Managing Forage Stands Damaged by Fall Armyworm

By:  Mark Sulc

Alfalfa Damaged Field, Photo Courtesy of Mark Badertscher

A severe and fall armyworm outbreak developed across Ohio and neighboring states. It has caused serious destruction in many forage fields. For more complete details on this pest, including how to scout for this pest and options for control, see the articles posted at https://forages.osu.edu/forage-management/pests-diseases.

This article addresses how to manage forage stands damaged by the fall armyworm.

Fields with minor to no damage seen.

If the hayfield or pasture shows any feeding damage at all and is reasonably close to having enough growth for harvest, cut or graze it as soon as possible. This is perfect timing to take the last cutting of the season (see article on that topic at https://agcrops.osu.edu/newsletter/corn-newsletter/2021-29/autumn-forage-harvest-management. If there are large numbers of fall armyworms present (more than 2 to 3 per square foot) and they are ¾-inch or larger, they will “harvest” the entire field for you while you sleep another night or two. So be aware of what is in your hayfield!

If your hayfield is not quite ready for harvest or is regrowing from a recent harvest, scout it now and continue to scout for fall armyworm every few days until you do harvest it. Be prepared to make a rescue treatment if fall armyworm numbers reach the threshold of 2-3 per square foot.

Fields with severe fall armyworm damage.

If an established hayfield or pasture has already been severely damaged by fall armyworm, cut it down and salvage what you can or mow off and remove the stems or graze it to prevent any windrows from smothering of the regrowth. This mowing will stimulate the plants to regrow.

But be aware that fall armyworms have been seen to survive a cutting, so they could continue to devour the crown buds and any regrowth. Those surviving fall armyworms could also move to adjacent fields including soybean and corn (especially non-Bt corn hybrids).

Alfalfa Field Partially Mowed after Fall Army Worm Damage. Photo Courtesy of Mark Badertscher

Established alfalfa should recover from having the leaves being stripped off. Essentially, the fall armyworm took the best half of your last harvest. Cutting of the remaining stems will stimulate the fall regrowth process.

The speed of recovery will depend on how many crown buds in alfalfa were devoured by the insect. Regrowth will be slower if crown buds were fed on and new crown buds need to be initiated. Be patient, but it is also very important to stop the feeding from continuing.

Be on the alert for any second infestation from another generation that might occur yet this fall. The Ohio State University Extension entomologists and extension educators across the state are monitoring for further fall armyworm moth flights and which could potentially lead to another generation.

Established grass hayfields and pastures will likely show variable recovery depending on the extent of fall armyworm feeding on new tillers and the soil moisture situation. With severe feeding and dry soil conditions, permanent damage and loss of stand could occur. With more limited feeding and good moisture conditions, recovery should occur this fall.

New seedings made late summer with severe feeding by fall armyworm in the early seedling stages are likely to be completely lost. Going forward, if your new seeding has no signs of fall armyworm, be monitoring every few days for fall armyworm until frost.

Summary of steps to help your forages recover:

• Stop the feeding of fall armyworm and continue scouting every few days to prevent re-infestation. This is critical in new seedings made late summer.
• Harvest or graze off the alfalfa and red clover stems and mow or graze grass stubble as soon as possible to stimulate regrowth
• Check soil fertility and make corrective applications of P and K if needed
• On grass stands, apply 50 to 75 lbs of nitrogen per acre as soon as possible, right ahead of a rain forecast (to wash the N into the soil). Our turf specialists recommend N application in September to help lawn grasses tiller through the fall and in early November to increase root growth (https://turfdisease.osu.edu/news/benefits-late-fall-fertilization-0). So a shot of nitrogen is probably a good practice this year in damaged grass hayfields and pastures. Recent preliminary work Chris Teutsch at University of Kentucky (personal communication) showed a positive response in grass pasture growth the next spring after a late fall N application (20 lbs of additional forage per 10 lbs of actual N applied). While this idea needs further testing, the evidence from University of Kentucky and a great deal of work in turfgrasses suggest nitrogen application in the fall will be beneficial to grass hayfields and pastures recovering from fall armyworm damage. Make sure there are green tillers present in the grass crowns before investing in the nitrogen.
• Do not harvest fields later this fall. Give them a rest and allow leaf area to capture sunlight so energy and protein reserves in the plant are replenished before winter.

It is essential to continue monitoring the forage stand and apply timely control of fall armyworm if 2 per square foot are present to prevent additional feeding. We have time for recovery this fall, assuming additional feeding does not occur and the damage already done is not so severe as to have killed the stand.

The fall armyworm egg laying could have been somewhat asynchronous over time, so eggs could have been recently hatched or are still hatching in and around your forage fields. Fall armyworm population numbers can grow exponentially with each advancing generation. So, we aren’t out of the woods even after cutting or after an insecticide treatment applied now. Continued monitoring this fall is very important.

Fall Army Worm Infestation. Photo Courtesy of Mark Badertscher