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Hosts

While not as omnivorous as other armyworms, this species is fond of many of the field and vegetable crops and some ornamentals. The host range of the pest includes broccoli, beets, beans, cabbage, carrot, Chinese broccoli, Chinese cabbage, corn, cotton, grain, green bean, head cabbage, lettuce, onion, sorghums, peas, pepper, potato, soybean, spinach, sweet potato, tomatoes, rose and chrysanthemum. All can sustain heavy beetworm damage.

Distribution

This European species was first found established on the Pacific Coast. It is now distributed in Arizona, California, Colorado, Kansas, New Mexico, Oregon, and Hawaii. First found in Hawaii in 1880, the beet armyworm is now present on Hawaii, Kauai, Maui and Oahu.

Damage

Young larvae feed in terminal clusters of host crops and often do considerable damage before they are noticed. Leaves may be skeletonized and almost completely consumed. Plant stems are seldom eaten clear through as in the case of several cutworms. Young seedlings can be completely destroyed; however, there is recovery in older plants.

Biology

In Hawaii overlapping generations occur at lower elevations. Little is known about its seasonal patterns at higher elevations. The life cycle is completed in about 21 days.

EGGS

Eggs are laid in clusters that may be several layers deep. On onions, eggs are deposited in clusters from as few as 20 to over 100 eggs. They are covered by the white, hair like that give a cotton ball appearance to the egg cluster. If the protective covering is lifted, the eggs are visible. Eggs are flattened half-spheres, white to pinkish, with fine radiating lines from the top center. The hatch in 5 - 7 days in warm weather. Individual female moths can lay over 1,000 eggs in their lifetime, but the average is probably much lower.

LARVAE

Larvae molt 5 times in a minimum of 16 days during summer months. Color of mature larvae ranges from bright green to purplish green to blackish. The most common phase is light olive green with a darker strip down the back and a paler stripe along each side. The maximum size of this caterpillar is 1-1/2 to 2 inches. 

PUPAE

Pupation occurs on or under the soil surface. Cocoons have been reported but apparently are rare. Pupae are brownish, typical of all close relatives, and about 3/4 of an inch long. The pupal stage is reported to average approximately 17 days.

ADULTS

Adults are smaller than other members of the cutworm-armyworm group, approximately a half an inch long with a wing spread of 1 to 1-1/2 inches. Body and wings range from silvery-gray to grayish-brown. Forewings have a lighter spot near the center. Hindwings are paler with darker borders, however, a light band occurs at the wing edges.

Behavior

Young larvae spin loose webbing between leaves and remain under it until the third or fourth larval stages when they move out to other parts of the plant.

In Hawaii beet armyworm caterpillars seldom migrate in armies as do other species of armyworms.

Management

Cultural ControL

Adoption of certain cultural practices might be helpful in reducing the impact of this pest. Management of broad leaf weeds and rapid disposal of crop residues after harvest are the most important practices.

Biological Control

Little is known about the occurrence and importance of natural enemies in Hawaii. Frontina archippivora is known to parasitize the pest, but its impact on control is unknown.

Chemical Control

Hawaiian populations of beet armyworm have been easily controlled with insecticides in contrast to mainland populations. Bacterial insecticides (Bacillus thuringiensis) are more effective today then in the past because of deliberate selection for activity against this pest. In using insecticides, it is very important that users achieve good coverage of targeted plants and make applications when the pest is in early larval stages.

References

Flint, M. L. 1985. Other Seedling Pests, Cutworms. pp. 32. In Integrated pest Management for Cole Crops and Lettuce. University of California Publication 3307. 112 pages.

Lockwood, S. 1950. Beet Armyworm, Spodoptera (laphygma) exigua. California Department of Agriculture, Bureau of Entomology. Loose-Leaf Manual of Insect Control. 

Lockwood, S. 1960. Beet Armyworm, Spodoptera (laphygma) exigua. California Department of Agriculture, Bureau of Entomology. Loose-Leaf Manual of Insect Control.

Zimmerman, E. C. 1958. Spodoptera exigua (Hubner), new combination. pp. 339-344. In Insects of Hawaii. A Manual of the Insects of the Hawaiian Islands, including Enumeration of the Species and notes on the Origin, Distribution, Hosts, Parasites, etc. volume. 7: Macrolepidoptera. The University Press of Hawaii, Honolulu. 542 pages.

MAR/1991.

S-EXIGUA

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