Cotton
Boll Weevil
Anthonomus grandis
Insect
In a Nutshell
- Small punctures on flower buds, which turn brown and fall off.
- Flowers turn yellow and also drop prematurely.
- Impaired growth of bolls or boll rot.
Can also be found in
Cotton
Symptoms
Adult boll weevils feed on flowering or fruiting structures such as squares and bolls and occasionally on leaf petioles and terminal growth. The early stage of attack is recognizable by small feeding punctures at the side of the flower bud or small wart-like growth corresponding to oviposition sites. Damage to the flower buds causes discoloration and may result in abortion or premature dropping of flowers or small bolls (with the larva inside). Large, punctured bolls usually remain on the plant and may not open. Alternatively, they can be attacked by opportunistic pathogens that may cause them to rot. Adults feeding on leaf petioles during the vegetative growth of the plant causes leaves to wilt and wither while remaining attached to the stems, a trait that is commonly known as "black flags".
Recommendations
Organic Control
Parasitic wasps like Catolaccus grandis can be introduced or promoted to control the weevil. Furthermore, bio-insecticides based on the fungus Beauveria bassiana, the bacterium Bacillus thuringiensis or the Chilo iridescent virus (CIV) can be applied.
Chemical Control
Always consider an integrated approach with preventive measures together with biological treatments if available. Synthetic pyrethroid insecticides and substances like deltamethrin can be used to control boll weevils. Humid conditions increase the efficacy of the treatment. Pheromone traps can be used for weevil monitoring and control (combined with an insecticide or bio-agent)
What caused it?
The damage is caused by both adults and grubs of the boll weevil, Anthonomus grandis. Adult beetles are about 6 mm long with long slender snouts and vary in color from dark, brownish-red to brown or near black. They overwinter in well-drained areas in or near the cotton fields. After this period of suspended development, they emerge and enter cotton fields from early spring through midsummer, with peak emergence in late spring, during the development of the boll. Females lay eggs in the growing cotton bolls, usually one egg per square. The creamy-white, legless, C-shape larvae feed within the square or boll for about 10 days and later pupate there. The life cycle from egg to adult spans about three weeks during the summer. The boll weevil can reach up to 8 to 10 generations per year.
Preventive Measures
- Check for quarantine regulations in your country.
- Plant late in the season to avoid peak populations.
- Choose resistant or resilient varieties, if available in your area (several have been put on the market).
- Monitor the cotton field regularly for symptoms of the pest.
- Avoid excessive fertilization and overwatering.
- Immature cotton weevil stages are sensitive to heat and desiccation.
- Therefore, plant debris should be removed from the fields to expose remaining larvae or pupae to adverse weather conditions.
- Do not plow plant residues after harvest, simply remove debris from the field instead.
- Do not transport potentially infected plant material between farms or fields.
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