Fall Armyworm Outbreak: Round Two

About a month ago, we experienced a large fall armyworm moth flight into the state from southern areas. This resulted in outbreaks in Western Kentucky in pastures, sorghum, and soybeans, primarily double-crop beans.  Some surrounding states consider this the biggest outbreak of fall armyworm since the 1970s. A large number of fields were treated with insecticides to mitigate losses.

Now, we are entering round two with this pest! This past week, Extension agents and entomologists received dozens of inquiries about mystery eggs masses appearing on homes, lawn décor, pools, decks, seemingly everything. These have turned out to be egg masses of fall armyworm, and many have already begun hatching. Females of this species can each lay up to 1,000 eggs, and the eggs can hatch within 48 hours of being laid. This is the next generation of the fall armyworm, and it is a threat to pastures, sorghum, soybeans, some vegetables, and lawns

Figure 1. Young fall armyworm larvae on the tip of a grass blade. (Photo: Ric Bessin, UK),

The early bird gets the worm

Earlier this year, when dealing with the pest in July, many producers didn’t notice they had a problem until defoliation became noticeable. As a result, they were fighting small and large larvae together, which meant that many needed to use insecticide pre-mixes to control the larger larvae. With this next generation, and with this pre-warning of many egg masses, we recommend that producers scout their fields weekly and treat as populations reach the economic thresholds. By catching the infestations while the larvae are small, there are more cost- effective options available as small larvae are much easier to control. The key is identifying fields and other areas needing treatment while the larvae are still small.

Figure 2. Fall armyworm egg masses may contain 200 or more eggs and may or may not be covered with scales. (Photo: Ric Bessin, UK)

Scouting and thresholds for action

In soybean, we use a sweep net to estimate insect population densities, but in the case of defoliating insects, like fall armyworm or Japanese beetle, we just estimate defoliation. In pasture, we use a 1-foot plastic square, place it on the ground, and count the number of worms inside the square. In field corn, we count the number of egg masses or larvae on 20 consecutive plants.  We need these samples to be representative of the entire field, so multiple samples need to be taken across fields; the larger the field, the more samples are needed. For more information on scouting field crops, check out UK IPM scouting manuals.

In terms of thresholds for treatment, if more than two to three larvae per square foot are found in pasture, it would be time to control them with an insecticide or cut the field. In soybeans, a threshold for defoliating insects is 30% defoliation throughout the plant canopy 2 weeks prior to blooming (R1) and 15% defoliation throughout the plant canopy 2 weeks prior to flowering (stage varies) until the pods have filled (R7-R8). Although corn can be defoliated by fall armyworm, it loses much of its attractiveness to this insect after it passes through the vegetative stages. As much of the corn has already tasseled, it is less vulnerable than it would have been 5 weeks ago. Products recommended for fall armyworm are available in UK soybean, corn, vegetable and pasture recommendations online.

Considerations for home landscapes

Fall armyworms are usually associated with issues in pastures and crops, but they will cross over into the home landscape as well and attack turf in lawns. Initially when they feed, the tips of the blades of grass will have windowpane-like damage. As the caterpillars grow, they will progress into consuming whole blades of grass. The term “armyworm” also comes from the fact that these pests move in a group across the grass, creating a distinct line of damage opposed to undamaged grass. Newly planted sod is more susceptible to being killed by these pests than established turf areas. If you have noticed a high amount of eggs on your property, you might consider watching for damage in your lawn. You can also monitor for an increase of bird activity in patches of your lawn; they would be there to feed on the numerous delicious caterpillars.

If you have a yard that was treated with Acelepryn or Scott’s GrubEx in the spring/early summer for grub control, then you are most likely protected from any caterpillar damage as well (these products work on both groups as a systemic insecticide). If you haven’t had your yard treated, though, and are seeing a concerning amount of damage, you can control fall armyworms with pyrethroids like bifenthrin, cyhalothrin, and others. There are also organic options, like Bt sprays or Spinosad.

What happens next?

In terms of the future, fall armyworm will remain a threat until the first frosts of the fall. Cold weather in the fall and winter kills this insect. It can only overwinter in the extreme southern parts of the United States and re-infests the state each summer. We should expect a possible third generation this year.


By Ric Bessin and Jonathan Larson, Entomology Extension Specialists 

Posted in Forages, Grains, Lawn & Turf
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