Fall Armyworms (plus other sorghum headworms) Make their Return to Field Crops, Pastures and Turfs - Aug. 28, 2024

Fall armyworms (FAW), Spodoptera frugiperda, are making their annual appearance across Oklahoma in various field crops, pasture, and turf. Fall armyworms are surface dwellers that occur in large numbers in late summer and fall. They will often “march” in large numbers from food source to food source, hence their common name. Fall armyworm can infest alfalfa, peanuts, soybeans, sorghum, cotton (and other row crops), as well as grass pastures and lawns. In all, this pest attacks more than 350 plant species from 76 plant families. Its ability to feed on so many plants, reproduce prolifically, and migrate over long distances in its adult moth form lend to FAW being a serious economic pest. Unfortunately, relief from this pest won’t come until we get our first “killing freeze” which can vary in occurrence from year to year. Below we share information on how growers, producers, and homeowners can recognize and manage these pests that tend to make an annual appearance in Oklahoma.

 

Fall Armyworm Description and Damage

FAW eggs are very small and can be difficult to spot on vegetation such as grasses, it is often the damaging larval stage that will be noticed first. FAW are generally light tan to light green caterpillars that have a brownish-black head (may appear orangish). Larvae have a pale line that will run down the center of their back as well as yellow and black bands that run down their sides. To separate them from other caterpillar pests, look for four large spots at the end of the abdomen and an upside down “Y” on their head (Figure 1). As the larva feeds and grows, it will progress through 6 instars that can last approximately 2 weeks in warm temperatures and up to a month in cooler temperatures. When done feeding, the larva will burrow into soil to pupate for just over a week and emerge as an adult moth.

 

Figure 1. Image of fall armyworm (FAW) with two common features used to differentiate FAW from other caterpillar pests. Image: Corteva Agriscience. The fall armyworm has large spots on the second last segment arranged in square pattern. 2. an upside down white "Y" on dark head.

 

FAW can cause damage to a variety of crops and grasses leading to economic damage. Larvae will rapidly consume vegetation in the early instars (1-3), they consume small amounts, often this damage goes unnoticed. But as they enter the later instars (4-6) the rate of consumption increases. Damage at this stage and the size of the caterpillars is usually when FAW are noticed (Figure 2). The reason for rapid crop loss at this stage is caterpillars do most of their feeding (80-90%) in the final two instars, about 2-3 days. FAW cause damage by defoliating leaves, feeding on flowering parts, and cutting seedlings at the surface level. Small larvae, unable to completely chew through the leaf, will often feed on vegetation by scraping it, lending to a windowpane appearance as illustrated in Figure 3.

 

Figure 2. Fall armyworm larval size difference. Note that the smaller the larvae, the more effective chemical control will be in managing the pest. Image: Corteva Agriscience.  Learning to differentiate larval sizes is extremely important for the correct deployment of control tactics and to estimate the development of active infestations.

 

Figure 3. Windowpane appearance on grass caused by small fall armyworm larvae. Image: OSU IPM.

 

Fall Armyworm Management Guidance

A couple years ago, Tom Royer (former Cropping Systems Extension Entomologist) and Kelly Seuhs put together a nice Pest e-Alert that included suggested FAW management for various Oklahoma field crops and turf. Below is the information that they compiled with updated guidance for additional crops. Keep in mind that the insecticide application rate used must fall within the range of labeled rates for that crop. Always follow the label and be mindful of label language regarding insecticide use in areas frequented by people and animals. 

 

Grass Pastures:

For those wanting to put up grass hay, scout fields by looking for caterpillars and windowpaned or chewed leaves. Bend a wire coat hanger into a hoop, place it on the ground, and count all sizes of caterpillars in the hoop (Figure 4). The hoop covers about 2/3 of a square foot. Take samples in several locations within the field, along the field margin and the interior.

 

Figure 4. Wire coat hanger bent into hoop shape for scouting fall armyworms in grass pastures. Image: OSU IPM.

 

Treatment threshold is an average of two or three ½ inch-long larvae per hoop sample (3-4 per square foot). They are easier to control with an insecticide when they are small (less than ½ inches). For control guidelines and specific insecticide information, consult OSU Fact Sheet CR-7193 Management of Insect Pests in Rangeland and Pasture

 

Alfalfa:

Infestations of FAW are most likely to occur late summer to early fall, before frost. During this period, regularly scout for FAW in your fields using a sweep net. The net will pick up larvae that are too small to find other ways. If you find fall armyworm larvae in the sweep net, determine how many caterpillars are present per square foot by looking at plants and in the litter on the soil. If you don’t have a sweep net, check areas that look dead, wilting or drought-stressed, or where birds are congregating. Look for evidence of feeding such as caterpillar frass and leaf feeding damage. Populations of 10 to 15 per square foot have been observed to destroy 12-to-14-inch alfalfa. Treatment threshold is an average of 2-3 per row foot. However, even greater care must be observed for newly seeded stands. One to two caterpillars per row foot can destroy seedling alfalfa For more information check out OSU Fact Sheet CR-7150 Alfalfa Forage Insect Control.

 

Peanuts:

The threat of yield reduction due to FAW defoliation exists primarily when plants are from 60 to 90 days old. Producers should check fields frequently during July and August for the presence of earworms and armyworms and apply insecticides if populations exceed 3-5 larvae/row ft. in dryland or 6-8 larvae/row ft. in irrigated peanuts. Refer to OSU Fact Sheet CR-7174 Peanut Insect Control in Oklahoma for suggested insecticides and rates.

 

Soybeans:

For stand reduction, do not allow FAW to reduce stands by more than four plants per row-foot. For larger soybean plants, base treatment thresholds by estimating percent leaf loss as well as the presence of defoliators. Research from various states has shown that soybeans can withstand 35% foliage loss up to one week before bloom. During bloom and pod fill, the threshold falls to 15% to 20% defoliation, and then increases to 35% to 40% defoliation once pods have filled (Figure 5). Consult CR-7167: Management of Insect and Mite Pests in Soybean for specific insecticide information.

 

Figure 5. Visual guide to soybean defoliation. Image: S. Brown, University of Tennessee.

 

Sorghum (+ managing other caterpillars in the sorghum headworm complex): 

FAW will feed in the whorl and on ripening seed in the head (Figure 6), along with many other caterpillar species (hence the term complex). While yield loss from whorl feeding is negligible, damage to seed in head can occur until grain reaches the soft dough stage. Open-headed varieties are more susceptible than tight-headed varieties. 

 

You will want to have a white or other light color bucket with you when you go into the field to scout for headworms. Knock around the sorghum heads in the bucket so you can count the worms when they become dislodged. The threshold is 2 or more larvae per head before hard dough. Instructions on how to scout FAW in sorghum, along with the quick count guide can be found in OSU Fact Sheet EPP-7087 Sampling for Sorghum Headworms Using the Headworm Decision Support System. One may be concerned when they see multiple kinds of worms/caterpillars in their sorghum, fortunately a single insecticide approach should take care of the different kinds of caterpillars that feed in sorghum heads. When worms are small, and we mean small, a pyrethroid may be a good option for headworm control. However, when larger worms are present then Prevathon may be a better option for caterpillar control. For more information on suggested insecticides consult CR-7170 Management of Insect and Mite Pests in Sorghum.

 

 

 

Cotton:

In prebloom, FAW may defoliate cotton plants but cause irreparable damage when they cut off branches and sever stalks. FAW will also feed on fruiting structures such as bolls and will be most damaging when feeding during boll filling. Suggested insecticides and their rates for managing fall armyworm in cotton can be found at the Texas A&M AgriLife Cotton Insect Management Guide website.

 

Turf and Lawns:

Many insecticides are registered for control of FAW that can provide excellent control. For turf farms, golf courses and athletic fields, the pyrethroid insecticides, which include Astro®, DeltaGard®, Demand®, Scimitar®, Talstar®, or Tempo® are quite effective. Conserve™, which is derived from a naturally occurring bacterium, is also an excellent product. Diazinon and Sevin® (carbaryl) are also labeled for some commercial uses to control FAW. Carefully read the label for use patterns, some products may be registered for all turf uses, while others may not.

 

FAW can be detected through close examination of the lawn, or by using a “soap flush.” A soap flush simply involves mixing about 2 tablespoons of dishwashing soap into a gallon of water. Pour the soapy water over several small areas in your lawn and wait for 30 seconds to one minute. Any hidden larvae in the thatch will become irritated by the soap and come to the surface of the lawn. If 3-4 larvae are found per square foot, treatment may be warranted in commercial turf or golf courses.

 

For homeowners, carefully consider the need to control FAW. Some cool-season lawns may be able to recover from a FAW infestation this late in the year without treatment, and bermudagrass and zoysia lawns may be so slightly damaged that they would not warrant treatment. There are many products labeled for armyworm control in lawns and gardens. If choosing between granular and liquid applications, keep in mind that granular products are a bit slower acting and require watering for activation. Carefully read and follow all label restrictions for application and use patterns. Specific control recommendations can be found in EPP-7306 Ornamental and Lawn Pest Control (For Homeowners).