Bollworm and Tobacco Budworm

Bollworm, Helicoverpa zea (Boddie)
Tobacco budworm, Heliothis virescens (F.)

Bollworm and tobacco budworm larvae look very much alike and cause similar damage. Full-grown larvae are about 1½ inches long and vary in color from pale green, pink, or brownish to black, with longitu- dinal stripes along the back (Fig. 21).

Bollworm and tobacco budworm moths are attracted to and lay eggs in cotton with lush new growth (Figs. 22 and 23). Moths usually lay single eggs on the tops or undersides of young, tender, terminal leaves in the upper third of the plant. However, in transgenic Bt cotton, moths frequently lay eggs on blooms, drying petals of blooms (bloom tags), or other tissues deeper in the canopy. Larvae survive better because these Bt cotton tissues have lower concentrations of Bt protein toxins.

Tobacco budworm and bollworm eggs are pearly white to cream color and about half the size of a pinhead (Fig. 24). They can be confused with looper eggs (which are flatter), can have a blue green- ish-white tint, and are usually laid singly on the undersides of leaves.

Bollworm and budworm eggs hatch in 3 to 4 days, turning light brown before hatching. 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 (Figs. 25 and 26). Small worms are most vulnerable to insecticides when they are in the upper third of the plant.

Moths sometimes deposit eggs on squares, bolls, stems, and lower parts of the plant when cotton plants are stressed and making little new growth, or during periods of high temperature and low humidity. Detecting eggs and controlling small worms is more difficult when the eggs are deposited in the lower plant canopy.

Budworms are typically less numerous than bollworms, and, in recent years, have rarely reached damaging levels. With the increased adoption of transgenic Bt cotton, insecticide sprays for bollworm and tobacco budworm control have become much less common in Texas, particularly in West Texas.

Scouting and Decision Making

In Bt cotton, search the entire plant for tobacco budworm and bollworm larvae and injury. A proper sample includes squares, white blooms, pink blooms, bloom tags, and bolls. Reduce the scouting intervals to 3 to 4 days during periods of increasing bollworm egg- laying, especially during peak bloom. The presence of eggs alone should not trigger treatment since hatching larvae must first feed on the cotton plant to receive a toxic dose.

Terminal and Square Inspection Method

• Divide the cotton field into four or more manageable sections, depending on the field
• Examine 25 plant terminals (upper third of the plant), selected at random from each quadrant, for small larvae and Also, from each quadrant, examine 25 half-grown and larger green squares as well as small, medium, and large bolls for bollworms and bollworm damage.
• Keep track of the number of undamaged and damaged squares

and bolls (Figs. 27 and 28). Select fruit at random and do not include flared or yellow squares in the sample.

• Pay attention to bloom tags and petals stuck to small bolls; they will often hide larvae that burrow into the tip of the

Whole Plant Inspection Method

• Divide the cotton field into four or more manageable sections, depending on the field
• Make whole-plant inspections of five randomly chosen groups of three adjacent cotton plants in each Look in every square, bloom, and boll.
• Thoroughly inspect dried blooms or bloom tags attached to small
• Count the number of undamaged and damaged fruit and calcu- late the percentage of damaged

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, base treatment decisions on damaged fruit and the presence of larvae.

Budworms are more resistant to certain insecticides (for example, pyrethroids) than are bollworms, but more sensitive to the Bt toxins in transgenic cotton. Aphids and other secondary pests may increase when broad-spectrum insecticides targeted at budworms or boll- worms disrupt natural control. When secondary pests are present during a budworm or bollworm outbreak, use a selective insecticide to help prevent a secondary pest outbreak. Insecticides in the diamide, oxadiazine, and spinosyn classes are more selective than the pyre- throid and carbamate classes. (See the list of suggested insecticides on page 31.)

 

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)
Prevathon	Chlorantraniliprole	14-27	0.047-0.09	9.14-4.74	Diamide (28)
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)
Silencer	Lambda-cyhalothrin^	3.2-5.12	0.025-0.04	40-25	Pyrethroid (3A)
Karate/ Warrior II	Lambda-cyhalothrin	1.60-2.56	0.025-0.04	80-50	Pyrethroid (3A)
Declare	Gamma-cyhalothrin	1.28-2.05	0.0125-0.02	100-62.44	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)