The Bollworm

K. P. Ewing.

The bollworm has been known since 1796 as a widely distributed and destructive pest. Unlike the boll weevil and the cotton leafworm, which attack cotton only, it feeds on many cultivated and wild plants corn, grain sorghums, tomatoes, peas, alfalfa, lespedeza, beans, soybeans, flax, peanuts, and other commercial crops. It has several common names. On cotton it is called the bollworm; on corn, the corn earworm ; and on tomatoes, the tomato fruitworm.

It has four stages in its life cycle egg, larva, pupa, and adult.

The eggs are white, ribbed, dome-shaped, and about half the size of a pinhead. They are laid singly and mostly on the tender growing tips of succulent cotton plants. Sometimes they are deposited on squares, bolls, and stems. In hot weather they hatch in 3 days. A female usually lays about 1,000 eggs.

The larva is the destructive stage. After the egg hatches, the young larva feeds on the nearest tender growth for a day or two and then moves toward the center or bottom of the plant, eating out the squares and tunneling into and eating the contents of the bolls. In color and markings, the larvae range from a pale green through rose and brown to almost black.

The full-grown worm, about 1 1/2 inches long, enters the ground to pupate, and overwinters in this stage. The moths emerge in the spring over a period of a month or two; the number at any one time depends on the locality, temperature, and rainfall. The adult is a yellowish or brownish moth with a wing spread of about 1 1/2 inches. The first-generation moths oviposit on such host plants as clovers, alfalfa, bluebonnets, winter peas, and young corn. The second generation usually comes along in time to infest corn ears. The third and succeeding generations are the ones that ordinarily attack cotton. Often they overlap.

THE BOLLWORM is troublesome in Texas, Oklahoma, Louisiana, Arkansas, and Mississippi. Sometimes it reduces the yields in every cotton-growing State. It is hard to control, for it is difficult to kill with insecticides after it has entered the bolls. If control measures are not applied when the larvae are small, considerable loss usually can be expected even when insecticides are used. The presence of an injurious infestation of the bollworm often is not detected until it is too late to get maximum control from the use of insecticides. Many growers therefore fear the bollworm more than any other cotton pest.

At least two other insects, the tobacco budworm and the yellow-striped armyworm, often cause damage to cotton similar to that caused by the bollworm and are often mistaken for it. Some of the failures of the recommended insecticides to control the bollworm may be due to the mistaken identity of the insect against which the control was directed.

NATURAL ENEMIES often are important in its control or in the reduction of the damage it causes. A heavy egg deposition or hatch of young larvae often fails to develop into a damaging infestation because of parasites and predators. Among the more important predators are Orlus insidiosus and several species of Coccinellidae and Chrysopidae.

Laboratory tests in 1941 disclosed that 12 species of predaceous insects commonly found on cotton readily fed and survived on bollworm eggs. The populations of beneficial insects vary from field to field and season to season and therefore cannot always be depended on to control developing bollworm infestations.

Insecticides used against other cotton insects also kill the beneficial insects that attack the bollworm. Their use immediately before the time the bollworms usually appear often causes increased infestations of bollworms unless insecticides are also applied later. In Texas, early season applications of insecticides for control of cotton insects are coming into general use. It is recommended that when possible the last of these be timed to come at least a month before the normal occurrence of bollworm infestations. If no insecticides are used on cotton during this period, the beneficial insects will have a chance to reestablish themselves so they can effect some control of the bollworm when it appears.

Cultural practices of use against the bollworm are deep plowing in fall or winter; early planting or use of early maturing varieties of cotton; and thorough preparation and cultivation of the soil to hasten maturity of the crop. The use of trap crops, especially corn, to prevent infestations on cotton has been suggested, but they are not very successful in actual use, partly because farmers hate to destroy the crops early enough to prevent maturity of the worms.

Severe infestations do not occur every year, but some areas, such as central Texas, usually have infestations of some degree annually. The causes are complex and the infestations are therefore impossible to predict. Most localities have a fairly well-defined period when infestations normally occur. Just before that time, growers should inspect their cotton several times a week until the crop matures. They should apply an insecticide when they find bollworm eggs and 4 or 5 small larvae per 100 terminals. Insecticides should be applied at 4 or 5 day intervals until infestations are brought under control. Important are proper timing of the applications and proper coverage of the plants. The worms are easily killed when they are hatching.

Paris green, calcium arsenate, and lead arsenate were used for many years. Cryolite and basic copper arsenate had limited use. Calcium arsenate was fairly effective when heavy dosages were applied at the time the worms were hatching, but was not very effective against heavy infestations of medium or large worms; when heavy aphid populations followed its use, bollworm infestations often increased and yields were reduced.

DDT is the most effective chemical so far used for the bollworm. DDT does not control the boll weevil, the cotton leafworm, or the cotton aphid, however. Because one or more of them often occur in damaging numbers at the time the bollworms occur, it is desirable to use an insecticide that will kill two or more of the insects at once.

A dust mixture containing 3 percent of the gamma isomer of benzene hexachloride, 5 percent of DDT, and 40 percent of sulfur, commonly called 3-5-40, was developed in 1946 for cotton insects. It is one of the best all-purpose insecticides for use on cotton insects.

Another combination of those insecticides, a mixture containing 2 percent of the gamma isomer of benzene hexachloride, 10 percent of DDT, and 40 percent of sulfur (2 10-40), has given excellent control of several pests; in heavy dosages of 15 to 20 pounds per acre it is preferred for extremely heavy infestations of medium to large-size bollworms.