Tobacco Budworm

The budworm is an important pest of cotton only in the desert valleys. Budworm moths are about 0.75 inch long, with a wing span of 1 to 1.5 inches. Eggs are spherical, flattened, with 10 to 15 perpendicular rows of toothed ribs. Newly hatched larvae have several rows of dark tubercles along the back, each bearing one or two bristles. Larvae range from olive green to dark reddish brown in color and can be best distinguished from most other caterpillars by the tiny spines, visible under a hand lens, that cover most of the body surface. To distinguish the budworm from bollworm larvae, they must be at least in the third instar. The budworm has a toothlike structure on the inner surface of the mandibles that is lacking in the bollworm, and it has the tiny spines of the skin extending onto the tubercles on top of the eighth abdominal segment; in the bollworm, these tubercles lack spines.

Heliothis larvae damage bolls and squares. Larvae chew holes into the base of bolls and may hollow out locks. Moist frass usually accumulates around the base of the boll. Larvae may also chew shallow gouges in the boll surface, which can become infected with rot organisms. Bracts of young squares flare outward and the squares become yellow and abort from the plant. Older squares may remain but usually have a round hole and frass near the base. Fifth-instar larvae are the most destructive; they not only damage more fruit than do earlier instars, but they damage larger fruit that are harder for the plant to replace.

The impact of a budworm infestation depends on the number of larvae present, the age of the larvae, and the timing of damage relative to the crop's fruiting cycle. Although large larvae do most of the damage, it is not possible to kill a significant proportion of them once they are older than the third instar. Monitoring and control must therefore be aimed at the eggs and small larvae.

Natural enemies are very important in managing populations of budworms. Damaging populations usually do not appear until late in the season, after treatments for other pests have disrupted natural controls. Insecticides are needed only if the population exceeds the treatment threshold while the crop has a significant number of squares or green bolls that will have time to develop into mature bolls by season's end. There is no need to treat once bolls begin cracking, because most bolls are too mature by that time to be susceptible and squares still present will not have time to mature. In the low desert valleys, there are two periods when injury can occur—one in each fruiting cycle.

Many predators and parasites combine to substantially maintain Heliothis populations at low levels. Insecticide sprays for other pests will disrupt this natural control.

Heliothis are attracted to succulent, rank-growing cotton plants; keep water, fertilizer, and plant density at recommended levels to avoid rank growth. Because populations seldom reach damaging levels before late summer, manage the crop for early maturing and plan to defoliate by late September.

The use of Bt cotton will help prevent damage by this pest. The use of transgenic cotton, Bollguard II, offers suppression of cotton bollworm, along with beet armyworms, pink bollworm, and tobacco budworm.

Biological controls, cultural practices that promote early harvest, and sprays of Bacillus thuringiensis are acceptable for use on organically grown cotton.

In desert valleys, start sampling in mid-July, about 1 to 2 weeks after peak squaring. Continue sampling until most bolls have matured. In crops with a second fruiting cycle, continue until top crop bolls have matured. For standard sampling, check for larvae on the terminal growth of at least 100 plants chosen at random. Divide fields of up to 80 acres into quarters and check 25 plants in each quarter. Divide larger fields into more areas and check 25 plants in each area. The treatment threshold is 10 to 12 small budworm or bollworm larvae per 100 plants. In fields that have been treated previously, treat when you find 8 small bollworms per 100 plants. Later instar larvae are the most destructive but are very resistant to insecticides; therefore, aim treatments at first or second instars.

See Integrated Pest Management for Cotton, 2nd edition, for detailed sampling information, including a sequential sampling program.