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
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).
Male Confusion
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.
Discussion of sex attractants and their use in male confusion.
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. Link to more
information on the uses to which Bt and its toxin are being put.
The Biological Control of Plants
Prickly-pear
Cactus (Opuntia)
Introduced into Australia, this cactus soon spread of 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.
Klamath Weed
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.
Biological control can work. Remember:
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.