Author: Mark Sulc
A severe and unusual fall armyworm outbreak has developed across Ohio and is causing destruction in many forage fields. For more complete details on this pest, including how to scout for this pest and options for cotrol, see the articles posted HERE. Below are additional comments based on the many questions coming in on how to manage these damaged forage fields.
If the hayfield shows any feeding damage at all and is reasonably close to having enough growth for harvest, cut it as soon as possible. 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! Be sure to read the articles in the link provided above and learn how to scout for this pest and the control options available.
If your hayfield is not quite ready for harvest, scout it now and continue to scout it every couple of days for fall armyworm presence until you do cut it. Be prepared to make a rescue treatment.
If an established hayfield has already been severely damaged by fall armyworm, cut it down and salvage what you can or mow off and remove if possible the stubble that is left. Established alfalfa should recover from having the leaves being stripped off and cutting it will stimulate the regrowth process. The speed of recovery will depend on how many crown buds in alfalfa and how many young tillers in grasses were devoured by the insect. Be patient, but it is imperitive to stop the feeding from continuing or happening again by re-infestation from another generation that might occur yet this fall.
Whether or not the hayfield was damaged before cutting it, monitor the regrowth carefully for the rest of the growing season. Live fall armyworms have been reported to still be present under windrows after cutting the forage. In addition, the egg laying could have been somewhat asynchronous over time, so eggs could still be hatching in and around the field. 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 to be prepared if another generation develops.
New summer seedings of grass, alfalfa, or red clover that are damaged severely in the early seedling stages by fall armyworm are likely to be completely lost. Be especially attentive for fall armyworm in any new seedings you have made late this summer!
The best time to take a last harvest of alfalfa and other legumes is in early September in Ohio, for the least risk to the long-term health of the stand. In many fields, the fall armyworm has sessentially taken part (or most) of your last cutting of the season. Forages and forage legumes in particular need a fall period of rest to replenish carbohydrate and protein reserves in the taproots that are used for winter survival and regrowth next spring. So controlling re-infestations of the fall armyworm is critical as we move into fall. Scouting and insecticide application when the economic threshold has been reached is critical to the long-term health of all forage stands and alfalfa in particular as we move into fall.
Apply any needed P and K fertilizer as soon as possible on damaged forage fields. Base the application rate on a recent soil test. A sufficient nutrient supply will help the crop recover and improve the health of the stand going into the winter.
A small application of nitrogen (100 lbs/acre of urea) will likely be helpful to the recovery and tillering of grass stands. Our turfgrass colleagues recommend nitrogen applications in early September and early November for maintaining healthy turf. So in this case where the grass hay or pasture was damaged by fall armyworm, a little nitrogen will likely promote more tillering and recovery this fall.