Biology of the Boll Weevil
by G.T. Bohmfalk, Texas Agricultural Extension Service
Pictures courtesy of Texas A & M University, Department of Entomology

Boll weevil (Anthonomous grandis Boheman)

Identification. The adult boll weevil is a brown to grayish-brown beetle. The body is covered with short, fine hair, giving it a fuzzy appearance. There is considerable variation in size from slightly more than 1/8 inch to almost ½ inch in length. The boll weevil's snout is approximately half as long as its body. It is slightly curved and has chewing mouthparts on the end. Immature stages are found inside squares and bolls. The boll weevil egg is seldom seen since it is deposited inside a square or boll. The larva is a small, legless grub with a brownish head and chewing mouthparts. This grub varies in size from very small to ½ inch in length. The pupal or "resting" stage of the boll weevil is 3/8 to ½ inch long and cream colored with eyes and an obvious snout.

Biology. The adult boll weevil spends the winter in hibernation, called "diapause," without food and returns to cotton in the early spring the following year. Overwintering quarters usually consist of fence rows, broadleaved plant litter along creek bottoms, ditch banks and other protected, wooded areas near cotton fields. In the spring overwintered boll weevil adults concentrate in early planted fields nearest overwintering habitat where cotton is squaring. Adult boll weevils feed on tender growth in plant terminals if the young cotton does not have squares. In the early season, boll weevils colonize localized spots and do not generally invade the entire field.

The boll weevil is a pollen feeder; its survival is diminished without squaring cotton, although adult boll weevils emerging from overwintering quarters may subsist on other plants for short periods (e.g., an average of 18 days on yellow woolywhite in the Rolling Plains area). After adult weevils feed on cotton for 3 to 7 days and mate, they lay eggs in squares that have reached at least the "one-third grown stage" (approximately 1/4 inch in diameter). Egg laying may occur in smaller squares; however, sufficient feeding material is not available for a high percentage of larvae to develop to the adult stage. Late in the season eggs may be laid in small bolls, but squares are preferred.

It takes the eggs 2.5 to 5 days to hatch into the grublike larva that feeds inside the square or small boll. After larval development begins the infested square turns yellow, bracts open or flare and the fruiting form falls off the plant. The larva feeds for 7 to 14 days before pupating inside the square or small boll. During the next 4 to 6 days the pupal stage changes into an adult boll weevil. The newly developed adult eats its way out of the square or small boll and feeds on other fruiting forms for about 5 days. During this time the weevil mates and females begin to lay eggs.

The entire cycle takes 16 to 18 days under ideal conditions. Six or seven generations may be produced each year with each female having the capability of laying approximately 200 eggs.

Evidence of infestation. Although adult boll weevil feeding causes little damage, it indicates the presence of weevils and that egg laying will soon follow. There are distinct differences between feeding and egg-laying punctures. As weevils feed, a small cavity is formed at the site of feeding. If a female weevil determines that the feeding site is suitable for egg laying, she enlarges the cavity slightly and inserts her ovipositor (egg-laying tube) to deposit a single egg in the cavity. When the ovipositor is withdrawn she secretes a sticky substance that covers the cavity. This sticky secretion hardens to form a wart-like protuberance that can be easily seen and felt. Feeding punctures usually have no sticky covering and therefore no wart-like protuberance.

Nature of damage. Most of the damage is due to larval development inside fruiting forms. Feeding larvae eventually cause cotton squares and small bolls to shed or damage developing lint in larger bolls. Heavily infested cotton may produce much foliage but few mature bolls.

Next...a look at the national boll weevil eradication program