Loxagrotis albicosta Smith Lepidoptera Noctuidae

Natural History

Distribution. This native cutworm occurs in the Rocky Mountain and western Great Plains regions of the United States and Canada, extending east to Nebraska, Oklahoma, and Texas. Its range has expanded, apparently following adoption of corn as a food plant in the 1950s, and likely will continue to spread in the Great Plains. Western bean cutworm appears to be absent from the Pacific Coast States, but is known from Mexico.

Host Plants. Originally known as a minor pest of bean, and perhaps of tomato, the host range of western bean cutworm apparently expanded during the 1950s to include corn. Acreage in the western states devoted to bean and corn also expanded during this period, so it is not clear whether host range expansion occurred, or whether damage became more noticeable as the availability of suitable hosts increased. It is now considered to be a locally important pest of grain corn and dry beans in western states, but also injures sweet corn, snap beans, and rarely tomato. Other legume crops may receive eggs and support partial larval development, but are considered to be poor hosts. Weed hosts include fruit of groundcherry, Physalis spp., and black nightshade, Solanum nigrum, though these are mostly suitable after larvae are partly grown (Blickenstaff and Jolley, 1982). (See color figure 31.)

Natural Enemies. Few natural enemies have been reported, though this is more likely due to lack of study than absence of predation and parasitism. The wasp Apanteles laeviceps Ashmead (Hymenoptera: Bra-conidae) has been reared from western bean cutworm. Common predators such as lady beetles (Coleoptera: Coccinellidae), minute pirate bugs (Hemiptera: Anthocoridae), damsel bugs (Hemiptera: Nabidae); big-eyed bugs (Hemiptera: Lygaeidae), green lacew-ings (Neuroptera: Chrysopidae), and spiders consume western bean cutworm eggs and larvae under laboratory conditions (Blickenstaff, 1979).

Life Cycle and Description. There is a single generation per year. Adults emerge in July and early August, with eggs found throughout this period. Larvae develop during late summer, and mature larvae overwinter. Pupation occurs in May or June of the following year.

  1. The eggs are deposited on the upper surface of leaves and within the whorl of corn plants. They are nearly spherical, but flattened at the point of attachment to the foliage. They are white with a thin red line around the top initially, and turn pinkish or purplish gray at maturity. The eggs are about 0.7 mm in diameter, and well-marked with ridges that radiate from the center. The mean number of eggs per cluster is reported to be 52 (range 21-195 eggs). The incubation period is 4-7 days. (See color figure 255.)
  2. Upon hatching, larvae remain near the egg mass for several hours and then disperse, often to the tassel of corn where they feed on pollen. Larvae normally feed at night, but they can be observed feeding during the daylight hours if it is cloudy. On small plants, particularly on beans, larvae may move to the soil during the daylight hours to seek shelter. The larvae initially are brownish and marked with weak stripes. As they mature they lose the stripes and become pinkish-brown dorsally, and grayish laterally. The thoracic plate bears broad dark brown or black stripes. The number of instars varies from 5-8, but six is normal. Mean development time requires 31 days (range 21-45 days). The mature cutworm measures about 35 mm long. (See color figure 64.)
  3. The larvae burrow into the soil in the autumn, mostly to a depth of 7-15 cm, and create a small pupal cell from saliva and soil particles. However, they remain in the larval stage until late May or June, when pupation occurs. The pupa is dark brown and measures about 20 mm long.
  4. The moth is brownish, with a wingspan of 35-40 mm. The forewing is distinctively marked with a broad whitish or tan stripe along the anterior margin. The center of the forewing generally is well-marked with a small round spot basally and a bean-shaped spot distally. The spots are white or margined with

Western bean cutworm larva.

Western bean cutworm larva.

Head and prothorax of western bean cutworm.
Adult western bean cutworm.

white, and connected by a short black bar. The hind wing is white, but with delicate brown lines at the margin and along the veins. Adults live for a relatively brief period, averaging 7.2 days for males and 9.2 days for females. The pre-oviposition period is about four days, with females depositing about 400 eggs. (See color figure 251.)

The biology was described by Hoerner (1948), Hagen (1962, 1976), and Blickenstaff (1979). A sex pheromone has been described (Klun et al., 1983). Western bean cutworm was included in the larval keys by Crumb (1956) and Capinera (1986), and is included in a key to armyworms and cutworms in Appendix A. The adult was included in the key by Capinera and Schaefer (1983).

Damage

On corn, larvae feed on developing pollen. As the tassel matures, larvae disperse and feed on pollen that has collected on the foliage and on the leaves. If pollen is not available, larvae feed on corn silk. As ears develop, larvae feed beneath the leaf sheath on both silk and kernels. Unlike corn earworm, Helicoverpa zea (Boddie), which tends to attack the ear tip, larvae of western bean cutworm are likely to burrow randomly into the ear, attacking all areas. Also, western bean cutworm larvae, unlike corn earworm, are not cannibalistic, so several larvae may be found feeding on a single ear. Leaf and stalk feeding are trivial, but silk feeding can inhibit pollination.

When feeding on bean, larvae initially remain near the top of the plant, feeding on buds and young leaves. Larger larvae feed on pods, but usually do not burrow into the pod. Instead they wander and feed on devel oping beans through the seed pod at several locations. When not feeding, large larvae may seek shelter in the soil.

Management

  1. Moths can be captured with both blacklight and pheromone traps. Blacklight traps capture moths earlier in the season than pheromone traps, but both trap types correlate well with damage potential. Although trap captures and overall damage potential are highly correlated, there is high among-field variation in damage (Mahrt et al., 1987).
  2. Application of insecticides to foliage is effective, though it is important to time the application early in the larval period, so applications are usually timed shortly after peak moth flight. If moth populations are not being monitored, insecticide should be applied before all the corn tassels have emerged from the whorl. Applications of insecticide-treated bait and soil-applied systemic insecticides are not very effective (Hoerner, 1948; Blickenstaff and Peckenpaugh, 1981).

Cultural Practices. Timing of planting affects susceptibility of corn to western bean cutworm infestation. Moths oviposit preferentially on plants that are silking, avoiding early and late planted corn. Emergence of moths from the soil is enhanced by high moisture conditions and high sand content; the rate of emergence from heavy and dry soil is low.

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