Least Wanted From flora, fauna, even arthropod, our region is under siege.

In a field somewhere in Chester County, a fiend by the name of Heracleum Mantegazzianum is lurking, poised to take over the region - and possibly unstoppable.

This bad guy is a plant, literally: a giant hogweed that grows to 15 feet, with pillowy, white flower heads. Woe betide anyone the sap touches. Skin turns photo-sensitive, sometimes for years; exposure to sun raises welts resembling acid burns.

Government agents have yanked the hogweed, cut its blooms, and drowned it in herbicide. But nothing has stopped its advance. In the early 1980s, it turned up in northwest Pennsylvania, far from home in Russia's Ural Mountains, then crept through 14 counties to this exurban Philadelphia field two years ago, and now to North Jersey.

In Morris County this past summer, a giant hogweed was found in the worst possible place: next to a creek that can express-mail seeds downstream.

If it seems like invasive species of plants, bugs, fish and varmints are besieging this region, and indeed the nation, they are. And if they seem invincible, experts fear many are, even as 40 federal agencies pour $1 billion a year and their most lethal technology into what the U.S. Geological Survey terms "one of the most serious ecological battles of the 21st century."

Across the country, invasive plants alone crowd out hometown flora on an estimated three million new acres annually - an area greater than two Delawares - and imperil the wildlife that hides and dines there.

"The problem's not new. What's new is the acceleration of the problem and the acceleration of the impacts" on native habitants and habitats, said Lori Williams, executive director of the National Invasive Species Council, created by the Clinton administration in 1999.

Blame "the three Ts," she said. "Trade, travel, tourism."

Among nature's malefactors that showed up locally just this year is the snakehead, nicknamed "Frankenfish," that was found in a South Philadelphia pond in July. Several adults and hundreds of juveniles were since spotted there. An air-breather, this Asian import can pull itself over land. It favors native fish, but also wolfs down small mammals, frogs and birds.

The flathead catfish, a Mississippi Basin monster of up to 90 pounds with a native-fish appetite to match, already was breeding in the Schuylkill when it appeared in July in a New Jersey canal near Lambertville.