Why is the ichneumon fly known as the farmer's friend?

Ichneumon fly - is known as the farmer's friend because it controls a great many plant pests. Species have even been transported by man to colonize areas where artificial pest controls have not been successful. Ichneumon flies of which there are thousands of species spread throughout the world are parasitic and their larvae feed on caterpillars pupae and larvae of other insects. Larva (plural larvae) is the name given to an insect from the time it leaves the egg till it is transformed into the pupa or grub. The pupa (plural pupae) is the name given to the chrysalis. The female fly lays her eggs in or on the larvae or pupae of the host species. When the maggot like parasitic larvae hatch out they feed on the body fats and fluids of the host until they are fully grown. Then the parasitic larvae spin cocoons within which they pupate and from whence the adult fly emerges. In the case the parasitic larvae breeding inside the host the latter behaves normally until shortly before the uninvited guest larva has fully developed. There are some ichneumon flies which live on other ichneumon flies and these are called hyperparasitic. The different species of ichneumon vary greatly in size and the range extends from 1/8" up to 1 1/2 - 2" in length.