Diseases of insects are important when the insects concerned are beneficial to man. A diseased insect may be suffering from an infectious disease caused by a microorganism or a noninfectious disease, such as a metabolic disturbance, a genetic abnormality, a nutritional deficiency, a physical or chemical injury, or injury caused by predators or parasites.
Beneficial insects, for which diseases have important consequences for man, include those which produce useful products, such as the silkworm and the honeybee, and those which are predators or parasitic upon various pests. Until recently, concern has focused solely upon the infectious diseases of insects which produce useful products, such as pébrine, flacherie, polyhedroses and muscardine of the silk worm, and foulbrood and nosema of the honeybee. These and other diseases still cause losses to the respective industries. Recently increasing attention has been paid to noninfectious diseases of insects, especially those where injury is caused by chemical agents such as insecticides. Whole populations of honeybees have been wiped out by drifting insecticidal sprays. More disturbing are reports that efforts to kill pests have been more effective in killing their natural predators (either directly through poisoning with pesticides or indirectly through feeding on poisoned prey or through the disappearance of adequate prey), which may result in population explosions of pests, either when chemical control is relaxed or when the pest develops some type of immunity. Furthermore, pesticides applied to the soil indiscriminately destroy harmful and beneficial insects. The latter may be vital in the regeneration of the soil.