Beet armyworm (Spodoptera exigua [Hubner]) and
Yellow-striped armyworm (Spodoptera ornithogalli [Guenee])
Description. The adult stage of the beet armyworm has a wing span of 1 to 1 1/4 inches, and the fore wing is grayish-brown with two yellow spots near the center. The hind wing is a translucent white with narrow brown borders. The adult yellow-striped armyworm is a larger moth with a wing spread of 1 ½ to 1 ¾ inches. The fore wing is blackish-brown, occasionally with a reddish tint, or may be yellowish-brown with oblique yellow markings near the center. There may be bluish-gray markings at the apex and the hind angle of the fore wing. The wing veins usually are lighter colored. Hind wings usually are iridescent white with a narrow brown border.
The females of both species lay an average of 500 to 600 eggs over a 4- to 10-day period. Individual egg masses typically contain 40-80 eggs. These eggs are covered with hairs and scalesfrom her body and may be laid on both leaf surfaces. Eggs take 2 to 5 days to hatch. The larvae then feed for about 3 weeks and pass through five instars (growth stages). Small larvae may spin a light web over the foliage. Small larvae feed in groups for several days but later spread out and become solitary feeders. After larval feeding, pupation takes place in the upper 1/4 inch of the soil in a cell formed by gluing soil particles and trash together with a sticky secretion. This entire life cycle from the egg to adult requires 30 to 40 days, depending on weather conditions.
Both species overwinter in the pupal stage.
The immature stage of the beet armyworm is a distinctive green larva up to 1 1/4 inch long.
There is a conspicuous black dot on each side of the second body segment behind the head, above the second pair of true legs. The larval stage of the yellow-striped armyworm is a smooth caterpillar with scattered short hairs. The yellow-striped armyworm has a pair of black, triangular spots below the top surface on e. These yellow lines give the yellow-striped armyworm its name.
Damage. Both of these species are primarily foliage feeders, but larvae will also feed on some squares, blooms and occasionally small bolls. The most obvious indication of a beet or yellow-striped armyworm infestation is plant defoliation. Cotton fed upon by beet and yellow-striped armyworms will take on a tattered look and is often described as “shot gunned”. Usually these damaged areas, termed armyworm hits, are 3 to 4 ft in row length and scattered. Young armyworms “web up” and feed together on leaves, but eventually disperse and become more solitary in their feeding habits.
In pre-bloom cotton, larvae tend to skeletonize leaves rather than chew holes in them and will occasionally destroy the terminal, causing extensive lateral branch development and delayed maturity. However, most infestations occur later in the season during fruiting. At this time, in addition to foliar feeding, these armyworms may also feed on squares, blooms and small bolls. Beet and yellow-striped armyworms are not as voracious fruit feeders as bollworms or fall armyworms, so higher numbers of these species can be tolerated. Beet and yellow-striped armyworms will often feed on the bracts of squares and bolls without feeding directly on the fruiting structure; this type of feeding causing little to no yield loss. The most severe damage occurs when squares and bolls are fed on and penetrated.
Each segment except those between the head and the abdomen. Similar to the beet armyworm, it has a distinctive pair of black spots behind the head, but located above the first pair of true legs. There are yellowish-brown or occasionally orange stripes with white lines along each side.
Cultural management. By far the most effective means to control beet and yellow-stripped is to plant cotton containing genes from the bacteria, Bacillus thuringiensis (Bt). These genes produce proteins that are toxic to most leaf and fruit feeding caterpillars after ingestion. Bollgard® cotton varieties were introduced in 1996 and contain only one gene (Cry 1Ac) for fruit feeding, leaf feeding caterpillar control. Research trials evaluating the Bollgard® transgenic Bt gene technology have determined these varieties to be moderately effective against beet and yellow-stripped armyworms. Licensing of Bollgard® ceased in 2010 and are no longer allowed to be grown. Transgenic technology is being updated continuously and varieties containing multiple genes to control fruit feeding and leaf feeding caterpillars now predominate Texas cotton acreage. Bollgard® II contains two genes (Cry 1Ac and Cry 2Ab) and WideStrike® also contains two genes (Cry 1Ac and Cry 1F). Bollgard® II and WideStrike® cotton varieties are more effective against beet and yellow-stripped armyworms compared to Bollgard®. However, large worms that move onto cotton after developing on weeds such as pigweed may be able to destroy some fruit before succumbing to the Bt toxins. Also, it is not unusual for these large larvae to develop by feeding on bloom petals which do not express the Bt toxin effectively. Bloom feeding rarely results in destruction of the developing boll.Several factors can contribute to late-season beet armyworm outbreaks. These factors are: mild winters (e.g., absence of prolonged freezing temperatures); late planting; delayed crop maturity; heavy early-season organophosphate or pyrethroid insecticide use; prolonged hot, dry weather conditions; presence of armyworms prior to bloom; and weather conditions that support long-distance migration, such as tropical storms.Additional characteristics of high risk fields that consistently appear to fit a pattern for armyworm problems are: sandy and droughty soils; skip-row planting; fields with skippy stands, open canopies; drought stressed plants; and fields infested with pigweed. The likelihood of a heavy outbreak increases as more of these factors occur in a given location. However, when armyworm populations are high in an area, all fields are susceptible.Biological control. Natural enemies play a key role in suppressing armyworm populations. At least 10 parasite species and 13 predator species have been identified (in Georgia) as attacking beet armyworm eggs and larvae. Most of these parasite/predator species, or close relatives, are also found in Texas. This parasite/predator complex normally helps keep armyworm populations at low numbers. For example, mortality of beet armyworm eggs, larvae and pupae in untreated cotton may exceed 95% due to natural mortality factors. Many of these armyworm parasite/predator species also attack tobacco budworm and bollworm in cotton.One of the most important predators of armyworm eggs, larvae and pupae is the red imported fire ant while one of the most effective parasites is a tiny wasp, Cotesia marginiventris, which attacks the first through fourth larval stages. It is especially effective at finding and attacking small larvae while they are still feeding in a group. C. marginiventris is a widely distributed parasite that attacks a wide range of caterpillar species in a variety of habitats. Its population dynamics are not exclusively dependent on insects in cotton. In fact, C. marginiventris is a common parasite attacking bollworm in cotton, corn and sorghum in the lower Gulf Coast region. The pupal stage of this parasite is small, white, fuzzy and oblong; and is often found attached to the underside of a cotton leaf. Another parasite, Chelonus insularis, has been found parasitizing beet armyworm eggs and larvae in cotton as well as bollworms in corn and sorghum in Texas.
Scouting. General observational scouting for detection of armyworm hits should be conducted throughout the growing season on a 5 to 7 day schedule. Infestations usually are spotty within a field, and careful scouting is necessary to determine the need for, and field area requiring, control. It is not uncommon for only a portion of a field to require treatment. Once armyworm hits are detected, more extensive scouting should be conducted. For extensive scouting, 10 to 20 plants from 3 to 4 locations throughout the field should be thoroughly inspected using a whole plant inspection technique (see page ??? for details on conducting whole plant inspections).
Weedy areas of the field, especially those infested with pigweed, should be scouted carefully since beet armyworms are strongly attracted to pigweed and may move onto cotton, especially if the pigweeds have been treated with a herbicide.
Chemical Control and Action Thresholds. The action threshold for beet and yellow-striped armyworms varies depending on the stage of growth of the cotton, the percentage of infested plants, whether or not fruit feeding is occurring and the size of the larvae.
Effective control of beet and yellow-striped armyworms is much easier to achieve with the introduction of more effective insecticides over the past 15 years. However, as with older insecticides, smaller larvae are easier to control than larger larvae and good spray coverage is essential to maximize control of larger worms feeding deep in the plant canopy.
|Suggested Insecticides and Rates for Managing Beet Armyworm and Yellow-striped Armyworm in Cotton
ingredient per acre
|Amount of formulated
|Acres treated per gal or lb of
|Mode of Action Group (IRAC)|
|Cry 1Ac, Cry1F
|0.045-0.09||3.5-7 fl oz||36.6-18.3||28|
(Demin 0.16 EC)
|0.0075||6 fl oz||21.3||6|
(Belt 2 SC)
|0.03-0.047||2-3 fl oz||64-42.7||28|
(Steward 1.25 EC)
|0.09-0.11||9.2-11.3 fl oz||13.9-11.3||22A|
|0.06-0.16||4-10 fl oz||32-12.8||18|
|0.058-0.078||9-12 fl oz||14.2-10.7||15|
(Tracer 4 SC)
|0.04-0.045||2.5-2.9 fl oz||51.2-44.1||5|