Forget about the Roach Motel. Call out the Army.

Cockroaches have invaded the Pentagon. By one count, 2 million of them.

From the inner sanctums of Defense Secretary Caspar Weinberger and the Joint Chiefs of Staff to the cafeterias and snack bars of the 23,000 Pentagon workers, the Blattagermanica (German cockroaches) have infiltrated the 17.5 miles of the Pentagon's corridors like armies of the night.

But the Army, which knows an invasion when it sees one, is on to the creepy critters at the Arlington military fortress, and has begun a counteroffensive. Over at the Walter Reed Army Medical Center in Washington, Capt. Brian Thoreson, chief of environmental health, has been waging a one-man war against the invaders.

Thoreson catches the enemy roaches off guard at various Defense Department buildings and puts them under surveillance in a glass-enclosed model of a seven-story Walter Reed building. Then, as about 500 to 2,000 of them frolic and dine on dog food in the roach motel he built, Thoreson plots ways to control them.

"They've always been there at the Pentagon and they always will be there," Thoreson said.

The 2 million figure was Thoreson's rough estimation based on the number of people who work there. "The numbers may be alarming to lay people, but in a multipurpose building of that size and age, it's not alarming at all," he said.

Thoreson doesn't like to refer to his work as "chemical warfare." He prefers to say that his job is to determine the roaches' susceptibility to various pesticides over a period of time. The pesticides involved are biodegradable and will not hurt humans, animals or plants, he said.

Roaches are so highly adaptive they can develop a resistance to poisons in a half-dozen generations, or about six months, the 33-year-old Thoreson said.

In fact, he said, there are scientific tests showing roaches "can resist increasingly higher amounts of radiation, far more certainly than man or animals can. It's fair to say they'll outlast man."

There are 55,000 known species of roaches, about a dozen of them common to the United States, Thoreson said. They can detect a human being within .054 of a second, and, besides dog and human food, they like glue, soap and paper, particularly book bindings. So far, they have not destroyed any top secret documents at the Pentagon, he said.

Declassified intelligence reports indicate it is the so-called German roaches that are stalking the 6.5 million square feet of the Pentagon.

While Thoreson continues his research, GSA has deployed Terminex Inc., a Springfield exterminator, which for $60,700 a year goes into the Pentagon every Monday, Wednesday and Friday to spray and set roach traps.

Dave Shimamoto, a Terminex employe, said yesterday that the Pentagon does have a roach problem. But, he added, "I'd say rats are really the main problem."