Boll Weevil
Anthonomous grandis grandis Boheman, Curculionidae, COLEOPTERA


DESCRIPTION

Adult - The adult weevil measures from 3 to 8.5 mm from the tip of the snout to the tip of the abdomen. It is usually reddish or grayish-brown though its color may vary (yellowish-brown to dark brown) according to its age and size. The conspicuous snout is about half as long as the body. A distinctive characteristic is the double-toothed spur on the inner surface of each front leg. Color plate.

Egg - The pearly white egg is elliptical and approximately 0.85 mm long. The soft shell permits the egg to fit into almost any cavity.

Larva - The newly hatched larva is inconspicuous, being only slightly larger than the egg. The mature larva is white, legless, and about 13 mm long. Its head and mouthparts are brown; its body is curved and wrinkled. Color plate.

Pupa - White at first, the pupa becomes brown as it develops.


BIOLOGY

Distribution - At some time prior to 1894, the boll weevil spread into the United States from Mexico. It now occurs throughout the cotton belt east of the high plains of Texas and may now be found in some areas of Arizona.

Host plants - In North Carolina, cotton is practically the only host plant of the boll weevil.

Damage - The annual loss to boll weevils in North Carolina has reached $7.5 million but varies greatly depending upon winter temperatures and their effect upon population levels. Injury is caused by both adults and larvae. Although adult females prefer squares, they oviposit into both squares and young bolls and seal the holes with excrement. Egg punctures become small, nipple-like protuberances. Larvae (developing within the cavities) then feed within the squares, causing the bracts to open or "flare," the color to fade to a yellowish-green, and the plant to shed the infested squares. Limited feeding on the squares and bolls by adults usually does not result in shedding, but cotton fiber is sometimes ruined. Boll-rotting fungi may enter through egg and feeding punctures. Color plate.

Life history - Boll weevils overwinter as diapausing adults sheltered under leaf litter, in woods, in weeds and along fence rows and ditch banks surrounding cotton fields. These adults begin to emerge as early as February in southerly areas and continue through early July, with peak emergence during late May and early June. In early spring the weevils feed primarily on terminals of cotton seedlings, but egg-laying does not occur until squares are present. Adult females make a small cavity in each square into which they deposit a single egg. First generation females lay an average of 200 eggs, while late summer generations may average only 5 eggs per female. Eggs usually hatch in 2 to 4 days depending upon the temperature.

After the larvae begin to feed, the infested squares yellow, flare, and drop from the plant in about 3 days. Depending upon the temperature and nutritional value of their food, larvae complete their development in 7 to 12 days and then transform into pupae within the squares. This stage lasts from 3-1/2 to 8-1/2 days. Newly formed adults remain in squares 1-1/2 to 2-1/2 days before chewing their way out. Because of the prolonged emergence of overwintering adults, distinct generations are difficult to recognize; however, in North Carolina there are usually four generations per year.


CONTROL

Satisfactory control depends upon a combination of cultural and chemical methods. Recommended cultural practices include (1) early planting, (2) stimulating rapid growth by thorough preparation of the seed bed, by adequate fertilization and by recommended weed control practices, and (3) selection of early maturing varieties specifically adapted to local areas. The main objective of these practices is to hasten the development of cotton plants and set a crop before weevils become abundant. The application of a chemical defoliant toward the end of the season speeds up harvesting and allows crop residue to be destroyed as early as possible. As a result, potentially diapausing weevils are left without a food source.

Insecticidal controls include in-season and diapause applications. Regular in-season applications are used to control weevils during the major period of fruit set and boll maturity; these applications should be based on weekly weevil counts and damage. Initial insecticide applications are made when 10 percent of the squares are punctured. Insecticide applications are frequently used to reduce the diapausing (overwintering) weevil population. This practice delays the need for in-season insecticides the following year. When warranted, treatments should start at the onset of diapause and continue until fields no longer afford the boll weevil food and breeding sites. For further control information, consult the current North Carolina Agricultural Chemicals Manual.