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Alien Invasion

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Nature Vs. Nature 4 pages: | 1 | 2 | 3 | 4 |

Photo of WortIn "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.

Photo of Chrysolina
The Chrysolina beetle's preference for St. John's Wort saved acres of American rangeland.

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

While biological control can be a successful way to control invasive species, it can also fail miserably.

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.

Photo of Giant African Snail
Giant African snails multiply quickly and are serious pests.

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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4 pages: | 1 | 2 | 3 | 4 |

Photos: CA Dept. of Food & Agriculture, Botany Lab; R. Underwood, Lethbridge Research Centre; FDACS, Div. Of Plant Industry/Jeff Lotzs

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