Cotton Bollworm

Cotton bollworm 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, first-instar 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 (except the budworm) by the tiny spines, visible under a hand lens, that cover most of the body surface. Bollworm larvae must be at least in the third instar in order to be distinguishable from the budworm. 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.

Cotton bollworm 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. Squares injured by cotton bollworm usually have a round hole 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 bollworm 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 bollworms, especially in the San Joaquin Valley. Damaging populations usually do not appear until late in the season, after treatments for other pests have disrupted natural enemies. 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. The same principle applies to long-season desert valley crops, except that there are two periods when injury can occur - one in each fruiting cycle.

Many predators and parasites combine to substantially maintain cotton bollworm populations at low levels. Insecticide sprays for other pests will disrupt this natural control and may result in severe outbreaks of this pest.

The use of transgenic cotton, Bollguard II, offers suppression of cotton bollworm, along with beet armyworms, pink bollworm, and tobacco budworms. Other forms of transgenic cotton (with ‘Cry' proteins) are being developed and introduced into the commercial market.

Cotton bollworms 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.

Biological and cultural controls and Bacillus thuringiensis applications are acceptable for use on organically grown cotton.

In the San Joaquin Valley, start sampling plant terminals for bollworms as soon as bolls are present and continue until most bolls mature. Check five adjacent plants at each stop as you pass through the field. Choose the first plant at random; then check its mainstem terminal and those of the four plants next to it. For standard sampling select at least 100 plants.

There are two treatment thresholds for bollworms in the San Joaquin Valley. In fields that have not been treated with broad-spectrum insecticides, treat when you find 20 small bollworms 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.

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.