For many farmers armyworms can be a very costly pest. By correctly identifying the armyworms and choosing the most effective insecticide for their crops, farmers can save time and money.
The three most common armyworms found in Florida are the southern armyworm (Spodoptera eridania), fall armyworm (Spodoptera frugiperda) and the beet armyworm (Spodoptera exigua) though several other species also exist. Armyworm species can be difficult to tell apart. All armyworm species are ravenous feeders that can cause serious crop damage. Armyworms tend to feed on foliage but may also attack other parts of the crop including the stems and fruit.
The southern armyworm is one of the larger armyworm species. It is sometimes referred to as climbing cutworm. When larvae are fully grown they can be up to 1.5 inches in length. Their head usually has a yellow to red tint, while their bodies have a tan to pink color. Southern armyworms can be serious pests of beet, cabbage, carrot, collard, cowpea, eggplant, okra, pepper, potato, sweet potato, tomato, and watermelon. They can also damage avocado, citrus, peanut, sunflower, velvet bean, tobacco, and ornamental flowers.
The fall armyworm often causes the most damage to vegetable and hay producers. Unlike the southern armyworm the fall armyworm has a black or brown head. Its body ranges in color from tan to gray to green. The fall armyworm can easily be identified by the inverted “V” found on its head. Mature larvae can be up to 1.5 inches in length. Fall armyworms can feed on numerous plant species. However their preferred food sources are grasses. They often cause the most damage to field corn, sweet corn, sorghum, and bermudagrass. Fall army worms can also attack alfalfa, barley, buckwheat, cotton, clover, oat, millet, peanut, rice, ryegrass, sugarbeet, sudangrass, soybean, sugarcane, timothy, tobacco, and wheat. Sweet corn is the only vegetable crop that fall armyworm causes significant damage to.
Beet armyworms are slightly smaller than the fall armyworm and the southern armyworm. The larvae are typically 1.25 inches when fully grown. Their bodies are typically green with dark stripes along their entire length. The distinguishing characteristic for beet armyworms is the large black spot behind their head. Beet army worms can be a serious pest for a wide variety of vegetable and agronomic crops. They can attack asparagus, bean, beet, broccoli, cabbage, cauliflower, celery, chickpea, corn, cowpea, eggplant, lettuce, onion, pea, pepper, potato, radish, spinach, sweet potato, tomato, and turnip, alfalfa, corn, cotton, peanut, safflower, sorghum, soybean, and tobacco.
The striped grass looper (Mocis repanda) is sometimes mistaken for armyworms. It is often found in the same crops as armyworms. Striped grass loopers are thinner than armyworms and move in a distinctive crawling motion similar to inchworms. Striped grass loopers also have 4 prolegs compared to the 2 found in armyworms. In addition, striped grass loopers can be distinguished from armyworms by the numerous vertical stripes that cover their head. Striped grass loopers are can be serious pests of forage crops particularly bermudagrass and stargrass.
It is important to use Integrated Pest Management (IPM) principles when trying to control armyworms or any other lepidopteran larvae. Scouting for armyworm population is critical to determine the timing for insecticide applications. If most of the larvae on the crop are greater than 0.5 inches in length, treating them with insecticides is usually not worth the cost since they will soon stop eating and pupate. However, if more than three to four larvae that are less than 0.5 inches are found per square foot, they need to be sprayed quickly. If the crop has been sprayed before the treatment threshold drops down to two larvae per square foot. Armyworms and other lepidopteran larvae have numerous natural enemies. When choosing a product to treat worms look for insecticides that have low toxicity to beneficial insects. Using broad spectrum insecticides that kill larvae’s natural enemies can lead to greater worm populations later in the season.
Insecticides for worm control have two routes of entry: stomach and contact. For stomach insecticides to be effective, the insect must first ingest the pesticide where it then moves to the gut and is absorbed by the insect. In contrast to stomach insecticides, contact insecticides do not need to be ingested by the insect. The pesticide is absorbed through the insect’s cuticle. Systemic insecticides move throughout the plant in the xylem tissue. In contrast, locally systemic insecticides move only within the leaf that was treated. Most locally systemic insecticides also have translaminar activity. Translaminar refers to the pesticide moving from the top of the leaf, through the leaf tissue, to the bottom side of the leaf.
The following insecticides are commonly used to control lepidopteran larvae. The Insecticide Restience Action Committee (IRAC) groups insecticides together according to their mode of action. It is critically important to rotate insecticide applications with products from different IRAC groups. Repeated use of insecticides with the same mode of action has and will lead to insecticide resistance. To simplify information available in this CEU series, it is sometimes necessary to use trade names of insecticides. No endorsement of these insecticides is intended, nor is criticism implied of similar insecticides not mentioned.