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.
Bt cotton is genetically altered by the insertion of genes from a common soil bacterium, Bacillus thuringiensis to produce certain proteins which are toxic to specific groups of insects. For example, currently available Bt traits in cotton specifically target worm pests such as cotton bollworm, tobacco budworm, and beet armyworm. On the other hand, conventional, or non-Bt cotton does not produce such insecticidal proteins and thus it is more vulnerable to worm damage.
Since its introduction into US agriculture in 1996, Bt technology has developed from a single-gene trait to multi-gene trait packages. The first-generation Bt cotton (Bollgard) had a single Bt gene that expressed (produced) Cry1Ac. The second-generation Bt technologies, such as Bollgard® II, TwinLink®, and WideStrike®, produce two Bt toxins, and the most recent third-generation Bt technologies (WideStrike® 3, Bollgard® 3, and TwinLink® Plus) are three-gene trait products.
|Bt Technologies||Proteins expressed|
|Bollgard® II||Cry1Ac + Cry2Ab|
|WideStrike®||Cry1Ac + Cry1F|
|TwinLink®||Cry1Ab + Cry2Ae|
|WideStrike® 3||Cry1F + Cry1Ac + Vip3A|
|Bollgard® 3||Cry1Ac + Cry2Ab + Vip3A|
|TwinLink® Plus||Cry1Ab + Cry2Ae + Vip3Aa19|
Cotton varieties with third generation Bt technologies have excellent activity against tobacco budworm, pink bollworm, cotton leaf perforator and loopers, and good activity against cotton bollworm, saltmarsh caterpillar, fall armyworm and beet armyworm. Some situations may require supplemental insecticide treatment for bollworm and fall armyworm. Recommended economic thresholds used to trigger insecticide applications on Bt cotton are the same as those used for non-Bt cotton but should be based on larvae larger than ¼-inch.
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 as 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, and small, medium and large bolls for bollworms and bollworm damage. Keep track of the number of undamaged and damaged squares and bolls. Fruit should be selected at random and flared or yellow squares should not be included in the sample. Pay attention to bloom tags, petals stuck to small bolls. These will often hide larvae which burrow into the tip of the boll.
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. Thoroughly inspect dried blooms or bloom tags attached to small bolls. Count the number of undamaged and damaged fruit, and calculate percent damaged fruit.
Chemical control and Action thresholds.
Thresholds in Bt cotton fields are based on how many worms survive to late first or second instar larval stage, not on newly hatched larvae or the presence of eggs. Since newly hatched larvae must feed on the plant for the Bt toxin to be effective, decisions should be based on damaged fruit and the presence of larvae.
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 pests may 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, use of a selective insecticide may help prevent a secondary pest outbreak. Insecticides in the diamide, oxadiazine and spinosyn classes are more selective than the pyrethroid and carbamate insecticide classes (see the list of suggested insecticides).
|Bollworm and Tobacco Budworm Action Threshold Based on Boll Damage|
|Cotton stage||Action threshold (both Bt and non-Bt cotton)|
|Before bloom||≥ 8 worms (≥1/4 inch) per 100 plants or when populations threaten to reduce square retention below 80 percent|
|After boll formation||≥ 6% damaged squares and/or bolls and worms are present|
|Fields that have accumulated 350 DD60s beyond 5 NAWF are no longer susceptible to first or second instar bollworm/tobacco budworm larvae. Action threshold should be adjusted according to yield potential and production system (dryland vs irrigated).|
Suggested Insecticides and Rates for Managing Cotton Bollworm
|Product Name/Common Name||Insecticide Active Ingredient/s||Formulated Rate (fl oz or oz/A)||lb AI/A||Acres Treated per gallon/lb||Insecticide Class (*IRAC Groups)|
|Lannate LV||Methomyl||24-36||0.45-0.68||5.5-3.5||Carbamate (1A)|
|Steward EC||Indoxacarb||9.2-11.3||0.09-0.11||14-11.5||Oxadiazines (22A)|
|Fanfare ES||Bifenthrin^||2.6-6.4||0.04-0.10||49.23-20||Pyrethroid (3A)|
|Brigade 2EC||Bifenthrin||2.6-6.4||0.04-0.10||49.23-20||Pyrethroid (3A)|
|Discipline 2EC||Bifenthrin||2.6-6.4||0.04-0.10||49.23-20||Pyrethroid (3A)|
|Karate/ Warrior II||Lambda-cyhalothrin||1.60-2.56||0.025-0.04||80-50||Pyrethroid (3A)|
|Mustang Maxx||Zeta-cypermethrin||2.64-3.60||0.0165-0.0225||48.49-35.56||Pyrethroid (3A)|
|Baythroid XL||Beta-cyfluthrin||1.6-2.6||0.013-0.021||80-49.23||Pyrethroid (3A)|