The Biological Control of Pests

The biological control of pests involves using natural enemies of the pest to control it — instead of chemical agents like insecticides and herbicides. Not only should this be safer for the environment, but — once established — the natural enemies might be able to sustain their population avoiding the need for future treatments. Most of the species that we consider pests are plants ("weeds") or animals (especially insects) that have invaded a new habitat without being accompanied by the natural enemies that kept them in check in their original home. With increasing international travel and trade, this problem becomes increasingly severe.
The Biological Control of Insects
In 1887, this insect — an import from Australia — was devastating the citrus groves of California. A U.S. entomologist went to Australia to find a natural enemy and came back with the vedalia beetle, a species of lady beetle. Released in California, the beetle quickly brought the scale under control. At least until 1946. In that year the pest made a dramatic comeback. This coincided with the first use of DDT in the groves. DDT not only killed the target pest insects but the vedalia beetle as well. Only by altering spray procedures and reintroducing the beetle was the scale insect again controlled.
The Sterile Male Technique
This technique was first applied against the screwworm fly, a serious pest of cattle. The female flies lay their eggs in sores or other open wounds on the animals. After hatching, the larvae eat the tissues of their host. As they do so, they expose a still larger area to egg laying, often finally killing the host.Prior to its eradication from the southeastern United States, the screwworm was causing huge annual livestock losses. The sterile male technique involves releasing factory-reared and sterilized flies into the natural population. Sterilization is done by exposing the factory flies to just enough gamma radiation to make them sterile but not enough to reduce their general vigor. Starting in early 1958, up to 50 million sterilized flies were released each week from aircraft flying over Florida and parts of the adjoining states. Each time a fertile female in the natural population mated with a sterile male, the female layed sterile eggs. Since the females mate only once, her reproductive career was at an end. By early 1959, the pest was totally eliminated east of the Mississippi River.
Success depended only on the sterile males. In fact, the presence of sterile females was a drawback (because they competed with the intended target), but it was difficult to separate the sexes. The southwestern states presented a harder problem because the fly winters in Mexico and with each new season could move across the border. Even so, by expanding the program to include Mexico as well, the screwworm fly was finally eliminated from both countries by 1991. The sterile male technique has also been used with success against several other insect pests, including the "medfly", a destructive fruit fly (not Drosophila) in California andthe tsetse fly, the vector of African sleeping sickness.
Using Genetic Engineering to Improve the Sterile Male Technique
There are two problems with the sterile male technique The factory produces both males and females in equal numbers. But if you release the females along with the males, many males will mate with them rather than with wild females. For this reason, the sexes are now separated — an expensive operation — and only males released. Irradiation may harm the males in subtle ways — reducing their breeding effectiveness.