Unwelcome Invaders Moving In and Taking Over.

There's a silent, insidious environmental invasion of the United States that claims billions in damages each year. And we're not talking about something Out of Invasion of the Body Snatchers or Creature from the Black Lagoon.

It's invasive species--plants, animals, insects and microbial pests that have been introduced to the United States from elsewhere via produce shipments, ships, trucks and other ways that can be found to "hitch a ride."

These "foreigners" have had adverse effects, such as reducing profitability of grazing land or choking out native flora and fauna. According to a recent study by researchers at Cornell University, invasive weeds and other species cause an annual $136 billion in economic damage every year.

Every state plays host to hundreds of non-native species. A 1994 state-by-state comparison by the University of California at Davis noted that 7.9 percent of the total flora of the state of New Mexico is introduced species; the percentage ranges up to 24.8 percent in Missouri, 27.5 percent in Illinois and as high as 47 percent in Hawaii.

Many of the introduced plants, such as corn, soybeans and wheat, are beneficial to the United States. Others, however, pose a threat to the environment and cost billions of dollars to control. Such pests include European purple loosestrife, which now grows in 48 states and costs $45 million a year to control. Leafy spurge can now be found in at least 36 states and causes more than $144 million each year in loss of grazing land in Montana, North Dakota, South Dakota and Wyoming.

Animal pests include the brown tree snake, which has killed nine species of Guam birds, as well as causing estimated annual losses of at least $1 million due to power outages from snakes getting into the transformers or wrapping around the wires. The Asian longhorn beetle forced New York and Chicago to destroy several thousand trees at a cost of more than $6 million. The zebra mussel clogs water intake pipes and threatens native species in the Great Lakes and throughout the United States (the cost of control in the Great Lakes alone has totaled more than $5 billion).

Illinois Senator Evelyn Bowles believes invasive species, like the zebra mussel, "are a plague. There is no question that zebra mussels have had a drastic effect on water intake lines and have taken over the habitat of indigenous mussels and sport fish in Illinois.

"Although zebra mussels were not intentionally imported into the United States [they are thought to have arrived on the hull of an oceangoing ship], there must be some way of controlling this bad, bad situation," she says.

A federal Invasive Species Advisory Council has been created to craft a unified approach to these foreigners and to issue a national management plan that will include recommendations for states. NCSL has slated a special session on the issue during its annual meeting in Chicago.