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The boll weevil
(Anthonomus grandis Boheman)
Adult -- A hard shelled snout beetle, about 1/4-inch in length, that varies in color from yellow, gray to brown, becoming darker with age. The boll weevil’s snout is about half as long as its body and is slightly curved with chewing mouthparts on the end. Characteristic two spurs or teeth near the end of the front femur, the thinner one being much longer than the other, and a single tooth on the middle femur.
Larva --White, strongly curved and wrinkled on the back; brown head, dark mandibles; 1/5- to 2/5-inch in length.
Pupa -- Varies in color from white to brown during development and has some adult features.
Eggs -- White, elliptical shape, 0.8 mm in length. Eggs are seldom seen because they are deposited inside squares or bolls and covered with a glue-like substance
Principal Host
Cotton is the only cultivated host in which reproduction is known to occur in Texas, although when forced, adults may feed on the buds and blossoms of other plants.
Life Cycle
Typically, the boll weevil life cycle adheres to the following pattern: Eggs are laid singly in both squares and bolls. Each female is capable of laying 100-300 eggs over a 30- to 40-day period. Squares are a preferred site for egg laying from early to midseason.
Development from egg to adult takes place within the square or boll. Infested squares generally flare and drop to the ground where the larva completes its development. Infested bolls usually remain on the plant. Eggs hatch in 3-5 days; larvae feed for 7-14 days before pupating; and pupation lasts 3-5 days. One generation can be produced in as few as 16 days. This produces the potential for up to eight generations per year under favorable conditions. Usually four generations of weevils occur during the season.
Behavior and Ecology
Adult boll weevils overwinter, or diapause, in a variety of sheltered areas: in bolls, trash and field debris, in shrubs and ground litter along waterways, etc. Harsh winters have a pronounced effect upon hibernating weevils in some areas -- up to 97 percent reduction in the Texas High Plains).
Diapause
This is a physiological phenomenon where the boll weevil enters a period of suspended growth, development and inactivity. This overwintering state is much like the hibernation of animals. The insect is more resistant to unfavorable environmental conditions such as the absence of host plants and harsh winters.
Diapause in the boll weevil occurs when reproduction in some adults ceases, and they gear their eating habits toward building up fat content and decreasing water content. The boll weevil’s breathing rate also slows down.
Scientists have identified five environmental conditions that encourage the boll weevil to enter diapause:
1. Exposure of larvae and pupal stages to 11 hours or less of daylight (photoperiod).
2. Exposure of adults to night temperatures 50 degrees Fahrenheit or less.
3. Adults eating bolls.
4. Fewer squares for adults to feed on.
5. More larvae growing and eating bolls.Studies have also shown that the first diapausing boll weevils can be collected from cotton plants in the Lower Rio Grande Valley in mid- to late June, which is about the time the first open bolls appear. The proportion of diapausing weevils to reproductive weevils increases rapidly during July as the plants mature and are defoliated. The diapausing weevils then start to decrease as hostable plants and their food supply -- squares, blooms, bolls -- begin to disappear.
Boll weevils move around primarily as airborne adults. In the spring, movement from hibernation sites is usually steady across a field, but adults may fly to distant portions of a field or to distant fields. Throughout the summer, movement within and between fields occurs randomly, but the greatest movement activity occurs in the spring and late summer. As cotton matures, weevils move from fields to surrounding fields seeking food and hostable cotton plants to deposit eggs, or they may move to hibernation sites.
Overall movement is by individual weevil flights and is influenced strongly by wind currents. Weevils moving into cotton fields in the spring are able to feed on young plants, even in the cotyledon stage, but females cannot reproduce until squares are large enough to feed on and to lay an egg in the feeding puncture. This requires flower buds about the size of a pencil eraser.
Males moving into a field containing fruiting cotton settle down to feed, releasing pheromone and attracting females. Females moving into the field respond to this pheromone and move little after mating. Early infestations, therefore, tend to occur in clumps.
Weevil Damage
Boll weevils damage cotton by feeding on squares and bolls. Adults puncture the fruit by use of chewing mouth parts at the end of the snouts. Females place eggs in some punctures and may cover the eggs with a glue-like substance. Eggs hatch into larvae that feed, develop and pupate within the square or boll. After emergence from the pupal stage, the adult chews its way out and, in a few days, begins to lay eggs for the next generation. Punctured squares in which eggs are laid usually fall off the plant with the resulting loss of that fruiting position. Boll damage may range from one damaged lock to loss of the entire boll. A common occurrence is boll rot because of weevil damage.
Identifying infestation
While cotton is in the fruiting stage:
1. Look for flared squares, which are often yellowed
2. Check blooms for feeding adult weevils
3. Look for damaged, misshapen bolls which may harbor weevils
4. Look for “cells” inside bolls which may contain weevils
5. Check squares for signs of feeding and egg laying damage shown by the presence of yellow-orange pollen grains
