Cotton Boll Weevil Biology
Guide A-232
Jane Breen Pierce, Assistant Professor
Agricultural Science Center at Artesia New Mexico State University
This Publication is scheduled to be updated and reissued 12/03.Cotton boll weevil causes more damage to cotton in the United States than any other insect. Native to Mexico or Central America, it was first reported in the United States in the fall of 1894 in Brownsville, Texas. Over the next 30 years, boll weevil spread rapidly through most cotton-growing areas in the Southeast. It advanced at a rate of 40-160 miles per year, primarily by dispersal flights. By 1922, it had infested more than 85 percent of the Cotton Belt, from central Texas to the Atlantic Coast and as far north as Virginia. Despite earlier beliefs about the limits of range expansion for this pest, it has continued to spread and adapt to new areas with a wide range of habitats, climates, and cultural methods. In the last 15 years, it became a pest in the High Plains of Texas despite relatively cold winters and in the irrigated deserts of Arizona.
Although up to 42 species of parasitoids and over a dozen species of predators attack boll weevil in various parts of the pest's range, few have had much impact in reducing or controlling boll weevil populations. Boll weevil eggs, larvae, and pupae are well protected from most natural enemies by host plant tissues. Adult boll weevils also are well protected from natural enemies by their tough exoskeleton. The more likely source of boll weevil mortality in the growing season is desiccation from high heat and low humidity.
DESCRIPTION
Adult boll weevils are gray, brown, or reddish brown beetles slightly less than a quarter inch long. The grooved wing covers (or elytra) completely cover the abdomen, and the grayish flying wings are tightly folded and tucked under the wing covers at rest. Boll weevils are further characterized by a curved snout almost half the length of the body. The chewing mouth parts are at the tip of this snout, which also bears the elbowed antennae. With magnification, a pair of spurs is visible on each front leg above the femur-tibia joint.
EMERGENCE
Spring emergence of overwintering boll weevils may last from March until July, but in southern New Mexico the majority of weevils emerge in May and June. Peak emergence depends in part on spring weather. In 1997, peak emergence was the two-week period prior to June 9. In 1998, following a mild late winter and early spring, peak emergence was earlier, particularly in Eddy County. In this county, peak emergence was the second week of May. In Lea County, peak emergence was mid- to late-May depending on habitat.
In the spring and early summer, boll weevils invade cotton fields close to their hibernation sites. Generally, they will not fly more than 2 miles and the majority will establish themselves in cotton fields nearest to overwintering sites.
FEEDING HABITS
In the spring before cotton begins to square, boll weevils feed on cotton terminals, leaf buds, and pollen of alternate host plants. Such weevils will not live more than a couple of weeks, under New Mexico conditions where humidity is often low and in-field temperatures in young cotton are high. However, those weevils that are able to find suitable alternate sources of pollen can live longer and possibly survive until cotton begins to square.
As soon as squares are available, boll weevils feed on them, boring into the immature anthers (pollen-bearing structures in the cotton flower) with their long snouts. Feeding on the squares induces the bracts to flare (open wide and usually yellow), and results in the plant shedding the injured square within a week. Injured squares can be identified by tiny punctures the diameter of a large needle and by the bright yellow frass (insect feces) and pollen deposits close to the punctures. Flowers that open from injured squares will have a series of tiny, round holes in the petals and damaged or missing anthers.
REPRODUCTION
Female weevils lay eggs in squares, although they will use small bolls if squares are scarce. Usually only one egg is laid per square, maximizing the weevil's chance of survival. When a female is ready to lay an egg, she first bores into the square, generally near the widest part of the stem. She pulls her snout out of this bored hole, then turns around and extends her tubular ovipositor into it. After laying the egg, she seals the hole with frass.
Oviposition marks are difficult to detect on squares. The mark is similar to a small wart-like protrusion that may be more easily felt than seen. If the egg hatches, the small larva will begin feeding inside the square. If the square is severely damaged at this point, the bracts will flare, turn yellow and drop from the plant. On small bolls, the fecal plug is about the size and color of a fly speck.
Overwintered females may produce less than 100 eggs. Females maturing in the spring or summer can produce 300 or more eggs, laying an average of 5-6 eggs per day.
Boll weevils produce 5-6 generations per year in New Mexico. During the summer, eggs hatch in 3-4 days with developing larvae feeding on anthers or pollen in squares, or on lint of small bolls. Depending on temperature and the nutritional value of their food, larvae complete development in 7-12 days. Boll weevil larvae then pupate within the squares or bolls for 3-6 days before emerging as adults. However, adults that develop in mature bolls late in the season may remain there for 4-5 months until the bolls are mechanically opened or softened by moisture and decomposition. Thus, field sanitation after harvest can help reduce numbers of overwintering boll weevils (as well as certain other pests).
HIBERNATION
Beginning about mid-August in New Mexico and continuing into fall, adult boll weevils begin to fly much longer distances, dispersing and migrating to hibernation sites. Timing of those flights is influenced by high weevil density, quality and abundance of cotton fruit, and other season cues such as diminishing day length. Weevils that migrate long distances at that time may feed and reproduce on suitably developed cotton prior to cut-out. This can establish boll weevil populations in new areas late in the season.
Alternatively, some of these migrating weevils may overwinter directly. These weevils have undeveloped gonads but well developed fat bodies that can sustain them completely for the next 10-11 months. As time goes by, increasingly more migrants will overwinter without reproducing. They are likely, however, to search for a food source to finish building up fat reserves before settling into an overwintering site. Consequently, cotton that is allowed to develop squares in late summer provides the weevils with an abundant supply of breeding sites and nourishing food. If boll weevils go into hibernation with high fat levels, they are much more likely to survive and successfully infest fields the following spring. Difficulty in finding appropriate food to accumulate fat reserves will lower dramatically their chances of survival.
Boll weevils overwinter as adults sheltered by ground litter or hidden in protected areas. In New Mexico, data collected from 1996-1998 indicate shinnery oak can be a significant overwintering habitat in eastern New Mexico, where this plant is common. However, in the Mesilla and Pecos valleys, important overwintering habitats are primarily urban backyards and weedy borders. Some boll weevils also have been found overwintering in buildings, under shingles, and even attempting to burrow into the hides of livestock.
MORTALITY
Winter mortality in some parts of the U.S. is quite high. Historically it has often been greater than 90 percent in the High Plains of Texas, although in the last few mild winters, that has not been true. Winter mortality is less likely to have a large impact in the Pecos and Mesilla valleys where winter temperatures are moderate.
However, low humidity and high temperatures, which are common in the spring prior to squaring, may have a large impact on boll weevil mortality. These conditions also may force boll weevils out of overwintering habitats in a suicidal emergence before cotton is available. Such conditions were likely a significant factor in reduced boll weevil numbers and early emergence in 1998.
New Mexico State University is an equal opportunity/affirmative action employer and educator. NMSU and the U.S. Department of Agriculture cooperating.
Printed December 1998Electronic Distribution March 1999