Japanese invaders cause global headaches

Masayuki Takata / Yomiuri Shimbun Staff Writer

Japanese invaders are wreaking havoc around the globe, in some cases more than a century after the initial invasion. It should be mentioned that these invaders are plant and animal species, not people.

Invasive species originating in Japan include the kudzu vine, which was introduced to the United States as a garden plant in the late 19th century but has become an uncontrollable menace throughout the southern part of the country since.

Itadori, or Japanese knotweed, is a perennial plant that has proliferated in many parts of Europe, endangering indigenous plant species. It is believed the plant was brought to Europe by Philipp Franz von Siebold, a German doctor, during the late Edo period (1603-1867).

Mutsuo Kobayashi, chief of the Department of Medical Entomology at the National Institute of Infectious Diseases, said hitosuji-shimaka, or tiger mosquitoes, the most common mosquito in Japan, are believed to have been transported to the United States in shipments of used tires. The mosquitoes have caused problems on the island of Oahu, Hawaii, because they can carry dengue fever. The West Nile virus has also been found on tiger mosquitoes caught on the U.S. mainland.

In 17 eastern U.S. states, tsugakasa-aburamushi, a species of aphid indigenous to Japan, has damaged Canadian hemlock trees in state parks and other places. The insects are believed to have been brought into the country with garden plants in the 1950s.

In the sea, wakame, or brown seaweed, has spread to coastal areas around the world, and research on the routes the seaweed used is under way. One team of researchers, led by Prof. Hiroshi Kawai of Kobe University, analyzed the genes of brown seaweed from several parts of the world. Their study showed that wakame collected in Australia and New Zealand were from not only Japan, but also South Korea and China. Kawai said wakame has also spread to South America.

Seaweed, shellfish and other marine species can be transported to faraway places when they get into the ballast water carried by cargo ships, or by sticking to fishing boats.

Last month, researchers from 11 Pacific Rim countries, including Japan and the United States, met and agreed to exchange information on alien species.

To get rid of these pesky invaders, enemy species from Japan have been introduced in some cases to eliminate them.

In the 1990s, the United States released the tsugahime-tento, a lady bug species that is a natural enemy of the tsugakasa-aburamushi aphid. However, the lady bugs did not reproduce in U.S. hemlock forests. Shigehiko Shiyake, chief curator of the Osaka Museum of Natural History, discovered in 2005 another beetle species that is a natural enemy of the aphid. The insect probably will be released in the United States in the coming winter.

The use of enemy species carries a risk of unpredictable negative impacts on ecosystems. For example, mongooses that were introduced to Amami-Oshima island and nearby areas to eliminate the poisonous habu snake endangered other species in the areas.

Daisuke Kurose, senior researcher at the Japan Society for the Promotion of Science, said that the British government, which is concerned about the damage caused by Japanese knotweed, is conducting an experiment this spring to eradicate the plant in cooperation with Kyushu University and other Japanese entities. In the experiment, a psyllid insect species that eats Japanese knotweed was released into the wild. The scientists will examine whether the insect damages other plant species over the next year, Kurose said.