When Koichi Goka heard rumors about the mysterious deaths three years ago, he started snooping around. What he found has put government officials on alert against a new plague, one that causes the limbs of its victims to rot and fall off.

The sick aren't filling hospitals -- this plague is restricted to beetles.

But in a country where it is not uncommon for collectors to spend thousands of dollars on a pet bug, Goka's discovery has become national news.

"I was stunned," said Goka, a biodiversity researcher at the National Institute for Environmental Studies in Tsukuba City, just north of Tokyo. "The way they died was very different from anything I had studied before."

The culprit turned out to be a large tick never before seen in Japan.

The amber-colored parasites, about 0.08 of an inch long, were found living in the joints and bellies of their beetle hosts and carrying deadly bacteria.

Goka said the tick is about 10 times the size of the ones that are most familiar here, the kind that inhabit tatami mats in Japanese homes.

He said it doesn't appear to be dangerous to people but poses a serious threat to the 50 known species of indigenous beetles. Several months ago, Goka published a warning in the major newsletters of the bug research and collecting industry.

"We have only tested beetles from Tokyo. We still don't know where the ticks are from or how far they may have spread," he said.

Japan's penchant for big and exotic beetles is likely to blame.

The unidentified tick doesn't appear to be native to this country, and Goka believes collectors dealing in the lucrative black market in insects may have unwittingly imported it along with their bugs.

Beetle-collecting is a popular pastime in Japan.

Department stores sell them, and children keep them as pets. Hard-core collectors dish out exorbitant sums for rare species and often hunt for wild beetles in Japan's forests or on expeditions to South Asia and Southeast Asia.

In recent years, many of the most popular and highly prized beetles have come from that region. For example, the Dorcus antaeus, native to India, Nepal, Vietnam and Malaysia, can sell for up to $3,330.

Experts say that since 1999, when Tokyo eased import restrictions on animals potentially harmful to crops, the number of foreign species has risen sharply.

Collectors, scientists and environmental groups say Japan must step up controls or risk introducing new species and diseases that could allow foreign species to become dominant and destroy the country's ecological diversity.

"This kind of tick invasion is especially dangerous because it is unpredictable. It can wipe out an entire species or spread and infect several different species, including plants," said Izumi Washitani, a Tokyo University professor who heads the Ecological Society of Japan's panel promoting tighter animal import laws.

"An island nation such as Japan has an ecology that can be upset very easily and very significantly by foreign species," Washitani said.

Fumiaki Uragami, editor in chief of Stag Beetle Magazine, said stores and Internet sites specializing in beetles are doing a brisk trade in imported species and crossbreeds.

"The fear is that they will lead to the disappearance of the native species," Uragami said.

Last year, collectors imported about 682,800 beetles, said Shoko Kameoka, an official with Traffic East Asia-Japan, a Tokyo-based wildlife trade watchdog.

She said the government has no system to track what happened to the 96 beetle species from 25 countries that entered Japan during the past year.

Not all beetles are susceptible to the new tick plague.

Goka said experiments on several indigenous species show the tick targets the rhinoceros beetle and stag beetle. It can kill them both in less than a month, he said.

Tomoyuki Nozuka, an employee at Scissors World, a beetle shop in Tokyo, said talk of new plagues surfaces so often that it rarely affects prices or hurts sales.

"Mostly we hear about people finding the egg or larvae or chrysalis of a tick or some other parasite latched onto their beetles," he said. "But in most cases they can be washed off with water without harming the beetle."

Japan's Environment Ministry has formed a task force to devise a plan to crack down on the invasion of all types of foreign animal species. Ministry official Ichii Ishiyama said it could be another year before details of the plan are fleshed out, however.

"Until the government enforces tighter controls," Goka said, "we may continue to see more of these kinds of cases."

Amber-colored ticks, about 2 millimeters are stuck in the joint and the belly of a Japanese stag beetle in this August 2001 photo. The ticks carry deadly bacteria.The parasites were found living in the joints and bellies of their beetle hosts and posing a serious threat to the 50 known species of indigenous beetles in Japan.