The Boll Weevil

R. C. Gaines

The boll weevil, undoubtedly a native of Mexico or Central America, is about one-fourth inch long and a third as wide. The amount of food the developing larva gets in the square (flower bud) or boll (fruit) causes differences in the size of the weevils. The color varies from light yellow to gray or nearly black, depending on its age.

The boll weevil was identified first by the Swedish entomologist Boheman from specimens he got from Mexico. We know little about its spread through Mexico. It crossed the Rio Grande River near Brownsville, Tex., in 1892 or so. By 1894 it had spread to six counties in southern Texas. It advanced 40 to 160 miles a year and by 1922 it had infested more than 85 percent of our Cotton Belt.

It is possible to produce cotton in the infested part of the Cotton Belt because about 95 percent of the hibernating adults die and many that actually survive the winter emerge and die before cotton produces squares in which eggs may be deposited. Even after egg deposition and weevil development have started, heat, dry weather, insect parasites and predators, and birds help materially to check the rapid multiplication. Without such natural interference, offspring of a single pair of boll weevils could amount to several millions in one season.

Much attention has been given to the Possible control of the boll weevil with parasites. About 8o percent of all the parasites reared in the different States have been Bracon mellitor. Other Species of importance are Triaspis curculionis, Eurytoma tylodermatis, Catolaccus hunteri, Zatropis incertus, Eupelmus cyaniceps amicus, and Myiophasia globosa. About 6 percent of the boll weevil larvae were parasitized. Occasionally parasitization has run as high as 30 percent. In a few localities it has remained as high as 20 percent throughout the season. Bracon kirkpatricki, from Kenya Colony, and Triaspis vesticida and Bracon vesticida, from Peru, have been released in cotton fields, but we have no evidence that any of them have become established in the United States.

The damage caused by the boll weevil varies greatly from year to year. During 1950, for example, boll weevil damage was greater than for any year since 1921 and 1922 over the Cotton Belt as a whole and was the greatest ever recorded in Arkansas, North Carolina, and Virginia. The supply of feeding and breeding material, which is affected by weather conditions during the late summer and early fall, determines the number and condition of boll weevils for hibernation. Low temperatures in winter greatly influence the survival and emergence during the following spring and summer. The effect of rainfall and temperatures is most important throughout the period of cotton production. Each of these factors constitutes a separate study. At Tallulah, La., a greater correlation occurred between the number of days on which there was 0.3 inch or more of rainfall from June 21 to August 19 and the percentage of increase in yield in experimental plots where boll weevils were controlled than between any other two variables.

RESEARCH to reduce the losses caused by the boll weevil was started by the Department of Agriculture in 1894. It has been continued up to now, except during 1898 1900, when Texas made a special appropriation and all the work was handled by the State entomologist. Research has also been conducted by the various States in the cotton-growing area. Starting in 1947, a conference of Federal and State workers concerned with studies of cotton insects has been held in November or December of each year. The conference reports bring together the results of research and are a basis for control recommendations.

Boll weevil.

INDIRECT OR CULTURAL practices are of vital importance. Successful control cannot be accomplished unless full advantage is taken of every possible indirect method. The control program must be based on a combination of the different methods rather than an attempt to concentrate all efforts on direct control. Some of the most important cultural practices are preparation of seedbed, early planting, seed treatment, planting of a recommended variety, soil improvement and fertilization, frequent shallow cultivation, clean-up of favorable hibernation quarters, and early destruction of cotton stalks.

Early destruction of cotton stalks, one of the first recommendations made by entomologists, is a sound practice. It removes the food from boll weevils, stops late-season breeding, and causes the adults to enter hibernation in a more or less starved condition, which increases winter mortality. The earliness of stalk destruction has a bearing on the effectiveness. In a long series of experiments at Tallulah, La., no survival occurred when the weevils were placed in cages during the first week in September, while the highest survival was in cages installed during the latter half of October and first half of November. That is the time when frost normally kills cotton and the weevils enter hibernation.

The stalk-destruction program practiced in the Rio Grande Valley of Texas and Mexico for control of the pink bollworm has greatly reduced the crop losses from the boll weevil there. More than I million acres of cotton in Cameron, Hidalgo, and Willacy Counties in Texas and the adjacent area in Mexico were in this cleanup program, which calls for the completion of cotton planting by March 31 and destruction of the stalks by August 31. Immediately thereafter the debris is plowed under and seedling and sprouted cotton plants are eliminated so as to create a host-free period between crops.

The program has reduced the infestation and increased the yield of cotton in this area. Records of infestation of squares were made in June in the three counties from 1944 through 1950. In 1944 and 1945, before the program was started, the infestation averaged 38.5 percent. In 1946 to 1950, following the clean-up, infestation averaged 10.5 percent. The average lint yield per acre was 213 pounds in the three counties for the 5 years before the clean-up and 342 pounds for the 5 years afterwards.

Chemical defoliation of cotton plants causes boll weevils to leave the treated fields almost immediately. Proper defoliation checks the growth of the cotton plant and accelerates the opening of bolls. The crop may be harvested earlier so that the cotton stalks can be destroyed earlier.