The cotton bollworm is a caterpillar. It eats inside the cotton stem and boll. The top of the cotton plant wilts and the leaves and boll fall. Sometimes the bollworm chews round holes into the stem. If you split the stem open you will see the droppings of the bollworms. The inside of the stem and boll has turned blackish or brown. You might also see the bollworm in top of the stem.
The bollworm eats and grows quickly for 5 to 10 weeks of the cotton season. It hatches from eggs laid in the upper leaves and squares of the cotton plant.
When we spray insecticides some of the bollworms do not die. Contact insecticides only kill the bollworms on the top of the plant. Even systemic insecticides do not kill all of the bollworms because the cotton plant protects them.
After the cotton harvest, some of the bollworms still live in the crop stubble and in the weeds of the field, and produce the next generation of pests.
Bt cotton has a special poison inside the plant which kills some of the bollworms. The poison is only for insects and does not harm people or animals.
Rather than spraying insecticides it may be better to prevent the bollworm from entering the cotton. When the rainy season starts, plant the seeds 30 to 40 centimetres apart, in rows 45 to 60 centimetres apart, so that when the plants are big you can walk down the line with spraying equipment.
- Hand pick the bollworms when they are still small, before spraying insecticide. Carefully open the tops of the plants and kill the small worms.
- Clean the field carefully before planting, to destroy the cotton stubble. Weed and mulch the field properly.
- After harvesting cotton, follow it with a different crop to avoid further bollworm attacks. Cover the cotton stubble with soil to avoid pests the next season.
- Plant Bt cotton or other varieties which are resistant to the bollworm.
- Put bamboo sticks in the field so that birds can stand there and eat caterpillars.
- Use light traps and sex pheromone traps in the field to control the bollworm.
- When the bollworm attacks, spray the cotton field with contact insecticides like cypermethrine or fipronil. Mix 2 to 3 millilitres of insecticide in one litre of water and spray every 2 weeks, up to a maximum of 2 applications per season, until the cotton becomes mature.
When using a pesticide, always wear protective clothing and follow the instructions on the product label, such as dosage, timing of application, and pre-harvest interval.