Armyworms and Cutworms

Adults (Noctuidae) are night-active moths and attracted to lights. At rest the top of the body and forewings are a mottled mix of blackish, brown, gray, tan, and whitish. The underside and hind wings are pale and the wingspread is 1 to 2 inches. Young larvae are mostly greenish. Older larvae have more coloration and markings useful for distinguishing the species. Last instars grow up to 1 to 1-1/2 inches long.

Beet armyworm is the most common of these pests. The female lays eggs in a mass on leaves, covered with fluffy, pale, hairlike scales. The first through third instars commonly feed in a group, chewing the surface of leaves and causing patches of skeletonized (windowpane-like) foliage. Larvae also feed on or inside buds, killing the bud. Late instars feed individually, chewing holes in leaves and flowers and clipping off plant parts. Late instars can vary in color and overall may appear brown, blackish, grayish, or green. Commonly they are dull green with wavy, lengthwise stripes of paler green, yellow, and whitish lines. A black spot on each side of the abdomen above the second pair of legs distinguishes beet armyworms from larvae of other pest noctuids.

Beet armyworm feeds and reproduces throughout the year in locations with mild winters and migrate into floriculture and nursery crops from numerous alternate hosts including cole crops, cotton, lettuce, strawberry, tomato, and numerous weeds. One generation occurs in about 31 days or 24 days at an average temperature of 75° or 80°F, respectively.

Yellowstriped armyworm larvae have a pair of black triangles on the back of most segments. Some larvae appear nearly completely black when viewed from above. A lengthwise stripe on each side is bright orange or yellow. The adult has a complex and highly contrasting pattern of brown, yellow, and white on the front wings, and wingspread is about 1-1/2 inches.

Variegated cutworm adults have a distinct kidney-shaped marking (an oval with an inward curve on one side) near the center of each front wing and their wingspread is 1-1/2 inches to a little over 2 inches. Larvae have yellow or orange spots or a broken, longitudinal stripe at the top of the body, which is otherwise gray. Often there is a dark triangle or W-shaped mark on the top of the eighth body segment. The variegated cutworm overwinters as a naked pupa in topsoil.

Western yellowstriped armyworm females lay eggs in clusters covered with a gray, cottony material. Adults are a mix of black, brown, tan, and whitish. Larvae grow up to 1-1/2 inches long and commonly are blackish to brown with yellowish and whitish, longitudinal stripes on each side of the body. The larval head is brown with pale lines.

Armyworms and cutworms chew or clip off flowers and leaves that would normally be marketed. Late instars in seedling flats can consume a large portion of small plants. However, moderate early-season armyworm feeding on gypsophila was found to increase tillering and yields.

Biological and cultural controls and insecticide application are used to manage armyworms and cutworms.

Numerous species of parasitic tachinid flies and wasps and various predators prey on armyworms and cutworms and significantly reduce their abundance in fields where broad-spectrum, persistent insecticides are not applied. Parasitized armyworms and cutworms commonly continue to feed through to the last instar and still damage crops, but will not mature to adults that produce subsequent generations. Naturally occurring viruses also kill the later instars.

To conserve natural enemies rely on insecticide baits, use Bacillus thuringiensis (Bt), and entomopathogenic nematodes to control armyworms and cutworms and avoid broad-spectrum, persistent insecticides for all pests in the crop where feasible. Control ants and dust that can interfere with biological control. For more information, see Biological Control, Protecting Natural Enemies and Pollinators, and Relative Toxicities of Pesticides Used in Floriculture and Nurseries to Natural Enemies and Honey Bees..

Keep production areas free of weeds, many of which can host armyworms and cutworms. Exclude winged adults by covering openings to greenhouses with screens of sufficient surface area to allow adequate ventilation. Screens are especially important when lights are used at night to control flowering because lights attract moths. Switch from always-on night lighting to motion-activated, security lights. Cover seedling flats with screens to exclude adults and larvae. Apply row covers to exclude moths in the field, but use hoops or otherwise hold the mesh above plant surfaces to eliminate egg laying through the fabric. Hand-removal may be practical for small-scale situations, such as when a few caterpillars are found when scouting plants for pest problems.

Biological and cultural controls are organically acceptable management methods. The botanical azadirachtin, entomopathogenic nematodes (Heterorhabditis and Steinernema spp.), the microbial insecticides Bacillus thuringiensis ssp. aizawai and Bt ssp. kurstaki, and certain spinosad formulations (Entrust Naturalyte, Entrust SC) are acceptable for organic production.

Visually inspect hosts regularly to detect larvae and their chewing damage and droppings (frass). Randomly inspect leaves, terminals, and entire plants throughout the growing area at least once per week. If growing areas are lighted at night and not effectively screened, devote extra time to inspecting plants near unscreened vents and on the outside of field blocks near unmanaged vegetation or other alternate hosts.

Use species-specific, pheromone-baited sticky traps to determine when adult males are flying (most pheromone traps only attract and trap males) and most abundant, especially when timing applications of Bacillus thuringiensis and other pesticides with short-residual activity. When males are trapped, expect females are laying eggs or will soon and Bt applications to control young larvae are likely warranted. Make at least a second application after 7 to 10 days to control larvae hatching later because Bt is not persistent. For guidelines on when to treat, see Establishing Action Thresholds.

Degree-days

Insect and mite development is highly dependent upon temperature, so accumulated ‘heat units' can be used calculate egg-to-adult generation times using degree-days (DD) based on a minimum developmental temperature of 54°F. For example, eggs hatch about 94 DD after they are laid, so expect young larvae to be present after this period has elapsed starting soon after the first adult males are trapped. The larval and pupal stages require accumulation of 470 DD and 318 DD, respectively, for females and 540 DD and 344 DD, respectively, for males. The total DD for eggs, larvae, and pupae is the period for one generation, which at cooler temperatures corresponds to more calendar days in comparison with fewer days per generation when temperatures are warm. Knowing the seasonal variations in generation time by monitoring DD helps to schedule monitoring and control actions, saving money and time and improving management efficacy. To calculate these development times based on prevailing temperatures, see Degree-days on the UC IPM website. See Using Degree-Days: Video Tours for more information on this technique.

Selected Products Registered for Greenhouse or Nursery Ornamentals

• Degree-day model for Beet Armyworm in Floriculture
• Video presentation Using degree-days to time insecticide applications