5 min read
Fall Armyworm Larvae

Current Situation

Alabama is experiencing a high number of migrating fall armyworm moths. Additionally, the state has experienced numerous rainfall events to put the yearly total above average. This frequency of rain creates a favorable environment for fall armyworm eggs and larvae to survive in large numbers. It also creates lush, green fields that attract armyworms. In some cases, producers are unable to access fields for control because of excess water.

Characteristics

Figure 1. Fall armyworm head capsule.

Figure 1. Fall armyworm head capsule. Image by Pat Porter, Texas A&M Agrilife Extension

Multiple generations of fall armyworms occur each year. Larvae, the damaging life stage, live for approximately two weeks, but this can change based on temperature. Small larvae do not each much for the first ten days of their life and are also much easier to control than larger life stages. The larger caterpillars cause almost all the damage during their last couple of days before pupation.

Fall armyworms are notorious for having a broad host range. People may find populations in home lawns, turf, field crops, and pastures. Infestations are especially common in well-managed bermudagrass. Two distinct features identify fall armyworms. The head has light markings that form an upside down “Y” (figure 1), while the opposite end has four black dots that form a square (figure 2).

Scouting

Scouting is the best way to determine if and how many fall armyworms are present. Grasses can be inspected by getting kneeling and looking at the top of the grass blades during cooler parts of the day and down near the soil surface during the hotter parts of the day. A soap flush in turf may also be used to detect infestations. For row crops and forages, a sweep net is an easy method to sample for armyworms. While scouting, people should note both the size and number of fall armyworms. This will help determine the proper control strategy.

Determining a Control Strategy

Figure 2. Four black dots on the back of a fall armyworm.

Figure 2. Four black dots on the back of a fall armyworm. Image by Pat Porter, Texas A&M Agrilife Extension

Caterpillars that are less than ½ inch long are easier to control and cause less feeding damage. Caterpillars more than ½ inch long are more difficult to kill and are responsible for most of the damage. If the population is a majority of one size or another, control decisions are somewhat easier. If most of the caterpillars are large (1 to 1.5 inches), then it is likely too late to control the population. They will cycle out and move on to “greener pastures.” If the population is mostly small caterpillars (0.25 to 0.5 inches), they may not require as extreme measures in order to manage them. However, things get tricky when there is a mixed population of sizes. This means that pressure is continuing and products that provide good initial knockdown and residual control may be needed.

Mechanical Control Options

If a hayfield is close to harvest, producer should go ahead and harvest early. Cattle can also be used to intensively graze the forage before the caterpillars consume it all. In turf systems, mowing may mechanically kill a few caterpillars but will not provide adequate control. However, turf should be mowed before applying an insecticide to reduce the distance the chemical must penetrate. If the thatch is dry, irrigation prior to treatment may bring more larvae to the surface for more effective control.

Chemical Control Options

Pastures

There are multiple control options for fall armyworms in pastures. They vary in price, residual, and level of rain-fastness. Products containing diflubenzuron (Dimilin) or methoxyfenozide (Intreprid) are insect growth regulators that disrupt the pest’s life cycle and work best on small caterpillars. Residual control is approximately 10 and seven days, respectively, but both will cease to provide control following a rain.

Pyrethroids (Mustang Maxx, Karate, Warrior II) will provide more immediate knockdown but have a shorter residual. They will also cease to provide control following a rain. If producers do not plan to harvest for another 10 to 14 days and have multiple generations of armyworms, they should consider mixing a pyrethroid and an insect growth regulator.

If rain persists, producers should consider applying products that are more rainfast. Products containing chlorantraniliprole (Prevathon, Beseige) are taken up by the plant, so they will still be effective after a rain. While these products are more expensive, producers can expect longer control, up to 21 days at the highest labeled rate. Products labeled for pastures can be found in the Alabama Extension Pastures and Forage Crop IPM Guide.

Many forage producers are still applying herbicides in certain situations for their pastures and hayfields. This leaves many of them wondering if the insecticide they are using for fall armyworms can be mixed with the forage herbicide. For all of the most commonly used forage herbicide products going out right now, Extension professionals do not know of any that specifically restrict this tank mix application. The only restriction for any herbicide/insecticide tank mix would be compatibility issues. Like any other mix, producers need to first test that the products are compatible.

Commercial Turf

Professional products for turf vary in mode of action, systemic properties, and residual. They may also have site-specific labeling. Granular insecticides are typically less effective than liquid insecticides. Treatments should be applied later in the day when the caterpillars are feeding or preparing to feed overnight. Do not mow for several days following an application. On golf courses, Extension professionals recommend treating the turf surrounding the greens. Worms may be feeding in these areas, and it will help prevent them from moving back onto the greens following an application.

Reduced risk insecticides containing spinosad (Conserve, Entrust) or Bacillus thuringiensis (Dipel Pro DF, Xentari) will preserve natural enemies. These products are most effective against small larvae and multiple applications may be necessary. Pyrethoids (bifenthrin, cyfluthrin, lambda-cyhalothrin) will provide effective control at the label rate, but with a short residual, weekly applications may be necessary. Diamides (chlorantraniliprole, cyantraniliprole) are more expensive, but provide much longer control. In some cases, people may see months-long control with a diamide.

Products available for commercial pesticide applicators can be found in the Alabama Extension Commercial Turf and Lawn IPM Guide: Insects.

Home Turf

Products for home lawns will usually be labeled for caterpillar pests rather than specifically for fall armyworms. Most products are granular formulations of pyrethroids. If using a granular formation or Ready-To-Use spray formulation, there is no dilution or equipment required. Products containing spinosad or Bacillus thuringiensis (Bt) will control smaller caterpillars without harming beneficial insects. However, Bt will only provide control for one to two days.

Products available to homeowners can be found in the Alabama Extension publication Controlling Fall Armyworms on Lawns and Turf.

Soybeans

The primary soybean fields at risk of economic fall armyworm infestations are Alabama’s late beans (i.e. wheat beans). This is because of the size of the soybeans. Fields planted earlier have much more foliage and fewer grass weeds in the field. The majority of the caterpillars are feeding on grasses that border the field or within the field. They then move to the soybeans either after consuming all of the grass or after an herbicide application that kills the grass. For soybeans prior to bloom, the threshold is 35% defoliation. However, if high numbers of fall armyworms are present (eight to 10 plus per 25 sweeps), do not wait. As the caterpillars grow, they can devastate a field.

Based on recent reports, Extension professionals recommend that producers avoid using pyrethroids if possible. If pyrethroids are going to be used, professionals in the mid-south are recommending to tank-mix a minimum of 0.5 pounds of acephate, and to be sure to follow up on the application two to three days to ensure it worked. The insect growth regulators Dimilin (4 oz), Diamond (6 oz), or methoxyfenozide (Intrepid, Trubador, etc.) are also options that could be tank-mixed with a knockdown insecticide or used alone under lower pressure situations.

Under high pressure situations, go-to products include the diamides (chlorantraniliprole: Prevathon, Vantacor, Besiege) or Intrepid Edge (5 oz). These are good options if rain is in the forecast over the next 2 to 3 days. The lower end of the labeled rates for diamides should provide adequate control.

Products labeled for soybeans can be found in the Alabama Extension Soybean IPM Guide.


Trade names are used only to give specific information. The Alabama Cooperative Extension System does not endorse or guarantee any product and does not recommend one product instead of another that might be similar.

Did you find this helpful?