Visit Your Local PBS Station PBS Home PBS Home Programs A-Z TV Schedules Watch Video Donate Shop PBS Search PBS
Scientific American Frontiers Logo
TV Schedule
Alan Alda
For Educators
Previous Shows
Future Shows
Special Features

Alien Invasion

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

The Double Edged Sword

Photo of Prickly  Pear Cactus
 
Prickly pear cacti grow in abundance in the American Southwest.

Native to the dry, desert regions of the Americas, the Prickly Pear Cactus was imported by Australian ranchers in 1839 for use as living cattle fences. Soon, the hardy, drought-resistant cactus covered some 16 million acres of land. Impenetrable as well as inedible by cattle, the invited guest quickly became an unwelcome guest in the Land Down Under.

In an effort to control the prickly pears, the cactus moth Cactoblastis cactorum, was imported from Argentina in 1925. The moth lays its eggs in the flesh of the cactus, which the young eat and destroy as they grow. The plan worked. The transplanted moths succeeded in ridding the outback of the prickly pear invader.


The plan worked. The transplanted moths succeeded in ridding the outback of the prickly pear invader.

As the Californians would do after them, the Australians were so grateful to the cactus-munching moth, they erected a commemorative structure in its honor. The Boonargo Cactoblastis Hall pays homage to the moth that saved rural Australia.

Although this story had a happy ending, a similar scenario did not work so well in another part of the world. The cactus moth was introduced to the island of Nevis in the West Indies in 1957, also to control pest cactus. The moth quickly spread to surrounding islands, and arrived in the Florida Keys by 1990.

Photo of Hall
 
Boonargo Cactoblastis Hall in Australia was built in honorof the cactus-eating moth.

Here, however, the moth does not merely dine on the unwanted pest cactus; it also feeds on a native cactus called the semaphore, pushing this rare species to the brink of extinction. Today, any remaining semaphore cactus will only survive if covered by a protective cage.

So far, the cactus moth has spread to Georgia, and ecologists fear it could soon establish itself in the western United States and Mexico. There, the moth could threaten the existence of 50 rare cactus species, some of which are an important part of the local traditional diets and can serve as cattle feed in times of drought.

Photo of Cactoblasts
Hungry moth larvae dine on a cactus pad.
 

So far, no suitable means of controlling the moth has been discovered. Pesticides as well as some forms of biological control could have a disastrous affect on rare moths and butterflies related to this invasive moth. The same moth that saved Australia, could be the ruin of parts of the U.S.

 

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


Photos: Bruce Elder, Walk About Travel Guide; Department of Botany, Univ. of Texas

 

return to show page

 

The Silence of the Birds Green Invader The Silken Tree Eaters Dust Busting Teaching guide Science hotline video trailer Resources Contact Search Homepage