Bollworm, Helicoverpa zea (Boddie)
Tobacco budworm, Heliothis virescens (F.)
Description. These two insects are sometimes described as the bollworm/budworm complex or the Heliothine complex because they often occur in the field at the same time. The adults of the two insects are easy to distinguish but the larvae can only be separated under magnification.
The adult bollworm is a relatively large moth with a wing span and body length of approximately 1.5 inches. Fore wings vary from light brown or tan to reddish-brown and are marked with dark areas near the tip and a dark spot usually near the center. The hind wing of the bollworm moth is white to light tan with an irregular dark band on the outer hind margin. The adult tobacco budworm is similar but with a slightly smaller wing span and body length of about1.25 inches. The fore wing of the tobacco budworm usually is light olive green with three or four light-colored, oblique bands (like sergeant stripes).
The larvae of both species have many very short hairs along the body which can be used to distinguish these species from other caterpillars. However, in the larval stages, bollworms and tobacco budworms look alike. Their size varies from very small (1/16 inch) when hatched up to 2 inches long as the larva reaches maximum size. Tobacco budworm larvae have a tooth-like projection on the inside surface of the mandibles and fine short hairs on the first, second and eighth abdominal projection (tubercle) which bear a single, prominent spine. If the projection and hairs are absent, this indicates a bollworm. A microscope or hand lens (10X) is necessary to observe these characteristics. Full-grown larvae vary in color from pale green, pink or brownish to black, with longitudinal stripes along the back. Both tobacco budworm and bollworm larvae are cannibalistic and thus usually not found in close proximity to one another.
Bollworm and tobacco budworm eggs are similar. When recently laid, the eggs are whitish colored and approximately the size of a pin head. Eggs are hemispherical in shape, resembling an inverted cup, with ridges running along the side from the top center to the point of attachment on the plant. These should not be confused with looper eggs, which are flatter and usually laid singly on the undersides of leaves. Many studies indicate that moths usually lay single eggs on the tops of young, tender terminal leaves in the upper third of the plant and as they mature, usually within 2 to 3 days, they become tannish-brown. The darkening indicates the approaching hatch. However, moths will deposit eggs on squares, bolls, stems and, in general, lower parts of the plant. This may occur when cotton plants are stressed and making little new growth, or during periods of high temperature and low humidity.
Budworms are less numerous than bollworms early in the crop season and rarely reach high numbers until mid- to late-season in the eastern portions of Texas. In the Rolling Plains, the Midland Basin and Trans Pecos areas, budworms may be found late-season most years, and are rarely recorded in substantial numbers in the South Plains and Panhandle areas.
Damage. Bollworm and tobacco budworm larvae are similar in appearance and cause similar damage. Tobacco budworm and bollworm moths are attracted to and lay eggs readily in cotton that is producing an abundance of new growth. Moths usually lay single eggs on the tops of young, tender terminal leaves in the upper third of the plant.
Young worms usually feed for a day or two on tender leaves, leaf buds and small squares in the plant terminal before moving down the plant to attack larger squares and bolls. When small worms are in the upper third of the plant, they are most vulnerable to natural mortality and to insecticides. Sometimes moths deposit eggs on squares, bolls, stems and, in general, lower parts of the plant. This may occur when cotton plants are stressed and making little new growth, or during periods of high temperature and low humidity. Detection of eggs and control of small worms are more difficult when eggs are deposited in these locations.
Management and decision making.
Cultural management. By far the most effective means to control bollworms and budworms 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 highly effective against tobacco budworms. Bollgard® cottons are also effective against cotton bollworm, but under heavy pressure from this species, insecticide treatment may be needed. In an agreement with the EPA (Environmental Protection Agency), Monsanto agreed that the licensing of Bollgard® would cease in 2010. Varieties containing Bollgard® will no longer be 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 tobacco budworm and bollworm compared to Bollgard®.
Bollworm/tobacco budworm management tactics are different in Bt-cotton relative to non-Bt cotton. However, field monitoring is still important in Bt-cotton because bollworm/tobacco budworm 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 bollworm/budworms. Bollworms and budworms 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.
Irrigation and fertilization can affect bollworm/budworm 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 bollworm 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 control of bollworms of budworms 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. Cotton fields should be scouted carefully every 3 to 5 days during periods of predicted moth egg-laying activity. In fields with fewer than five squares per row foot (approximately 67,000 squares per acre), bollworm populations often collapse and cease to be a problem.
Eggs and newly hatched worms are usually found in plant terminals and indicate possible outbreaks. In non-Bt cotton, frequent examination of the upper third of the plant (leaves, stems, squares, blooms and bolls) is often all that is needed to make a sound management decision. However, when eggs are being laid all over the plants or when 60 percent or more of the bolls are mature or in transgenic fields, whole plant counts should be used. Mature, unopened bolls are firm, cannot be dented when pressed between the thumb and forefinger, and cannot be cut easily with a sharp knife.
In Bt cotton, the entire plant should be searched for tobacco budworm and bollworm larvae and injury. A proper sample includes squares, white blooms, pink blooms, bloom tags and bolls. Scouting intervals should be reduced to 3 to 4 days during periods of increasing bollworm egg laying, especially during peak bloom. Treatment should not be triggered by the presence of eggs alone. Hatching larvae must first feed on the cotton plant to receive a toxic dose.
Terminal/Square Inspection Method. Divide the cotton field into four or more manageable sections depending upon field size and examine 25 plant terminals (upper third of the plant), selected at random from each quadrant, for small larvae and eggs. Also, from each quadrant, examine 25 one-half grown and larger green squares for bollworms and bollworm damage. Squares should be selected at random and flared or yellow squares should not be included in the sample. If previous insecticide applications have eliminated natural enemies (see guidelines for scouting for predators), fewer bollworms/tobacco budworms can be tolerated before economic damage occurs.
Whole plant inspection method. Divide the cotton field into four or more manageable sections depending upon field size. Make whole plant inspections of five randomly chosen groups of three adjacent cotton plants in each section. This entails looking in every square, bloom and boll. Particular attention should be paid to dried blooms attached to the bolls (sometimes called “bloom tags”). Count the number of eggs, worms and key predators encountered per 100 plants or estimate the number of eggs, worms or key predators per acre.
In the High Plains and Panhandle, thresholds are based on number of larvae per acre, while in the eastern part of the state thresholds are based on the number of larvae per 100 plants. The number of larvae per 100 plants appears to be more conservative; however, in practice the thresholds are very similar and nearly interchangeable.
When using the number of larvae (worms) per acre, simple calculations need to be made for each field inspected. Count the number of eggs, worms and key predators encountered and estimate the number of eggs, worms or key predators per acre using the following formulas:
Number of worms, eggs, or key predators counted multiplied by (the plant population divided by the number of whole plants checked)= the number of worms, eggs, or key predators per acre.
The plant population can be calculated on 40 inch rows by: counting the number of plants in approximately 13 feet 1inch row feet. Do this in at least 4 locations and average. Multiply that average by 1000. This results in the number of plants per acre. For 30 inch rows, count the number of plants on 17 feet 5 inches row feet; for 38 inch rows count the number of plants on 13 feet 9 inches row feet.
For example, 45 plants were counted on average in 13 feet 1inch from 4 spots a field. Multiply 45 by 1000 to determine that the field has 45,000 plants per acre. The field inspection looked at 40 whole plants and found 10 bollworm eggs, 3 small worms, and 12 key predators such as pirate bugs and lacewing larvae. Use the 45,000 plants/acre and divide by 40 plants checked to get a multiplier of 1125. Multiply the number of eggs, worms or predators by the 1125. The field has approximately 11,250 bollworm eggs/acre; 3,375 small bollworms/acre; and 13,500 key predators.
Chemical control and Action thresholds.
A range of treatment thresholds is provided under both the terminal and whole plant inspection methods because many factors in addition to density of larvae determine the need to treat with insecticides for bollworms/budworms. One of these factors is the number of predatory insects and spiders which feed on bollworm/budworm eggs and small larvae. If a previous insecticide treatment has eliminated these beneficial insects, then a lower treatment threshold should be considered. However, if two or more key bollworm predators are found for each small worm, control measures may not be needed. The number of bollworm/budworm eggs can also be considered along with worm densities in making treatment decisions. The treatment threshold will also vary according to the ability of the individual scout to locate small larvae, the age structure of the infestation, the stage of crop growth, the percent fruit set, the cost of insecticide treatment, the duration of the infestation (1 to 2 weeks vs. 3 to 4 weeks), the type of production system (high input/high yield or low input/low yield) and the market value of the crop.
When the worm populations is composed of small larvae primarily in the plant terminal, natural mortality agents such as weather and predators frequently control these before any damage occurs. These worms are also the most vulnerable to insecticides. Additionally, mortality of newly hatched larvae in cotton that has cutout (≥5 NAWF) is often particularly high due to poor host quality.
Once worms have grown to larger than 1/2-inch long, natural and foliar insecticidal control are less effective. Insecticides applied to control 1/2-inch long worms are only moderately effective. If treating a bollworm or budworm population that is actively feeding on bolls, consider using a long residual contact insecticide that the worm is more likely to become exposed to when moving from one boll to the next.
Budworms are generally more resistant to certain insecticides (e.g., pyrethroids) than bollworms, but are more sensitive to the Bt toxins in transgenic cotton. Aphids and other secondary pest will often increase following applications or broad-spectrum insecticides targeting budworm or bollworms due to the destruction of natural enemies. When secondary pests are present during a budworm or bollworm outbreak, using a more selective insecticide may be advisable to prevent a secondary pest outbreak.
|Suggested Insecticides and Rates for Managing Bollworm and Tobacco Budworm in Cotton
ingredient per acre
|Amount of formulated
|Acres treated per gal or lb of
|Mode of Action Group (IRAC)|
|Cry 1Ac, Cry1F
(Baythroid XL 1)
|0.013-0.02||1.6-2.6 fl oz||80-49.2||3A|
(Brigade 2, generics)
|0.04-0.10||2.6-6.4 fl oz||49.2-20||3A|
(Ammo 2.5, generics)
|0.04-0.1||2.0-5.0 fl oz||64-26||3A|
(Asana XL 0.66E)
|0.03-0.05||5.8-9.6 fl oz||22-13.3||3A|
(Declare 1.25, generics)
|0.01-0.015||1.02-1.54 fl oz||125.5-83.1||3A|
(Karate 1 EC, generics)
(Mustang Max 0.8 E)
|0.018-0.025||2.8-4 fl oz||45.7-32||3A|
(Hero 1.24 EC)
|—||10.3 fl oz||12.4||3A|
|Foliar sprays—Bollworms or Tobacco Budworms|
|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|
(Tracer 4 SC)
|0.033-0.045||2.14-2.9 fl oz||59.8-44.1||5|
1rates vary depending on formulation.