Order Lepidoptera: Family Noctuidae
Western Bean Cutworm (Striacosta albicosta)
Adult: Brown bodied moths, about ¾ inches (19 mm) long with a wingspan of 1.5 inches (38 mm) and marked with creamy white stripes on the leading edge of the forewings. Adjacent to the stripes, towards the center of the body, and in the middle of the wing lengthwise, is a circular white and tan spot. A crescent shaped mark is also located between the spot and the tip of the wing. The hind wings are light colored with no distinct markings (Fig. 10.25).
Figure 10.25 - Western bean cutworm adult stuck on sticky card from Delta trap.
Egg: Dome-shaped, and pinhead-sized, white with a thin, red ring around the top when newly hatched. Eggs change color with age from white to brown, and then finally turn a dark purple just before hatching.
Larva: Brown with faint crosshatching on their backs when newly hatched. As larvae mature they lighten to a gray-pinkish color and are about 1.5 inches (38 mm) long with three short dark stripes on the first segment behind the head (Fig. 10.26).
Figure 10.26 - Full-grown cutworm larva.
Pupa: Dark brown, oval shape.
The western bean cutworm is a late-season pest of corn. Adult moths emerge mid-summer and mate shortly afterwards. Females lay eggs in July and August on a variety of non-cultivated and cultivated host plants including sweet corn. Females are attracted to fields with corn that is in late whorl or tasseling stage. They lay eggs in masses primarily on the upper surface of leaves. Egg masses contain an average of 50 eggs, but can range from 5 to 200 eggs per mass. Eggs mature in about a week. Newly hatched larvae feed on their egg shells before moving to other protected feeding sites. Larvae feed on corn plants for about 30 days. When feeding and development is complete, fully mature larvae drop to the ground and burrow 3 to 9 inches beneath the soil. Once in the soil, larvae construct earthen overwintering chambers with their salivary gland secretions. These larvae remain in a dormant state throughout the winter. As temperatures rise the following spring and early summer, larvae pupate and complete development into adults. Western bean cutworms have a single generation each year.
Larvae feed on leaf tissue, fallen anthers/pollen, and silks on their way to the ear where most of the feeding is concentrated. Larvae enter the ear through the tip or by chewing through the husk and feeding directly on developing kernels. Damaged kernels are more prone to molds and mycotoxin infection. Injury from larval feeding can result in lower quality and reduced yield. Larvae from a single egg mass can invade nearby plants within a 6 to 10 ft circle, causing patchy infestations throughout the field. Several larvae may also feed on a single ear of corn, especially during high infestations.
Pale Western Cutworm (Agrotis orthogonia)
Adult: Mottled gray with yellowish and brownish spots on the forewing and a wingspan of 1.25 inches (32 mm) (Fig. 10.27).
Figure 10.27 - Pale western cutworm moth adult.
Egg: Spherical and about 1/16 inch (1.6 mm) in diameter. Eggs appear white when first deposited, and then turn a yellow-gray color.
Larva: Young larvae are yellow-brown to slate gray with three pairs of greenish-gray stripes along the back and sides. Head is amber with black markings that resemble an ‘H’ on young larvae and a ‘V’ on mature larvae. Mature larvae are 1.25 to 1.5 inches (30-40 mm) long (Fig. 10.28).
Figure 10.28- Mature pale western cutworm larva.
Pupa: Yellowish initially, then dark brown, and about 5/8 inches (10 mm) long.
Adult moths emerge from the soil in late summer and early fall. Following flight and mating, females begin laying eggs with peak egg laying occurring in midSeptember. Females prefer to lay eggs in dry, sandy or dusty soil in the late afternoon before sunset. Eggs are laid about 0.25 to 0.5 inch (7 to 10 mm) deep in clusters of 30 to 40 eggs. Pale western cutworms overwinter as eggs and hatch between late winter and early spring. Newly hatched larvae feed on corn stems throughout the spring and are most commonly found in the driest parts of the field. After feeding is complete, larvae burrow deeper into the soil and construct pupal chambers several inches below the soil surface where they become dormant. Larvae pupate in these chambers in late July or early August and adult emergence follows shortly afterwards. One generation occurs per year. If conditions are dry during egg-laying, cutworm densities may be high.
The pale western cutworm is a subterranean cutworm that feeds on the crown just below the soil surface (0.25 to 1 inch (10.4 to 25.4 mm) deep), severing stems of small seedlings and causing them to wilt and die. In larger corn plants they enter the plant and cause the growing point to die. Larvae will typically attack consecutive plants where soil is loose and dry.
- Use pheromone traps. Simple pheromone traps made from milk jugs are an easy way to monitor adult activity. Check traps weekly and begin examining plants when multiple moths are caught frequently (see links in the “more information” section).
- Scout fields by examining the upper leaf surface on the upper third part of the plant for egg masses and/or small larvae. Other signs of cutworms include leaf feeding, wilted leaves, and dead tillers. Larvae will be difficult to find once they enter the ear, so the treatment window is restricted to the period surrounding egg hatch.
- Check multiple plants and locations. Inspect 10 consecutive plants at several locations (at least five) per field. Make sure enough locations are used to represent all areas of the field.
- Check fields multiple times. Infestations can be patchy, and egg laying occurs over several weeks.
- Manage weeds. Remove or eliminate cool-season weeds with cultivation or herbicides at least 1 to 2 weeks prior to planting. This starves cutworm larvae by reducing food sources.
- Avoid fields with cutworm history. Both the western bean and the pale western cutworm overwinter in the soil and can be a problem if populations were high in previous years. Pale western cutworms are more likely to be found in corn where a wheat field was grown the previous year.
- Use tillage. Tilling one to two weeks before planting and after harvest may help reduce cutworm infestations by exposing overwintering cutworms to weather and predators and reducing available food sources such as weeds or plant debris.
- Use transgenic hybrids. Transgenic hybrids with the Cry1F gene will offer adequate to near-complete control of western bean cutworm. Hybrids with the Cry1F gene include ‘Herculex I’, ‘Herculex Xtra’, and ‘SmartStax’.
If an application is necessary, it must be properly timed for cutworm activity. Western bean cutworms spend considerable time inside the husk, while pale western cutworms are primarily in the soil. Chemical control of WBC is recommended when about 8% of the plants have egg masses or small larvae. If most eggs are hatched, treat when the crop is at least 95% tasseled and before larvae begin to feed on the silks. If most eggs have not hatched and the crop is completely tasseled, then treat to coincide with egg hatch (i.e., when most eggs have reached the purple stage, egg hatch usually occurs within 24 hours). Chemical treatment of PWC should be considered when larvae average 2 or more per foot of row.
Predaceous ground beetle larvae, damsel bugs, ladybird beetle adults, spiders and song birds are natural predators of western bean cutworms. Additionally, western bean cutworm larvae are susceptible to a naturally occurring disease caused by the microsporidian, Nosema sp. Pale western cutworms are less affected by natural enemies because of their subterranean nature. Wet weather, however, can cause larvae to move to the soil surface where they can be attacked by parasitoids and predators. There are several types of wasps (Braconidae, Ichneumonidae, Chalcididae) and flies (Tachinidae and Bombyliidae) that parasitize pale western cutworms (See the References section, Capinera 2001, for a list of specific insect parasitoids). Several predators have been observed to feed on pale western cutworm larvae such as the leaf-footed bug, assassin bug, ambush bug, and ground beetles.
Fall Armyworm (Spodoptera frugiperda)
Order Lepidoptera: Family Noctuidae
Adult: Mottled ash-gray in color with white or light gray spots near the tips of the forewings. Hind wings are iridescent silver-white with a narrow dark brown edge. Wingspan of about 1.5 inches (38 mm) (Fig. 10.29).
Figure 10.29- Fall armyworm adult.
Egg: Dome shaped, light gray, and laid in clusters. Eggs become dark just before hatching.
Larva: Light tan or green to nearly black with three white stripes running along the back. Dark spots run along the upper top edge of each segment and spots are arranged in a square on the next-to-last segment. Black head capsules turn an orange-brown color and have a distinct light-colored inverted “Y” on the face. Mature larvae may be up to 1.5 inches long (Fig. 10.30).
Figure 10.30 - Fall armyworm larva.
Pupa: About 0.5 inch (13 mm) long, reddish brown then darkening as it matures.
Fall armyworms overwinter as partly grown larvae in southern states along the gulf coast region where the ground does not freeze in the winter. After pupation, adult moths emerge and migrate northward throughout the summer and into the fall as temperature and weather conditions permit. Adults are most active during warm evenings, when females lay egg masses on corn leaves and other vegetation. They deposit most of their eggs during the first four to five days of life but can continue for up to three weeks. Larvae hatch in 2 to 10 days and then feed in the whorl or in the ears during the daylight. After 2 to 3 weeks, they drop to the soil to pupate. Adults emerge 10 to 14 days later. The fall armyworm life cycle lasts about 30 to 50 days depending on temperature, with one to three generations typically occurring in Utah.
Most of the damage in corn is caused by mature larval feeding. Young larvae begin consuming leaf tissue and create holes in leaves (Fig. 10.31). As larvae mature, they can cause extensive defoliation, often with only the leaf ribs and stalks remaining. This intense consumption of leaf tissue makes plants look ragged and torn. Corn plants in the late whorl stage, just before tasseling, are most sensitive to injury from fall armyworm feeding. Larvae can also feed on undeveloped tassels of young plants, bore into stalks, and attack immature ears by burrowing through the husk and feeding on kernels. Stunting of plants can occur when larvae feed on the growing point, but most corn plants can recover from moderate armyworm feeding injury if the growing point is not damaged.
Figure 10.31 - Feeding by young armyworms cause holes in leaves
- Use traps, including blacklight and/or pheromone, to detect presence of moths.
- Scout plants. When moths are detected, look for armyworm eggs and larvae.
- Search 20 plants in five locations or 10 plants in 10 locations.
- Continue to check plants until silks begin to dry
- Plant early and plant early maturing varieties. Late planted corn is more susceptible to larval feeding injury because more plants are in the seedling stage when larval feeding occurs.
- Use transgenic varieties. These hybrids can offer partial resistance to armyworm injury.
Insecticides should be applied before larvae burrow deep into the whorl or ear and are protected. Consider chemical control options when egg masses are present on 5% of the plants or when 25% of the plants show damage and live larvae are still present. Apply insecticides early or late in the day, since fall armyworm larvae are most active at these times.
Numerous species of parasitoids and generalist predators affect fall armyworms. The most common species that parasitize fall armyworm include braconid wasps and tachinid flies. Predators include various ground beetles, spined soldier bug, the insidious flower bug, and vertebrates such as birds, skunks, and rodents. During favorable seasons, natural enemies can suppress fall armyworm populations; however, in cold, wet springs their effectiveness is limited and fall armyworm population explosions may occur.
Sources:Figure 10.26, Adam Sisson, Iowa State University, Bugwood.org
Figure 10.27, John Capinera, `University of Florida, Bugwood.org
Figure 10.28, Frank Peairs, Colorado State University
Figure 10.29, Robert J. Bauernfeind, KSU, Bugwood.org
Figure 10.31, David Riley, University of Georgia, bugwood.org