Western Bean Cutworm
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.
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.
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
Adult: Mottled gray with yellowish and brownish spots on the forewing and a wingspan of 1.25 inches (32 mm).
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.
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.
- 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.