Fall armyworm larvae in grass pasture. (Photo by J. Villegas)

Fall armyworms, Spodoptera frugiperda, are chronic insect pests in the state, with more than 60 plants reported as hosts, including various pasture grasses (and lawns) and agronomic crops including corn, alfalfa, cotton, soybeans, grain sorghum, and rice. They migrate to Louisiana from neighboring regions like Florida, Texas, Caribbean islands, and Central-South America, with infestations most common from late July to early August. This pest belongs to the Noctuidae family of insects, which also includes other destructive insect pests like corn earworm, soybean looper, and tobacco budworm. Drought conditions favor the fall armyworm, and their activity continues until the first killing frost. While established pasture grasses can survive their attacks, newly established grasses and winter annuals face a higher risk of stunting and death.

Lifecycle and Feeding Habits

Fall armyworm larva identifying characteristics. (Photo by J. Villegas)

The fall armyworm's life cycle is tied to seasonal variations, leading to multiple generations throughout the year. Although this pest can be detected in low numbers during early-to-mid summer, the most severe outbreaks occur during the late summer and fall, attacking native grasses (barnyard grass, broadleaf signal grass, etc.), bermudagrass, summer and fall-seeded grasses, and established winter annuals. They have a more widespread presence as the summer progresses affecting pastures and lawns statewide. Fall armyworm moths lay eggs (white, domed shape,) in light-colored fuzzy masses. The larvae emerge from eggs within a few days. It takes two to four days per larval instar to complete their growth, with warmer temperature accelerating their development. The young larvae (cream to light green in color) initially feed on the underside of grass leaves, creating small "windowpane" effects on the foliage. As they grow, the larvae become more obvious, with noticeable dark and light stripes running down their bodies, four dark spots forming a square in the last abdominal segment, and the characteristic inverted “Y” on the head. The younger larvae (1^st–3^rd instars) consume some foliage but become more destructive as they mature into larger caterpillars. Large numbers of these insects may migrate to non-infested areas once they exhaust their current resources. Infestations often occur in waves, resulting in mixed larval sizes in the field, indicating overlapping generations. The larvae are most active early in the morning, during the late afternoon, and early evening. On tall, unmowed, or ungrazed grasses, the larvae can feed the whole day. In closely grazed or newly mowed fields, the larvae tend to retreat deep into the sod during the warmer hours of the day.

"Windowpane" feeding injury in grass blade (A) and heavy infestation in grass (B). (Photos by Dr. Blake Layton, Mississippi State University)

Injury caused by larvae appears in brown patches that spread from the field borders, increasing in size as the larvae move to non-infested areas. The damage may not be evident until large amounts of defoliation occur. Established pastures rarely get completely killed, but newly established grasses and winter annuals are at high risk of severe stunting and death. Drought-stressed grasses are more susceptible to damage.

Scouting and Management Strategies

To limit fall armyworm injury to forage and pasture crops, regular scouting is essential. The presence of birds in the field is a typical indicator of fall armyworms, however, using birds as indicators alone may be too late to prevent significant damage. Early signs of infestations include a "windowpane" appearance in the leaf blade. In severe cases, windowpaning can give fields a "frosted" appearance.

The LSU AgCenter has established an action threshold of when to treat for fall armyworms in forage or pastures. The general recommendation to initiate a spray is when there is one larva per sweep net sample or two or more larvae per square foot. Scouting random areas (10 or more) in a field is crucial, with samples taken from dense vegetation as these areas are preferred by moths for egg lay. Scouting should be performed at least every two weeks to prevent large populations and identify overlapping generations. Correct treatment application timing is vital for effective control. Several factors such as larval size and maturity of the pasture or forage crops should be considered before applying any insecticides. It's prudent to prepare the sprayer for an impending application if sub-threshold levels of worms are detected. When applying insecticides, using a minimum of 10 gallons per acre ensures adequate coverage for the best control.

There are various insecticides available for armyworm control, with pyrethroid insecticides offering faster action on small to medium larvae. Diamide and insect growth regulators provide longer residual activity compared to pyrethroids. For smaller worms, insect growth regulators are more effective as they disrupt the worms' molting process. Larger larvae are generally harder to control than smaller ones with any insecticide compounds. Choosing the right insecticide depends on factors such as harvest or grazing restrictions, residual activity, and availability. Some chemistries have little to no grazing or harvesting restrictions and prices may vary significantly. Producers must carefully read and follow all label specifications, including application rate, pre-grazing, and pre-harvest interval restrictions. Additionally, rotating between different modes of action within a season can help prevent resistance buildup. During years of severe infestations, multiple applications may be needed. Be sure to read the label for restrictions on re-entry periods, pre-harvest intervals, and re-treatment intervals.

If the pasture is used for hay and is near the time for harvest (mowing and baling), mowing a bit earlier than the ideal time can preserve the existing forage and prevent or even delay the need for an insecticide application.

*restricted use products require current pesticide applicator certification in order to buy and use the products

Written by

Dr. James Villegas
Assistant Professor - Extension and Research
Field Crops Entomology
LSU AgCenter
Email: jvillegas@agcenter.lsu.edu | Mobile: 225-266-3805