Fall Armyworms in Hay Fields and Pastures

By Blake Layton

Fall armyworm populations were unusually heavy the past two years.  They have been much lower so far this year, but this can change quickly.  Fall armyworms can cause heavy forage losses, especially in highly managed bermudagrass hay fields and growers will need to check hay fields and pastures regularly for this pest for the remainder of the season.  It is also a good idea to go ahead and get sprayers ready (cleaned, repaired, and calibrated) and to make plans for what insecticide you will use if you need to treat and where you can get it.  Failure to detect and treat a developing fall armyworm infestation in a timely manner can result in the loss of a cutting of hay or loss of valuable grazing.  Protecting forage from fall armyworms can require a lot of spraying, with some producers making six or more treatments during heavy fall armyworm years.

Checking for Worms:  The best way to avoid losing a cutting of hay to fall armyworms is to visit fields every three or four days and purposefully check for fall armyworms.  Get out of the truck, get down on your hands and knees, and look closely.  Scout for fall armyworms by vigorously shaking the grass then carefully counting the larvae that have fallen to the ground in a one square foot area.  Do this at several locations in the field and average your results.  Treatment is recommended when counts reach 5 to 7 larvae per square foot.  Be sure to look carefully for small caterpillars.  You want to find and treat them when they are small because small caterpillars are easier to control, and more importantly, have not eaten nearly as much as they are going to eat.

Like most caterpillars, fall armyworms do 90% of their eating during their last 2 to 3 days as a larva—when they are “teenaged” caterpillars.  This means that if a field has reached threshold on Friday, but is not scheduled to be cut until the middle of next week, it still needs to be treated.  A moderate to heavy population of large fall armyworm caterpillars can eat a whole field of grass in just a couple of days.  Be sure to pay attention to pre-harvest intervals when choosing an insecticide to use on a field that is near cutting!

Fall Armyworm Warning Signs:  Experienced forage producers learn to recognize the early signs of fall armyworm feeding.  Newly hatched caterpillars are too small to feed all the way through a leaf blade.  They begin by feeding on the underside of the leaf, but leaving the clear upper epidermis intact.  This creates tiny “windowpanes” in the grass blades that appear silver or white in color.  When large numbers of small larvae are just beginning to damage a field the grass often has a subtle “frosted” appearance because of these windowpanes.   This phenomenon is easier to see on the wide leaf blades of barnyard grass, which is a favorite food of fall armyworms.  Learning what this looks like on barnyard grass can help you recognize it on bermuda.  Learning to watch for this early sign of infestation can help save a cutting of hay.

Many producers watch for egrets and other birds feeding in a field as an indicator of fall armyworm problems.  If you see birds in the pasture you definitely want to check out what they are feeding on, but do not use this as your only way to check for armyworms.  This sign often comes too late to avoid damage because the birds are not attracted until the caterpillars are already pretty big. 

Spray for Worms or Just Go Ahead and Cut the Field?: One option for dealing with an infestation of fall armyworms in hay fields is to just go ahead and cut the field.  This can be a good choice, but only if the field is nearly ready to cut anyway and the forecast promises good hay-curing weather.  Be careful with this decision.  If there are a lot of big caterpillars they will go ahead and “cut the field for you” if you give them another couple of days.  The point is that if you decide to go ahead and cut the field you need to do it very soon—as in this afternoon or first thing tomorrow.  If there are threshold numbers of large caterpillars and it will be two days or longer before you can cut the field, then you probably need to go ahead and spray.    

Control Options: Over the past few years there have been significant improvements in the treatment options available for control of fall armyworms in pastures and hay fields.  Some traditional fall armyworm products, like Sevin and Malathion, are still available, but there are also several new products that are cheaper, more effective, provide longer residual control, or have more favorable pre-harvest intervals.  These include several pyrethroid insecticides (Mustang Max, Baythroid, and Karate Z), as well as Tracer and the insect growth regulators, Intrepid and Dimilin.

The following table lists treatment options for fall armyworms in hay fields and pastures.  Although a rate range is given for most products, the low rate is generally adequate in most situations.  When selecting insecticides pay close attention to the pre-harvest interval, and note that pre-harvest intervals can vary depending on whether the grass will be grazed or cut for hay (note that “days to harvest” means days till cutting, not bailing).  You may also want to consider the number of acres one gallon of product will cover, the size of container the product is sold in, and the size of field you need to spray.  Often the most economical option is to choose a product/container size that will best “fit the field.”

It is also worthwhile to compare costs on a per acre basis.  When comparing costs, keep in mind that some products provide longer residual control than others, and during years of heavy fall armyworm pressure, this can make the difference between having to spray once or twice vs two or three times to make a cutting of hay.  In general, the growth regulator products (Intrepid and Dimilin) provide longer control than the pyrethroids (Mustang Max, Baythroid, and Karate Z), but growth regulators are slower-acting and are best used when caterpillars are small.  Because rapidly growing grass results in new, untreated leaf area and dilutes insecticide residues, few treatments will provide more than about two weeks of control.

Control Options For Fall Armyworms in Pastures