Cotton is subject to attack by numerous insects, pathogenic fungi, bacteria, virus and nematodes. Its succulent foliage, large flowers and extended fruiting period make cotton an attractive host for many insects and the diseases they carry. In all, 500 species of insects attack cotton and among them are some of the most destructive known to agriculture, including the boll weevil, jassid, whitefly, pink bollworm, bollworm, cotton aphid, cotton stainer, cotton fleahopper, cotton leafworm, spider mites, grasshoppers and tarnished plant bugs. The most widespread disease which attacks growing seedlings is a complex of disease organisms, the predominant organism differing with environmental conditions. A number of weeds also compete with cotton, damage the crop, and reduce its value. The average crop losses worldwide due to pests and diseases are 60% of potential production.
All cotton-producing areas suffer high losses from insects and diseases. Due to insects, the annual losses in the USA alone has averaged more than US$ 243 million since 1929 with an all time high, in 1950, of over $900 million. To this must be added the cost of insect control measures which in the USA vary between $58 million and $95 million annually.
Losses due to disease have been estimated to be as high as 50% in Brazil and some African countries. Losses in India have fluctuated between 7% and 20% and in the USA have been reported as high as 15%. One widespread disease is bacterial, known as angular leaf spot, and in African countries this is the most serious disease. Fungus diseases which attack the growing cotton plant include fusarium and verticillium wilt and Texas root rot. Fusarium wilt is an important disease in the USA and the major disease in Egypt. Texas root rot is limited in distribution to areas of Mexico and the USA. Of the insect pests, the pink bollworm is the most widely distributed. It has become established in the 8 the countries that produce 90% of the world's cotton. In the USSR, it has caused severe losses. In India and Egypt the average annual loss caused by pink bollworm is between 15 and 25% and in Brazil, between 20 and 25%. It is the most serious cotton pest in China. The most serious cotton pest in the USA is the boll weevil. It is restricted to North and Central America, but another closely related species occurs in South America. Insects which feed on the leaves and buds, including the cotton leafworm and the sucking insects (fleahoppers, leafhoppers and aphids), do considerable damage to crops throughout the world. Of the weed pests, the most severe and widespread are cyperus rotundus, cynodon dactylon, and portulaca oleracea.
In China, the main cotton pest is the boll weevil - also a danger to cotton crops in other countries. Efforts to develop new methods of biological control have a certain urgency, because farmers and scientists are increasingly troubled by the costs and dangers of chemicals pesticide use. The University of Hubei in China, looking for natural predators to control cotton pests, found that of 600 predators, more than 100 were varieties of spiders. By maintaining populations of spiders in their fields, farmers find that their crop yields increase. At the same time, they have cut down on chemical use by 80%.
In 1889, California orange growers were losing their crops to a bug known as the cottony cushion scale. They successfully responded by enlisting the help of a small but hungry insect recently arrived from Australia, the seven-spotted ladybug.