Fall Armyworm in Vegetable Crops
Classification: Lepidoptera: Noctuidae, Spodoptera frugiperda (J.E. Smith)
Distribution: The fall armyworm (FAW) is native to the tropical regions of the Western Hemisphere from the United States to Argentina. This species overwinters in the Gulf Coast states and Florida and continuously migrates north during the spring and early summer. Adult FAW are strong fliers, and are able to disperse long distances annually during the summer months. They have been documented in virtually all of the states east of the Rocky Mountains. However, as a regular and serious pest of vegetable crops, its range tends to be mostly in the southeastern states.
Identification: Larvae are hairless and smooth skinned and vary in color from light tan or green to dark brown (nearly black). Three yellowish-white lines traverse the sides and back from head to tail and four dark circular spots are evident on the upper portion of each abdominal segment. The front of the head is marked with a prominent inverted white Y, but this characteristic is not always a reliable identifier.
Larvae vary in length from 1/2 in (2mm) as first instar larvae to 3/4 - 1 in (35 - 50mm) as mature larvae. The forewing of adult male moths is generally shaded gray and brown, with triangular white spots at the tip and near the center of the wing. The forewings of females are less distinctly marked, ranging from a uniform grayish brown to a fine mottling of gray and brown. The hind wing is iridescent silver-white with a narrow dark border in both sexes.
Life cycle: Seasonal FAW activities in Virginia begin with egg laying by moths migrating northward from out of their ranges in the southern United States and Mexico. The moths persistently continue to migrate and lay eggs throughout the summer. The female FAW moth can produce approximately 1,000 eggs over her life span, and deposits them in clusters containing up to 400 eggs each (Fig. 2). FAW generations can occur every 23 to 25 days. Neonates are able to produce a silken thread, which allows them to drop or be blown (called ballooning) to other areas. In Virginia, FAW are most active in the late summer/fall (See Fig. 3), beginning in early July. Caterpillars can cause severe damage and will eventually move into adjoining fields. FAW feed more in the daylight hours than other armyworms and feeding by large populations can rapidly lead to severe damage.
Host plants: FAW feed on a wider range of plants than do true armyworms, Pseudaletia unipuncta (Haworth) (Lepidoptera: Noctuidae). The FAW is a major pest of sweet corn, tomatoes, and peppers in Virginia and other southeastern states. However, FAW have a wide host range with over 80 plants recorded, but clearly prefer grasses.
The field crops frequently injured include alfalfa, barley, Bermudagrass, buckwheat, cotton, clover, corn, oats, millet, peanuts, rice, ryegrass, sorghum, sugarbeets, sudangrass, soybeans, sugarcane, timothy, tobacco, and wheat. Among vegetable crops, only sweet corn is regularly damaged. Other vegetable crops attacked include crucifer crops. Weeds known to serve as hosts include bentgrass, Agrostis spp.; crabgrass, Digitaria spp.; Johnsongrass, Sorghum halepense; morningglory, Ipomoea spp.; nutsedge, Cyperus spp.; pigweed, Amaranthus spp.; and sandspur, Cenchrus tribuloides. There is some evidence that FAW strains exist based primarily on their host plant preference. One strain feeds principally on corn, but also on sorghum, cotton, and a few other hosts if they are found growing near the primary hosts. The other strain feeds principally on rice, Bermudagrass, and Johnsongrass.
Worm Injury to Crops: FAW larvae primarily cause damage by consuming foliage. In pepper and tomato, the fall armyworm can cause serious damage to the fruit, resulting in premature drop and fruit rot. The young larvae first feed near the ground where the damage goes unnoticed. They initially consume leaf tissue from one side, leaving the opposite epidermal layer intact. By the second or third instar, larvae begin to make holes in leaves, and eat from the edge of the leaves inward. Larval densities are usually reduced to one to two per plant due to cannibalistic behavior. Older larvae cause extensive defoliation, often leaving the plant with a ragged, torn appearance. Outbreaks typically happen in the fall, and are worse when rains are frequent and temperatures are cooler. When larvae are very abundant, they can defoliate entire plants. The larvae disperse in large numbers, consuming nearly all vegetation in their path, thus the "armyworm" name.
Control of Fall Armyworm in Vegetable Crops
Sampling: Moth populations can be sampled with black-light and pheromone traps. Pheromone traps are more efficient and sensitive to regional changes. Pheromone traps will only trap male moths and should be suspended at canopy height in the crop. Catches are not necessarily good indicators of density, but indicate the presence of moths in an area. Pheromone trap catches of 10 to 20 per night (70 to 100 per week) signal the need to begin insecticide applications to protect fruit. Once moths are detected, it is advisable to search for eggs and larvae. A search of 20 plants in five locations or ten plants in ten locations is generally considered to be adequate to assess the proportion of plants infested. Sampling to determine larval density often requires large sample sizes, especially when larval densities are low or larvae are young, so it is not often used.
Biological Control: Numerous species of parasitoids affect FAW. The most frequent wasp parasitoids reared from larvae in the United States are Cotesia marginiventris (Cresson) and Chelonus texanus (Cresson) (both Hymenoptera: Braconidae) species that are also associated with other noctuid species. Among fly parasitoids, usually the most common species is Archytas marmoratus (Townsend) (Diptera: Tachinidae). However, the dominant parasitoid often varies from place to place and from year to year.
Chemical Control: Insecticides are frequently used to protect crops against FAW damage. Some crops, like sweet corn, require as many as four applications per week during the silking and ear stages. Some resistance to insecticides has been noted, with resistance levels varying regionally. Treatments using insecticides should be made when insect populations and/or damage levels reach economic thresholds. For control recommendations on vegetables, refer to the most recent Mid-Atlantic Commercial Vegetable Production Recommendations VCE Publ. No. 456-420 (SPES-103P) https://pubs.ext.vt.edu/456/456-420/456-420.html
Luginbill P. 1928. The Fall Armyworm. USDA Technical Bulletin 34. 91 pp.
Pair, S.D., and H.R. Gross Jr. 1984. Field mortality of pupae of the fall armyworm, Spodoptera frugiperda (J.E. Smith), by predators and a newly discovered parasitoid, Diapetimorpha introita. Journal of the Georgia Entomological Society 19:22-26.
Sekul, A.A., and A.N. Sparks. 1976. Sex attractant of the fall armyworm moth. USDA Technical Bulletin 1542. 6 pp.
Vickery R.A. 1929. Studies of the fall armyworm in the Gulf coast region of Texas. USDA Technical Bulletin 138. 63 pp.
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March 18, 2019