Management and Control of Boll Weevils

Biology

Both females and males are attracted by an aggregating pheromone. Female and male (there are equal numbers) must be within one inch of each other before recognition takes place.

Adult, overwintered females feed for 3 to 7 days, mate, and start laying eggs. Females find squares by sense rather than vision and starting in the spring lay one egg per square or several in a boll; one female during her lifetime usually lays an average of 150 eggs that hatch in 3 to 5 days.

Grubs or larvae feed about 7 to 11 days inside squares or bolls before changing into pupae; this stage lasts 3 to 5 days. Adults develop from pupae and cut their way out of squares or bolls. New adults feed from 3 to 5 days, mate, and begin laying eggs. Weevils increase rapidly in number, often resulting in a need for intensified spraying later in the season.

Males feed on two or three squares every 3 days, while females puncture 10 to 12 squares per day (for egg deposition). Squares damaged by egg-laying or feeding will flare in about 3 days after puncture, but squares may remain on the plant for 5 to 7 days before falling to the ground.

Cycles are repeated during the season until cotton plants are destroyed or killed by frost. Theoretically, the off-spring from a single pair of weevils could amount to 12 million in one year but probably number about 2 million.

Control of Overwintered Boll Weevils

Overwintered-weevil populations will be lower than subsequent populations in untreated cotton. Proper timing of effective insecticides can lower this population, thus reducing in-season weevil problems. After square initiation (pin-head size) but before squares are one-third developed is the best time to apply insecticides. The recommended action threshold is 25 weevils per acre. To determine the number of weevils per acre, examine all plants on 50 feet of row in five locations in the field and multiply the number of weevils counted by 50. If one or more weevil is found in the sample, you have an action threshold. Before taking action, ask

1. Does squaring occur before the peak emergence of weevils?
2. Are bollworm eggs present in the field?

Midseason Control

Check for weevil populations by randomly pulling unflared squares. If 10 percent or more of collected squares are punctured, a schedule of up to five applications (at no more than 5-day intervals) is required to maintain satisfactory control.

Diapause Control

Weevils entering diapause have been found as early in season as July; however, peak development of diapause usually coincides with maturity of cotton plants. Development of diapausing weevils in fields usually continues until food supplies are destroyed, either by a killing frost or by defoliation and stalk destruction. To enter diapause in fall months, weevils must feed for 14 to 21 days; then they move into hibernation in woody or trash-filled areas. Recent research indicates that feeding periods may be fewer than once believed.

In Mississippi, you must protect bolls during August and early September to make a maximum cotton yield. This is essential for diapause control, because it takes care of any weevils entering diapause early and it prevents a large number of weevils from developing and entering diapause from late September onward.

Defoliation is not altogether necessary for effective diapause control; but defoliation enhances effectiveness of control by reducing food and breeding sites for later development of weevils. Where defoliants usually are applied within 10 to 14 days after the last application of an insecticide, in routine 'in-season" control programs, each cotton producer should add a phosphate insecticide to the defoliants. If a clean defoliation job results, this one insecticide application (with the defoliant) may be the only extra treatment necessary for effective diapause control, and would, if used on a community wide basis, at least, reduce the total weevil population.

Soon after harvest, destroy cotton stalks with a shredder.

Insecticides recommended for application with phosphate defoliants are .5 pound methyl parathion, .25 pound Guthion (emulsifiable concentrate or ultra low volume), or 1.25 pounds of ultra low volume malathion.

Fall Destruction Means Fewer Weevils Next Spring

1. Lessens the total number of insecticide applications needed next season for maximum protection from boll weevils and lessens control costs for next season.
2. Minimizes the destruction of beneficial insects that suppress bollworms, spider mites, and other insects and reduces control costs for next season, and cuts losses caused by harmful insects.
3. Minimizes the risk of weather interfering with control measures and reduction in effectiveness of control measures.
4. Creates a condition favorable to elimination of weevils as a pest of economic importance. When research reaches the goal of developing an effective mop-up technique, it could be put into immediate use in early spring to clean up the few surviving boll weevils.
5. Increases yields by reducing the total insect damage.

Facts about Boll Weevils

1. Boll weevils are snout beetles and have chewing mouthparts.
2. They develop through egg, larvae, pupae, and adult stages.
3. All life stages, except the adult stage, are spent inside cotton squares or bolls.
4. In Mississippi, cotton is the only known host plant for boll weevils.
5. Boll weevils may stay alive for extended periods of time by feeding on vegetative cotton plant parts, e.g., seedling cotton; but cotton must be squaring to provide reproductive structures. Later in the season, eggs are deposited in bolls.
6. Female weevils deposit only one egg in a square but may place several eggs in bolls.
7. Boll weevils overwinter as adults in ground trash around cotton fields.
8. After emerging from ground trash, weevils may survive for 11 or more days without food.
9. After entering cotton fields, weevils are reported to live about 30 days; but marked weevils have survived well into July and August, and some were captured in September.
10. On an average weevil numbers increase about ten-fold from one generation to the next.
11. About 60 percent of overwinter female weevils were mated the previous fall; therefore, reproduction can occur without spring mating.
12. Male boll weevils release an aggregating pheromone after feeding on cotton squares. Both sexes are attracted to this pheromone.
13. The objective of pin-head square treatment is to kill weevils before reproduction occurs; therefore, treatments must be made before squares are one-third grown.
14. Baited pheromone traps placed around cotton fields are the best indicators of overwintered weevils, and the traps provide valuable information on necessity on timing of treatments.
15. Females lay about 12 eggs per day for two weeks or more.
16. Eggs hatch in about 3 days.
17. When eggs hatch, the squares flare.
18. After the first "molt" of weevil grubs, squares fall to the ground.
19. Larvae feed in fallen squares for 7 to 8 days, after which they pupate.
20. Pupae with dark eyes emerge as adults in about 48 hours.
21. The total life cycle may be as little as 14 to 15 days, to well over 3 weeks. A good average development time is 17 to 21 days.
22. Newly emerged female adults feed for about 3 days, mate, and start laying eggs.
23. The best indicator of weevil damage and a need for treatment is punctured squares.
24. Females are five to six times more destructive than are the males.

By Dr. Robert B. Head, former Extension Entomologist

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Publication 1830
Extension Service of Mississippi State University, cooperating with U.S. Department of Agriculture. Published in furtherance of Acts of Congress, May 8 and June 30, 1914. Ronald A. Brown, Director

Copyright by Mississippi State University. All rights reserved.

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