The larvae of the cotton bollworm can feed on leaves and stems but they actually prefer buds, inflorescence the lower part of the flower (square) and the cotton bolls. In the cases where the bolls are attacked, they gradually hollow them, leaving abundant moist frass, usually around the base. Larvae may also chew shallow gouges in the boll surface, which can become infected with opportunistic organisms. A the places where the larvae have entered the squares or the bolls, black holes can be found. The open holes in the plant tissue can lead to additional damage caused by fungal or bacterial infections. Leaves and shoots may also be consumed by larvae in some cases.
The damage is caused by the larvae of the cotton bollworm, Helicoverpa armigera, a common pest in a number of crops. The moths are light brown, 2 cm long, a wingspan of 3 - 4 cm and can be detected on flowers or close to light sources. Their forewings are yellow to orange in females and green to gray in males, with dark marks in both cases. Hindwings are whitish, with dark veins and dark elongated spots on the lower edges. Eggs are spherical, flattened, with 10 to 15 vertical rows of toothed ribs and hatch within 3-4 days of the ovoposition. The young larvae are white in color with little black spots and dark heads. As they develop, they can reach up to 40mm in length and are olive green to dark reddish brown with several black and yellow stripes on the body. Larvae can be best distinguished from most other caterpillars by the tiny spines, visible under a hand lens, that cover most of the body surface. As they reach maturity, the larvae drop off the host plant and pupate in the soil. Under tropic conditions a generation is complete within 30 days.
Bollworm management is based on scouting for eggs or small larvae. Treatment is recommended when 10 eggs or five small worms per 100 plants are present during early bloom. Some species of the parasitoid insects of the family Ichneumonidae, Braconidae, Scelionidae, Trichogrammatidae and Tachinidae can be used as biological control method. Also try predatory insects from the following orders: Coleoptera, Hymenoptera, Hemiptera and Neuroptera. Make sure to promote these species (or even introduce them to the field), and avoid use of broad-scale pesticides. You can apply bioinsecticide sprays containing Bacillus thuringiensis, Spinosad or Helicoverpa armigera nuclear polyhedrosis virus (HaNPV) to control population peaks.
Always consider an integrated approach with preventive measures together with biological treatments if available. As larvae become increasingly resilient to insecticidal treatment while growing, scouting for eggs and young larvae is crucial. Treatment is recommended to be applied during egg stage. Insecticides containing chlorantraniliprole, flubendiamide, methomyl or esfenvalerate can be applied. Chemical treatment may be inviable in low value crops.