Spodoptera frugiperda (J.E. Smith)
Description. There are two host strains of fall armyworms. The rice strain is associated with rice, Bermuda grass, and other pasture grasses. The other strain is the corn strain which predominates on corn, sorghum and cotton. Other than feeding habits, these strains are indistinguishable unless using molecular identification techniques. The fall armyworm isa tropical insect and can overwinter only in southern Texas, except during mild winters. The adult migrates northward as temperatures increase in the spring. The adult fall armyworm is a dark brownish-gray, mottled moth with oblique markings near the center of the fore wing. There is an irregular white or gray patch near the wing tip on the males. Female moths are darker than males. The hindwing is white in females with a pearly or pink luster and a brown border. The wing span of the fall armyworm is approximately 1 ½ inches.
Fall armyworm eggs are similar to those of the beet armyworm. They are laid in masses of 50 to 150 a day for 8 to 10 days. Similar to other armyworm species the eggs are covered with hairs and scales from the female’s body. The eggs are difficult to find and are distributed throughout the plant on the undersides of leaves. Approximately 3 to 5 days are required for the eggs to hatch. The larva of the fall armyworm, when newly hatched, is white with a black head. Early instar fall armyworms disperse into a solitary feeding habit more rapidly than beet armyworms. First and second instar larvae are very difficult to distinguish from other armyworm species and bollworms. As feeding progresses, the larva becomes darker. Young fall armyworms often curl up on a leaf or suspend from a silken thread. Larvae will feed for 2 to 3 weeks and can be 1 to 11/2 inches long with various color patterns depending on the food source. However, they are usually greenish-brown with a white line below the top of the back, usually a brownish-black stripe above the midline and a pale stripe with a reddish-brown tinge below. The most distinct character used to distinguish fall armyworms from other caterpillar pests is the prominent white inverted “Y” on the front of the head. Additionally, fall armyworm larvae can be distinguished from beet armyworms by the presence of black hairs on the body; these hairs may cause confusion with bollworms. The fall armyworm larva also has four large spots that form a square on the upper surface of the last segment of its body. Pupation occurs in the ground near the cotton plant and usually requires 1 to 2 weeks.
Damage. Depending on the strain of fall armyworm encountered, damage may be non-existent to severe. Although rare, the rice strain has been observed dispersing as larvae from pasture into cotton where they fed exclusively on grassy weeds. The corn strain, however, can be extremely damaging. Fall armyworms that feed on cotton are typically more damaging than other armyworm species infesting cotton because they have a greater propensity to feed upon fruiting structures. However, they do not appear to be as voracious of fruit feeders as bollworms. When abundant in pre-bloom cotton, fall armyworms may cause defoliation but the greatest damage comes from the topping of plants. Branches may be cut off and sometimes the stalks may almost be completely severed. The most damaging populations of fall armyworms are those that occur during boll filling. Because fall armyworms tend to quickly disperse away from the egg mass, and because the egg masses are often laid inside the plant canopy, it is not uncommon to find small larvae individually feeding on squares and bolls much like a bollworm. Also similar to bollworms, it is not unusual to find small larvae feeding under bloom tags. Because they exhibit a cryptic feeding behavior similar to that of bollworms, they are easy to miss and often become most evident when observed feeding in blooms.
Fall armyworms are occasionally observed in fairly high numbers feeding on blooms in Bt cotton. Blooms do not produce the Bt toxins at high enough level to induce mortality. There have been a number of instances where these larvae appeared able to complete their entire life cycle feeding exclusively on blooms without causing significant boll damage or pollination issues.
Management and decision making.
Cultural management. Currently, the most effective means to control fall armyworms 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 fall armyworms. To boost effectiveness and aid in resistance management, new varieties of cotton have been developed which produce multiple Bt toxins, and the single toxin varieties are no longer available.
Fall armyworm management tactics are different in Bt-cotton relative to non-Bt cotton. However, field monitoring is still important in Bt-cotton because fall armyworm populations can sometimes develop, particularly on blooms and late in the season on stressed cotton where the Bt toxin may not be being produced in sufficient quantity. Thresholds in Bt cotton fields are based upon surviving second and third instar larvae and not newly hatched larvae. Newly hatched larvae have to feed on the plant for the Bt toxin to be effective so decision making is delayed until survivorship of larger larvae (worms) can be determined. In addition, large acreage of cotton in Texas, particularly in the South Plains and Panhandle, is still planted in varieties that do not contain the Bt genes.
Planting date and variety maturity rates can influence losses due to fall armyworms. Fall armyworm populations are usually at their highest level late in the season. Early planting and/or choosing an earlier maturing variety can often avoid the late-season bollworm/budworm populations. Additionally, the attractiveness of other crops in the landscape can influence abundance of fall armyworms in cotton. When corn or sorghum are still attractive to fall armyworms, it is not uncommon for these crops to act as trap crops, attracting large numbers of fall armyworms, yet leaving nearby cotton fields virtually untouched.
Irrigation and fertilization can affect fall armyworm infestations by affecting the attractiveness of the plant to egg laying females. Avoiding excesses fertility and post cutout irrigation can drastically reduce the number of eggs laid in a cotton field.
Biological control. There are a vast number of general predators and parasitoids that prey on fall armyworm eggs and larvae. Common predators include big-eyed bugs, minute pirate bugs, damsel bug, lacewing larvae, assassin bugs, spiders, lady beetles and collops beetles. An important egg parasitoid is the Trichogamma wasp. Although releasing predators or parasitoids for worm control is not considered economically beneficial, conserving natural enemies through selective insecticide use is highly beneficial. When possible, avoid treating cotton with broad-spectrum insecticides that will negatively impact the predators and parasitoids that prey on insect pests.
Scouting. Small fall armyworms are very difficult to distinguish from bollworms and budworms and unless these larvae are found feeding in a mass, they should be included with the bollworm/budworm counts. Refer to page ??? for details on scouting for fall army worms. As for bollworms, it is often necessary to thoroughly go through the entire plant. Fall armyworms are cryptic feeders, often concealed by square bracts and bloom tags. Fall armyworms tend to be most noticeable when feeding in blooms.
Chemical Control and Action Thresholds. When larger, distinguishable worms are detected, it is important to note whether they are bollworm/budworms, fall, beet or yellow striped armyworms because control tactics between armyworm, bollworm/budworms or mixed populations will differ. Bollworms and budworms are most often controlled with pyrethroids, which are generally weak against armyworms, which are typically controlled with a variety of insecticides that tend to be weak against bollworms. Thus, when mixed populations of armyworms and bollworms are encountered, it may be necessary to use tank mixes of insecticides to achieve adequate control of multiple species.
Because of the similarity in damage and difficulty in distinguishing small fall armyworm larvae from bollworms, they should be included along with bollworm/budworm counts and the thresholds used should be accumulative with the bollworms and budworms. Refer to page ?? for threshold details.
|Suggested Insecticides and Rates for Managing Fall 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|
(Belt 2 SC)
|0.03-0.047||2-3 fl oz||64-42.7||28|
|0.058-0.078||9-12 fl oz||14.2-10.7||15|
1use a medium to high rate or a low rate tank mixed with a pyrethroid; pyrethroids alone at high rates only offer suppression.