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 In
"The Silence of the Birds,"
biologist Don Nichols
hopes to use a snake-killing virus to control the brown tree
snake explosion on Guam. The use of natural enemies to control
pest populations is called "biological control," and it's
an old practice. More than a thousand years ago, Chinese farmers
relied on green weaver ants to protect their citrus trees
from crop-destroying caterpillars and beetles. More recently,
biological control has been hailed as a safer alternative
to chemical pesticides, and a more humane solution to hunting.
But, while this method can be a successful way to control
invasive species, it can also fail miserably. Here are a few
hits and misses in the United States and around the world.
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Monumental
Success
St.
John's Wort, also known as Klamath weed, is perhaps the most
celebrated biological control success story. Introduced to
Pennsylvania in 1793, the yellow-flowering plant had both
medicinal and religious meaning for the Europeans who brought
it here. By the 1940's, the plant- toxic to livestock- had
spread across the continent, rendering more than five million
acres of otherwise valuable rangeland useless.
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The
Chrysolina beetle's preference for St. John's Wort saved
acres of American rangeland.
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Scientists
recruited natural predators to control the weed, and Chrysolina,
a group of Eurasian beetle species, were released in the western
United States in 1945 and 1946. As soon as the insects established
themselves in their new home, St. John's Wort populations
crashed in California, freeing up hundreds of thousands of
acres and saving an estimated $3.5 million per year throughout
the 1950's. The Directors of the Humboldt County Wool Grower's
Association and the Humboldt County Cattlemen's Association
in northern California were so thrilled with the results,
they erected a statue of Chrysolina in the town of
Arcata.
Accidental
Extinction
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While biological control can be a successful way
to control invasive species, it can also fail miserably.
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The giant African snail is a serious agricultural pest. Known
to attack more than 500 plant species-including cocoa, papaya,
peanut and rubber plants- the giant snail can also carry and
spread a form of meningitis. Stowed away in food shipments,
attached to vehicles or plucked by human collectors, the giant
snail spread from its native east Africa to large parts of
Asia, the Pacific Islands including Hawaii, and - more recently
- the West Indies.
In 1966, a boy visiting Hawaii brought two or three of the
giant snails home to his grandmother in Miami, where she released
them in her garden. The snails, each capable of laying 1,200
eggs each year of their nine-year-long lives, rapidly reproduced.
Within three years, some forty-two city blocks of Miami were
infested with more than 18,000 snails and millions of eggs.
Twenty-five miles north of Miami, another infestation broke
out in Hollywood, Florida.
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Giant
African snails multiply quickly and are serious pests.
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To control the exploding population, the predatory snail
Euglandina rosea, which preys on the giant snail was brought
into the affected Florida cities. While E. rosea did
eat some giant snails, humans were forced to take matters
into their own hands.
The giant snails were eventually eradicated through a costly
and labor intensive effort involving the use of bait and hand-collection.
By the late 1970's, after a four-year, $1 million campaign,
Florida was finally free of this invasive species. It will
require continued vigilance to keep this hardy traveler at
bay. 
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Photos:
CA Dept. of Food & Agriculture, Botany Lab; R. Underwood,
Lethbridge Research Centre; FDACS, Div. Of Plant Industry/Jeff
Lotzs
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