With all the jokes about northern snakeheads, it's easy to forget that there are nasty critters and plant life out there just itching to move in like some freeloading relative.
At first, they might appear to be welcomed guests with some redeeming qualities. The easy-on-the-eye, tough-on-plants mute swan comes to mind. But it doesn't take long before Swan Lake turns into the Black Lagoon, home of another web-footed creature.
Which brings us to the next unwanted visitor lurking just over the horizon: the zebra mussel.
We've all heard about the species mucking up the Midwest's ecosystem, clinging to native wildlife until it overwhelms it.
The bad news is that the nasty boys from Eastern Europe were detected in the Chesapeake Bay watershed in upstate New York two years ago, putting experts on high alert. A reservoir in Madison County has the fast-breeding mussels. A creek runs from the reservoir to a Susquehanna River tributary. You can connect the dots from there.
"In the Damoclesean sense, they're at the very top of our watershed, hanging over us now," says Jonathan McKnight, invasive species guru for the Department of Natural Resources. "This is very disturbing to us.
"It's not like we're the land of 1,000 lakes. But this is important for everyone who puts in a boat at Deep Creek Lake. It's close to infected waters and it has a lot of boat traffic. That's likely to be ground zero," says McKnight.
As the boating and fishing season cranks into high gear, the department has launched a public awareness campaign, asking folks to be vigilant about preventing the mussels from hitchhiking on boat hulls from infected waters to Maryland. Biologists have placed large orange warning signs at all public ramps and are asking private marina owners to do likewise.
The good news - if you like your cup half full - is that the Chesapeake Bay below the Bay Bridge is believed to be too salty for the mussels' liking, McKnight said. The bad news is there's plenty of bodies of freshwater with gigantic welcome mats on their shores.
Since they were detected in the mid-1980s, zebra mussels have spread to waters that border or cut through 21 states, Quebec and Ontario. Because they're not from around these parts, the mussels have no natural enemy. That, coupled with their ability to reproduce like an Internet virus, makes them particularly tough to eradicate.
A couple of isolated quarries in Pennsylvania and Virginia have zebra mussels in them. Biologists say they made themselves at home after "rogue introductions" by scuba divers to filter the water and improve visibility.
But while the black-and-white striped mollusks clear the water - one liter of water a day for each adult - they don't stop there. They also vacuum out other organisms that are food sources for critters.
"They make rare species even rarer," says McKnight.
Imagine Deep Creek Lake or Piney Run as lifeless as C-SPAN2's Book TV. Not a pretty picture, eh?
Invasive species experts estimate the cost of cleaning mussels from the Virginia quarry at $150,000, which is in the same ballpark as what it cost DNR to remove the snakeheads from a pond in Crofton two years ago. It's not the kind of money the agency has just sitting around.
Maryland officials began preparing the zebra defense more than a decade ago, spending millions of dollars to establish monitoring sites on waterways and building defenses at reservoirs.
The Baltimore Department of Public Works spent $3.6 million in the mid-1990s to install equipment to inject potassium permanganate into water intake pipes at Liberty and Loch Raven reservoirs and the Susquehanna River. Other controls were installed at Prettyboy Reservoir.
Freshwater anglers and other recreational boaters have a responsibility to make sure they're not giving zebra mussels a lift from an infested lake or river to a clean one. That means washing off the hull, prop, anchor and drive unit. It means making sure the cooling system, live well and bilge are clean.
It means not dumping your bait bucket overboard.
But that action may not be enough to ensure Deep Creek Lake and other fishing holes remain zebra-free. At some point, the state may require anglers who use live bait to buy only from sources that certify it as clean of zebra mussel larvae.
If you're really serious about keeping Maryland free of the nasty bivalves, take some time on a rainy weekend to Google the Zebra Mussel Information System web site, run by federal and New York officials.
But at the very least, do what the orange signs at the ramps advise.
I'm a sucker for the William Tell story, going as far as to see a three-hour outdoor theater version in Switzerland done in German, a language I know only from Hogan's Heroes.
The play was pretty interesting until the climax, when Willie takes a bead on the apple on his son's head and the audience leans forward. A sword fight breaks out across stage and the distracted audience fails to see Willie's son perform the old switcheroo with the apple.
There will be none of that sleight-of-hand next weekend at the Baltimore Bowmen's Traditional Classic, when long bow sharpshooter Byron Ferguson performs four shows.
The classic is held in the portion of Gunpowder Falls State Park that used to be Graham Memorial Park in Carney. It has three 3-D courses with a total of 80 targets. A number of vendors of traditional archery equipment will be displaying their wares.
But Baltimore Bowmen spokesman Mike Horst agrees that the real draw is Ferguson, an Alabama archer so precise that he can put an arrow through a diamond ring and knock down eight dimes with eight arrows.
Ferguson will perform four times: 6 p.m. Friday, 10 a.m. and 4 p.m. Saturday and 10 a.m. Sunday.
Details of the Classic and Ferguson's appearances are at www.baltimorebowmen.com. Or for more information, call Bob Domulevicz (410) 866-3697.