By Marty Morgan, Cooke County AgriLife Extension Office County Agent

With the past couple week’s rains and cooler weather, Fall Armyworms have been reported in pastures and hayfields. So be on the lookout for fall armyworms. The fall armyworm, Spodoptera frugiperda, is a common pest of bermudagrass, sorghum, corn, wheat and rye grass and many other crops in north and central Texas. It is one of the most problematic and common pests of pastures and hayfields in North Texas.

Larvae of fall armyworms are green, brown or black with white to yellowish lines running from head to tail. A distinct white line between the eyes forms an inverted “Y” pattern on the face. Four black spots aligned in a square on the top of the segment near the back end of the caterpillar are also characteristic of the fall armyworm.

Armyworms are very small (1/8 inch) at first, cause little plant damage, and as a result, infestations often go unnoticed. Larvae feed for 2-3 weeks and full grown larvae are about 1 to 1 1/2 inches long. As their name eludes, they can march across your field within days and wipe out forage before you realize they are present.

Look for fall armyworm larvae feeding in the crop canopy during the late evening and early morning, and during cool, cloudy weather. During hot days, look for armyworms low in the canopy or even on the soil surface where they hide under loose soil and fallen leaves.

A sweep net is very effective for sampling hay fields for fall armyworms. When fields are wet with dew, armyworms can stick on rubber boots worn while walking through the field. Also, small larvae chew the green layer from the leaves and leave a clearing or “window pane” effect and later notch the edges of leaves. At times, you might drive by a field that has just been hit with armyworms and see a white sheen across the green field.

When trying to decide if it is worth your time and money to control armyworms, there are several factors to look at.

It is important to note that in the last 2-3 days of feeding, an armyworm consumes 80 percent of its total forage intake for development. With that being noted, it is very important to notice the armyworm when in the beginning stages of development and act quickly to control it. For example: If you broke your field into a grid of one foot squares, there is a threshold of 2-3 armyworms. Once your field reaches or exceeds the threshold, it is best to apply insecticides early in the morning or late in the evening when the larvae are most active and therefore more likely to come into contact with the spray. If your field is near harvest, your best option may be to go ahead and harvest the field to prevent further damage.

Insecticides Labeled for Armyworm Control in Pastures and Hayfields

Always read and follow all label instructions on pesticide use and restrictions. Information below is provided for educational purposes only. Read current label before use.

Karate Z. 13.1% lambda cyhalothrin. Fall armyworms and grasshoppers. Pasture and rangeland grass, grass grown for hay and silage, and grass grown for seed. Pasture and rangeland grass may be used for grazing or cut for forage 0 days after application. Do not cut grass to be dried and harvested for hay until 7 days after the last application. Restricted use insecticide.

Lambda-Cy. 11.4% lambda cyhalothrin. Fall armyworms and grasshoppers. Pasture and rangeland grass, grass grown for hay and silage, and grass grown for seed. Pasture and rangeland grass may be used for grazing or cut for forage 0 days after application. Do not cut grass to be dried and harvested for hay until 7 days after the last application. Restricted use insecticide.

Mustang Max. 9.6% zeta-cypermethrin. Fall armyworms and grasshoppers. Applications may be made up to 0 days for forage and hay, 7 days for straw and seed screenings. Restricted use insecticide.

Tombstone Helios. 25% cyfluthrin. Fall armyworms and grasshoppers. Pasture, rangeland, grass grown for hay and seed. Zero days to grazing or harvesting hay. Restricted use insecticide.

Warrior II. 22.8% lambda cyhalothrin. Fall armyworms and grasshoppers. Pasture and rangeland grass, grass grown for hay and silage, and grass grown for seed. Pasture and rangeland grass may be used for grazing or cut for forage 0 days after application. Do not cut grass to be dried and harvested for hay until 7 days after the last application. Restricted-use insecticide.

Baythroid XL. 12.07% cyfluthrin. Fall armyworms and grasshoppers. Pasture, rangeland, grass grown for hay and seed. Zero days to grazing or harvesting hay. Restricted-use insecticide.

Dimilin 2L. 22% diflubenzuron. Fall armyworms and immature grasshoppers. Dimilin must be applied before armyworm larvae reach ½ inch or larger. Provides residual control for up to 2-3 weeks, as long as forage is not removed from the field. Label does not list a restriction on grazing.

Prevathon. 5% chlorantraniliprole. Fall armyworms and grasshoppers. Prevathon has a 0 day waiting period for harvest or grazing and is not a restricted-use insecticide.

Besiege. 9.26% chlorantraniliprole and 4.63% lambda cyhalothrin. Fall armyworms and grasshoppers. Pasture and rangeland grass may be used for grazing or cut for forage 0 days after application. Do not cut grass to be dried and harvested for hay until 7 days after the last application. Restricted-use insecticide.

Sevin 4F, Sevin XLR, Sevin 80S, Generic Carbaryl. Fall armyworms and grasshoppers. When applied to pastures, there is a 14-day waiting period before grazing or harvesting.

Malathion 57% and Malathion ULV. Fall armyworms and grasshoppers. Zero days to harvest or grazing.

Intrepid 2F. Fall armyworms (not grasshoppers). Begin applications when first signs of armyworm feedings appear. Use higher rates for heavier infestations. Do not harvest hay within 7 days of application. No pre-harvest interval for forage. 0 days to grazing.

Tracer. Treat when armyworm eggs hatch or when larvae are small. Use higher rates for larger larvae. Do not graze until spray is dry. Do not harvest hay or fodder for 3 days after treatment. Do not allow cattle to graze until spray has dried.

As always, if you have any other questions or concerns, please give me a call at the Extension Office (940)668-5412, or visit our website at cooke.agrilife.org