Unusual Armyworm Outbreaks are Taking Many by Surprise

Kelley Tilmon, Mark Sulc, Andy Michel, James Morris

Figure 1. Fall armyworm feeding damage. Photo by James Morris, OSU Extension

We have received an unusual number of reports about fall armyworm outbreaks particularly in forage including alfalfa and sorghum-sudangrass, and in turf.  Certain hard-hit fields have been all but stripped bare (Figure 1).  Armyworm is not typically a problem in Ohio in late summer, so we encourage farmers to be aware of feeding damage in their fields.  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.

Figure 2. Fall armyworm egg mass, with cover peeled back. Photo by Ric Bessin, University of Kentucky.

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.

Figure 3. Fall armyworm caterpillar, with an inverted “Y” near the head. Photo by James Morris, OSU Extension

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, but 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.

In corn, armyworms can randomly feed on leaves, with holes occurring throughout the leaf surface. The more damaging stage is when they feed on developing silks and kernels after entering the ear. Once they enter the ear, control by insecticides is much more difficult.  Most Bt corn varieties with above-ground protection is labeled for armyworm control, but resistance to several Bt traits has appeared in the US.  While we have not found Bt resistance in armyworms in Ohio, we would recommend growers scout ALL corn (Bt or non-Bt) for any evidence of damage or resistance. If feeding is found, please contact us (tilmon.1@osu.edu, or michel.70@osu.edu) or your local extension educator.

Figure 4. Fly Free Dates in Ohio. Wheat planted after this date have lower risks of damage from Hessian Fly as well as other pests, including fall armyworm and aphids that spread wheat viruses

Fall armyworm does not overwinter in Ohio.  Moths come up from the South early in the season and temporarily colonize the area, especially in grassy areas.  The current caterpillars are second-generation.  If we have a warm fall we could possibly see a problem third generation, especially in forage, cover crops, and winter wheat planted before the fly-free date (see Figure 4).

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

 

 

 

 

 

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