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
Cottony Cushion Scale Insect
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
- The 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.
Genetic engineering may solve both these problems.
A group of British entomologists (see Thomas, D. D., et al., in the 31 March 2000 issue of Science) have engineered Drosophila so that
- Only males are produced.
- When these mate with normal females, the females give birth only to males (thus ending the population).
Figure 126.96.36.199 Genetic engineering to improve male sterile techniques
The system works like this:
- Transgenic flies are created containing a chromosome with an enhancer (En) for a gene (tet TA) that encodes a transcription factor (green disk) that binds the response element (tet RE), which is part of the promoter for a gene (Toxin gene) encoding a protein whose product is lethal to the insect.
- Only females produce the transcription factor that binds En.
- If the antibiotic tetracycline is given to the insects, it binds to the tet TA transcription factor, producing an allosteric change that prevents the transcription factor from binding the tetracycline response element (tet RE).
- The toxin gene is repressed and viable females are made.
- In the absence of tetracycline, the tet TA transcription factor (green disk) turns on the toxin promoter and no females are produced.
- Because the tet TA enhancer (En) responds to a transcription factor made only by females, males are produced whether tetracycline is present or not.
If this system could be applied to an insect pest (and most seem to produce the same female-specific transcription factor [red oval]),
- removing tetracycline from the food of a batch of flies in the factory would produce a new generation containing only males.
- Released into the wild, these would pass their transgenic chromosome on to the offspring of all the wild females they mated with.
- The genes are dominant so even though the next generation would be heterozygous, only males would be produced.
- Thus the pest population would soon die out.
In 2010, release of male mosquitoes Aedes aegypti, the vector of dengue fever - genetically modified (GM) with a similar system reduced the resident mosquito population in part of Grand Cayman (Caribbean) by 80%.
Techniques have now been developed which greatly increase the frequency of any desired gene in a population. So far, the uncertainties of quickly spreading an engineered gene through an entire wild population has kept the process strictly confined to the laboratory. The process, called gene drive or the mutagenic chain reaction, is described on a separate page.
Insect sex attractants have also been enlisted in the fight against harmful insects. Distributing a sex attractant throughout an area masks the female's own attractant so the sexes fail to get together. This is called "communication disruption" or "male confusion". In some cotton-growing areas, male confusion with the sex attractant of the pink bollworm reduced the need for conventional chemical insecticides by 90%. It has been used successfully against pests that attack tomatoes, grapes, and peaches.
Parasites vs Insect Pests
Parasites, as well as predators, have been used to achieve control over destructive insects.
- The bacterium Bacillus popilliae is supplied commercially to help control the Japanese beetle by infecting it with "milky disease".
- Bacillus thuringiensis ("Bt") is sold commercially to aid in controlling a number of harmful insects. In some cases, the bacterium itself infects the pests and eventually kills them. But in most cases, it is the toxin manufactured by the bacterium while it is growing in culture that does the job.
The Biological Control of Plants
Prickly-pear Cactus (Opuntia)
Introduced into Australia, this cactus soon spread over millions of hectares of range land driving out forage plants. In 1924, the cactus moth, Cactoblastis cactorum, was introduced (from Argentina) into Australia. The caterpillars of the moth are voracious feeders on prickly-pear cactus, and within a few years, the caterpillars had reclaimed the range land without harming a single native species. However, its introduction into the Caribbean in 1957 did not produce such happy results. By 1989, the cactus moth had reached Florida, and now threatens 5 species of native cacti there.
In 1946 two species of Chrysolina beetles were introduced into California to control the Klamath weed (also known as St. Johnswort, and the same plant that yields the popular herbal concoction) that was ruining millions of acres of range land in California and the Pacific Northwest. Before their release, the beetles were carefully tested to make certain that they would not turn to valuable plants once they had eaten all the Klamath weed they could find.
The beetles succeeded beautifully, restoring about 99% of the endangered range land and earning them a commemorative plaque at the Agricultural Center Building in Eureka, California. (Photo courtesy of John V. Lenz.)
Rules to Live By
- Pick only candidates that have a very narrow target preference; i.e., eat only a sharply-limited range of hosts
- Test each candidate carefully to be sure that once it has cleaned up the intended target, it doesn't turn to desirable species.
- Don't use bio controls against native species.
- Avoid introducing alien species into the environment.