Anthonomus grandis
Description. The adult boll weevil is a small, brown to grayish-brown beetle. It has a distinctive gently curved, black snout which is about ½ the length of the body.
Boll weevils can be distinguished from other weevils by the two spurs on its front legs
Just behind the head, the thorax normally has a light whitish stripe that ends where the thorax joins the abdomen. The body of the boll weevil is covered with short, tan brown and grey scales which give it a slightly fuzzy appearance. The boll weevil can be distinguished from many other weevils by the spurs on its front legs. Many weevils have a spur at this location, but only the boll weevil and a few of its close relatives have a double spur. Adult weevils vary in size from slightly more than 1/8 inch to almost ½ inch in length.
Boll weevil grub feeding in a square
Immature stages of the boll weevil live inside squares and bolls where they are protected from most natural enemies and insecticides. Eggs are seldom seen since they are small and are deposited inside a square or boll. The larva is a small, white, c-shaped, legless grub with a tan to brown head and chewing mouthparts. Grubs vary in size from very small to ½ inch in length. The pupae or “resting” stage of the boll weevil is 3/8 to ½ inch long and cream colored. With dark eyes and a snout already formed, it has begun to develop the characteristics of the adult stage.
The adult boll weevil spends the winter in hibernation, called “diapause,” without food and returns to cotton in the early spring the following year. In areas where winters are warm and host cotton is available, reproduction can occur throughout the 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.
Despite having boll in it’s name, the boll weevil prefers to feed on and lay eggs in squares. 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.
Boll weevil egg punctures
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.
Boll weevil feeding sites can usually be distinguished from caterpillar feeding by the bright yellow-orange frass which does not contain webbing typical of most caterpillar frass
Damage. Adult boll weevil feeding causes little crop damage. It does, however, indicate the presence of adult weevils which are (or will soon be) laying eggs. As weevils feed, a small hole is formed at the site. If a female weevil determines that the site is suitable for egg laying, she enlarges the hole 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 hole. This sticky secretion hardens to form a wart-like protuberance on the outside of the square or small boll that can be easily seen and felt.
A few days after a boll weevil egg is laid in a square, the egg hatches and the larvae begins feeding. Subsequently, the square begins to dry out and develop a lighter yellowish green color. The square bracts flare open and the square falls to the ground. In contrast with oviposition punctures, feeding punctures in squares usually do not change color, flare or fall to the ground. Also, bolls which are damaged by boll weevil feeding or egg laying normally do not fall from the plant. Square and boll damage often result in yield loss. Most of the yield loss comes from damage associated with oviposition and larval feeding inside cotton fruit. Heavily infested cotton may produce rank vegetative growth with few harvestable bolls.
Management and decision making.
Cultural management. Cultural control strategies have been the cornerstone of boll weevil suppression programs formany years and they are essential elements of boll weevil eradication. A number of decisions that affect boll weevil population development are made before planting. Choices of tillage systems, varieties, fertilization, planting dates and irrigation amount and timing affect boll weevil survival, reproduction and damage potential. To optimize boll weevil suppression, cotton should be planted as soon as the soil has warmed sufficiently to support strong germination and stand establishment (65 degrees F at the 8 inch depth for 10 days with warm weather forecast). Bt cotton varieties should be strongly considered to suppress caterpillar pests in fields that are expected to receive several boll weevil eradication treatments.
The best management system for boll weevils (and most other cotton pests) is to begin the season with a compact planting window for all fields in the area. Well adapted, determinant or intermediate maturing varieties should be selected. Boll weevil colonization in cotton is closely related to the fruiting of the plant, with the greatest numbers of overwintered boll weevils entering cotton fields after squares are present. Therefore, the extent of overwintered boll weevil infestation depends on the size of the emerging weevil population and the availability of squaring cotton. Thus, in the more temperate portion of the state, early planted cotton and fields adjacent to ideal overwintering habitat are much more likely to have a significant boll weevil infestation than cotton planted later in the season or fields farther away from good overwintering habitat.
Excessive fertilizer and water inputs should be avoided. The cotton crop should be managed to set and mature fruit rapidly. Managing the crop for earliness during the production season makes early harvest possible and minimizes the risk of crop loss from tropical storms. Competitive yields with reduced risk of crop loss can be produced using these practices and the stage is set for early, thorough stalk destruction to deprive boll weevils of late season food.
It is critical during boll weevil eradication for farmers to keep fields and non-crop areas free of cotton plants both during the growing season and after harvest. Cotton plants in these situations can be destroyed using herbicides and/or tillage equipment.
Biological control. While biological control has been a large part of the management system for many crop pests, its role in the management and eradication of boll weevil has been limited. Predators, parasites and pathogens of boll weevil exist, but cannot be relied upon to significantly limit boll weevil populations in Texas. It is important, however, to monitor and manage natural enemy populations in pest management systems that involve intensive insecticide use, such as active boll weevil eradication. Natural enemies suppress populations of secondary pests such as whiteflies, aphids, beet armyworms, bollworms and tobacco budworms. Populations of these pests can increase to damaging levels when populations of their natural enemies are low.
Scouting. Pheromone traps are a very effective tool for boll weevil population detection and they are the primary means of boll weevil detection used by boll weevil eradication programs. However, visual inspection of squares, blooms and small bolls is important as well. When checking fields one should inspect white blooms and inside the bracts of squares and small bolls for the presence of boll weevil adults. The square and the boll walls (inside the bracts) should be inspected for boll weevil punctures. When boll weevil punctured squares or small bolls are found in a field (either on the plants or on the ground) boll weevils are present and the find should be reported the local boll weevil eradication office.
Small open holes in fruit may be boll weevil feeding punctures or they may be the result of feeding by other insects. The hole left by boll weevil feeding is small (about 1/32 inch in diameter), but larger than the pin-prick type damage left by plant bugs or stinkbugs. Feeding damage left by small bollworms is often similar to and easily mistaken for boll weevil damage. This confusing small worm damage is often associated with Bt cotton. The toxin kills the worms before they grow and leave larger feeding holes which are easily identified as worm damage. The presence of feeding holes 1/16 inch in diameter or larger indicate the damage was caused by one of the caterpillar pests (commonly bollworm or tobacco budworm). When smaller holes are present and fine webbing can be seen at the site, a small bollworm or budworm was responsible (since boll weevils do not produce webbing).
Insect droppings within the bracts of bolls or squares provide less certain, but none-the-less useful indications as to which insect produced the damage. Boll weevils produce scattered, powdery or grainy flecks of bright yellow or orange droppings. Worm droppings are usually clumped at the site of the damage, less powdery, and less bright yellow or orange in color. When only one or two damaged squares are found, it is often difficult to definitively determine whether the damage was caused by a small worm or a weevil. It is usually helpful to spend a little more time looking at fruit on plants or picked up from the ground nearby. If more damaged fruit can be found or boll weevils are seen, you can determine with greater confidence what caused the damage.
Boll weevil eradication.
All Texas cotton acreage is now in the boll weevil eradication program, http://txbollweevil.org. Any detection of boll weevil adults or boll weevil punctured squares should be reported to the local Texas Boll Weevil Eradication Foundation office or the state Foundation office at (325) 672-2800. If you determine that you would like to make an insecticide application targeting boll weevil in addition to efforts by eradication directed applications, please contact the Texas Boll Weevil Eradication Foundation before doing so.
There are a number of things a producer can do to aid boll weevil eradication effort; some of which are critical.
- Communicate to boll weevil eradication personnel the location of all cotton fields or areas where volunteer cotton exists.
- Allow boll weevil eradication personnel vehicular access to all cotton fields for trapping and mist blowing.
- Ensure that pheromone traps are kept standing and operational.
- Promptly alert eradication personnel of any adult weevils or suspected weevil damage found.
- Destroy volunteer cotton from fence rows, ditch banks, other crops, and any other place where it is found.