In addition to the flowers, the underside of each leaf of the cotton plant contains a small cuplike structure holding nectar. These deposits and the succulent stem make the plant attractive to a variety of insect pests. Chief among these is the boll weevil. The use of early maturing strains of cotton plus the application of several chemicals and control methods have greatly reduced losses from boll-weevil infestation. The bollworm, the pink larva of a small moth, is believed to have been a native of India but is now parasitic on cotton all over the world. Quarantine, fumigation of seed, and destruction of trash removed from the cotton in ginning are control measures. The bollworm-tobacco budworm also is one of the most damaging cotton pests in terms of losses and control costs. Armyworm, thrips, lygus, and red spider are among other significant pests.
Among the serious diseases to which the cotton plant is subject is the wilt caused by a fungus which enters the roots from the soil and manufactures a poison. No treatment is known, but wilt-resistant strains of cotton have been developed. Another fungus disease is boll rot or anthracnose, caused by sac fungus. The best control is using seed from fungus-free fields.