Casey Reynolds, PhD, Mike Merchant, PhD and Diane Silcox Reynolds, PhD
Fall Armyworm: Spodoptera frugiperda Smith
Armyworms belong to the insect order Lepidoptera and family Noctuidae. Common species of armyworms present in Texas include: the fall armyworm (Spodoptera frugiperda) the yellowstriped armyworm (Spodoptera ornithogalli) the beet armyworm (Spodoptera exigua) and the true armyworm (Mythimna (=Pseudaletia) unipuncta). The fall armyworm (Spodoptera frugiperda) is the most common cause of damaged turfgrass on golf courses, athletic fields, and home landscapes.
The larval stage (Figure 1) of armyworms can cause rapid,significant loss of leaf tissue in turfgrass. They feed primarily on bermudagrass, ryegrass, fescue, and bluegrass, but can also be important pests of agricultural crops. The name ‘armyworm’ originates from agriculture, where infestations sometimes resemble an army as they move across large agriculture fields. The same behavior can sometimes occur in turf, where areas as large as a football field can be consumed in the course of 2-3 days (Figure 2).
The fall armyworm has four life stages: egg, larva, pupa, and adult. Adult moths (Figure 3) are generally gray in color, with a 1½-inch wingspan and white underwings. Forewings are mottled with flecks of white, and males may have a triangular white spot near the wing tip, and another spot in the middle of the wing.
Fall armyworms are unusually susceptible to cold, and populations are thought to die out each winter except in the southern region of Texas. Infestations of fall armyworms often occur during “outbreak years”, when exceptionally high populations of the insects survive the winter and make their way north.
Armyworms fly and mate at night, after which the female will lay up to 1,000 eggs in masses (Figure 4) on suitable host plants or indiscriminate surfaces including the undersides of tree leaves or on structures near turfgrasses. These structures can include bleachers, fences, light posts, golf flags, and even water coolers (Figure 5). The presence of lights around athletic fields, parks, and golf courses can increase the likelihood of moths being present near these turfgrass settings and should be considered when scouting for these pests.
After hatching, newly emerged larvae may spin a silken thread to lower themselves to the turf to feed. The earliest (1-4) instars eat relatively little leaf material, while the fourth and fifth larval stages eat over 93% of the total foliage consumed over its life span. This usually means that early damage is often overlooked, and most defoliation takes place over a relatively short period during the later development stages. Caterpillars feed throughout the day but are typically most active in the early morning and late evening hours where they can often be easily observed.
Fall armyworm caterpillars range in color from shades of brown to gray, green or yellow-green. Their most distinguishing characteristic is a whitish inverted Y between the eyes (Figure 6), and three whitish stripes on the pronotal shield behind the head. Development from eggs to full-grown larvae often takes 2-3 weeks, at which point the larvae will burrow into the soil to pupate (Figure 7) and emerge as adults 10-14 days later when the life cycle begins again. Multiple generations occur each year, with most generations occurring in southern regions of Texas where development can often take place year round due to the warmer climate.
Damage by fall armyworm caterpillars (larvae) initially appears at the tips of the grass blades where they appear transparent due to the plant cells being eaten. If left uncontrolled, caterpillars may continue feeding, stripping tissue from turfgrass leaves and leaving brown areas adjacent to green turf.
Damage may initially resemble drought stress, but will progress to complete loss of foliage if numbers are sufficient and the turfgrass is left untreated. There may also sometimes be a distinct line between damaged and undamaged areas. Healthy and actively growing bermudagrass typically recovers after infestation and defoliation due to its aggressive rhizomatous and stoloniferous growth habit. However, newly established bunch-type grasses, such as ryegrass or fescue, may be stunted or killed by armyworm feeding.
While scouting for damage is important for all insects, careful inspection is especially important for this pest due to the rapid nature of its destructive feeding. If armyworms are present in turfgrasses in large numbers, it is important to treat as soon as possible to avoid further injury. There are several active ingredients that are effective in controlling fall armyworms, but many variations exist in formulation, use site, applicator requirements, etc. As always, be sure to follow the product label for specific instructions on timing, use rate, and application methods. For a complete list of products labeled for fall armyworm control, consult the Texas Turfgrass Pest Control Recommendations Guide.