattern,” said Kenyon.
If the following for conditions are present during spring, armyworms could cause economic damage. One, high numbers of true armyworm moths. Two, cool and wet weather. Three, lush growth of grasses (especially tall fescue). And four, lack of beneficial insects.
IDENTIFICATION
True armyworm moths have grayish-brown to tan colored forewings, with a white spot located in the center of each forewing, and grayish-white to pale hindwings.
Larvae are almost hairless with smooth bodies. Small larvae are often pale green, but change to yellowish-brown or tan bodies with tan to brown heads mottled with darker brown patterns. Three distinct broad, dark stripes run the length of the body with one occurring on the back and one running down each side. Additional orange lines can be found running the length of each side of the body from head to tail.
“Look for four pairs of abdominal prolegs in the center of the body and a single pair of anal prolegs at the tail end of the larva. Each abdominal proleg will have a dark brown to black triangle located on the foot; few other larvae possess this characteristic,” said Kenyon.
True armyworm larvae hatch from spring laid eggs and rapidly grow through about seven or more worm stages (instars) as they develop from egg to adult moth. The early instars avoid light and spend much time close to the soil surface and on lower plant foliage.
“When scouting, look for small larvae under plant debris during the late afternoon, evening, and early morning hours. As the larvae increase in size, they will feed during both night and day periods and move upward to host plants as they consume foliage,” said Kenyon.
WHEN TO TREAT
Treatment is justified when an average of four or more half-grown or larger worms (one-half inch to one and one-half inch larvae) per square foot are present during late spring. Do this before more than two to three percent of seed heads are cut from stems in tall fescue seed fields.
Mustang Max or Warrior II are recommended to control armyworms at threshold levels.
“Scout fields at least two times weekly to determine if larval numbers and damage are increasing to intolerable levels. True armyworms cause destruction of plant foliage and cut seed heads to fescue pastures. Heavy infestations may defoliate and consume 100 percent of the grass foliage and seed heads and then move to adjoining grass pastures before continue feeding and eventually reaching maturity,” said Kenyon.
For more information, contact any of these MU Extension agronomy specialists in southwest Missouri: Tim Schnakenberg in Stone County, (417) 357-6812; Jill Scheidt in Barton County, (417) 682-3579; John Hobbs in McDonald County, (417) 223-4775 or Sarah Kenyon in Texas County, (417) 967-4545.