Treating Fall Armyworm Invasions in Pastures

Keith Fontenot  |  7/12/2010 8:07:32 PM

Local cattlemen and other livestock and hay producers are seeing an invasion of their pastures and hay fields by an army that is devouring their grass and chewing into profits.

This invading force is made up of millions of armyworms, searching for lush, tender, green forage that they consume at an alarmingly fast rate. One of the best indicators of worms in a pasture will be large numbers of white cattle egrets walking around and feeding on these worms, leading to the joke that our cattlemen have become white “chicken” farmers.

The main pest in these pastures is the Fall Armyworm. This worm is brown with a black head, yellow stripes down the back and a light colored, inverted Y on the front of the head. They are usually about 3/4 to 1 1/2 inches long for the older larvae and have four pair of prolegs. The coloration of this pest may also vary with the material it is feeding upon. This description also fits the regular armyworm which along with the Fall Armyworm feed on several types of vegetables, as well as soybeans, corn, sweet potatoes, grasses and other crops at different ages and stages of growth.

These worms can produce from five to 10 generations in a year. We usually see the most damage from the summer and early fall hatch-outs. The adult female moths may lay up to 2,000 eggs in clusters of 25 to 100 on the blades of grasses. These eggs are round, white and side by side and may be partially covered with some of the scales from the female moths body which may give them a “hairy” appearance. Moths will also be attracted to lay eggs on white, yellow and red test plot flags just as they will on red markers around a football field full of lush tender grass. These egg clusters are an indication of large numbers of worms to come.

Eggs normally hatch in six to ten days and the young caterpillars begin feeding, especially at night or during cloudy weather. This larval or caterpillar stage is the stage we most often see and also causes the damage to our forages and lawns. The larvae progress through six instar growth stages in thee to fou weeks, with the last instar being the largest and consuming 80-85 per cent of the foliage eaten during the insect’s life. Full grown larvae then pupate in a silk cocoon usually under leaf litter or in cracks in the soil for seven to 10 days. The moths will emerge from this stage and the cycle is started over again. Normally ,when the moths emerge, they tend to fly northward, sometimes very long distances, 100-200 miles before they begin to lay eggs. The entire life cycle of the insect may take anywhere from 24-30 days and is somewhat temperature dependant.

The LSU AgCenter entomologists recommend controlling these worms when population levels average one per sweep of a sweep net or two worms per square foot when checking several different areas in the field. This also varies when this invasion moves from the field to the lawn where homeowners show little tolerance and areas to control are much smaller with more intense applications of pesticides.

Pesticide application rates are given at a low and a high range with the judgment call for the rate being size of the worms, severity of the infestation and how thick or tall the grass being treated is.

There are several different pesticides that can be used to control armyworms in pastures and hayfields. One of these is Sevin, which is composed of the active ingredient carbaryl. Sevin comes in several different forms and strength formulations: a four pound liquid, an 80 percent wettable powder and an XLR four pound formulation. Sevin can control armyworms when applied at the half pound active ingredient rate per acre. This usually works out to one pint but can be up to a quart depending on the strength you wish to apply and the formulation. One important word of caution, Sevin is a stomach poison, not a contact killer. The worm will have to eat the Sevin on the plants before it will kill so don’t be too hasty after application to decide if it worked or not . Give it a couple of days to show some good control. Another note, don’t cut the grass on a lawn or shred a pasture right after treatment. Remember the insecticide is on the leaf and the worm has to eat the leaf to be affected. They will not eat cut, dead, drying out grass.

Another pesticide that many homeowners use is Malathion. Malathion does control armyworms when used at a rate of one and a half-tw pints per acre. It is also an excellent mosquito product when used as a two percent or five percent spray and is probably one of the most frequently used pesticides by homeowners.

Another product used the last couple of years on various caterpillar pests is Confirm. Confirm is a biological pesticide that interferes with the caterpillar’s ability to shed its skin capsule. As caterpillars grow, they go through several molts, or skin sheddings, increasing in size with each molt. Once the worm eats the Confirm, it will not be able to shed the head capsule of its old casing when it molts. As such it will die in short order being unable to form a new casing and also unable to feed. Confirm is applied at a recommended rate of four to six ounces per acre active ingredient. Due to the container size and cost of this pesticide it may appeal only to individuals who have very large acreage to treat. However it is one of the safer pesticides with regards to proximity of homes and other animals to the treated area.

Other products available at local agribusiness dealers that are labeled to control armyworms include:

  • Tombstone applied at 1.6 to two ounces per acre; the active ingredient is Cyfluthrin.
  • Permethrin applied at four to eith ounces per acre; permethrin is the active ingredient.
  • Methyl Parathion applied a one to 1.5 pints per acre.
  • Penncap applied at two to three pints per acre.

Another biological type insecticide to use is one of the Bt insecticides which contain the active ingredient Bacillus Thuringiensis. This is actually a live Bacillus or bacteria tat, when the insect eats it, forms an enzyme in the digestive tract and will kill the worm in a two to three day period. This is one of the safer pesticides that have been developed, as it is non-harmful to humans, pets or beneficial insects. There are many different trade names for the Bt’s such as Dipel , Biobit, Foray, Javelin, Vectobac, and others.

As always anytime you use any type pesticide there may be a grazing or harvest intervalwhich can be found on the label of the pesticide. For the pesticides mentioned in this article, the following grazing or haying restrictions exist:

Methy Parathion / Penncap M: a 15 day grazing or harvest restriction.
Sevin: 14 days grazing or harvest on improved pasture; 0 days for rangeland (unimproved.)
Malathion: 0 days; allow spray to dry before grazing or harvesting.
Confirm: 0 days; allow spray to dry before grazing or harvest.
Bt products: no Grazing or haying restrictions

With any of these pesticides, remember that the label is the law when it comes to rates as well as labeled uses for these products.

If we may assist you in any way with these insect or any other problems you may have in your field, lawn, plants, garden or home, contact our office of the LSU Agricultural Center at 230 Court Street in Ville Platte, LA, or come by and visit with us. If necessary, we can come out and visit with you to better identify and assist you with these problems.

The LSU Agricultural Center prohibits discrimination in all its programs and activities on the basis of race, color, national origin, gender, religion, age, disabilities, political beliefs, sexual orientation, and marital or family status. Persons with disabilities, who require alternative means for communications of program information or other assistance should contact the Evangeline Parish office of the LSU AgCenter at (337)-363-5646.

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