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Venomous Spider Found in All but Two of Japan’s Prefectures

The Yomiuri Shimbun
A female redback spider caught by Hiroyuki Shimizu of the Kansai Kumo Kenkyukai (spider study group) in early April in Takaishi, Osaka Prefecture

An invasive species of poisonous arachnid called the redback spider, which was first discovered in Japan 28 years ago in Osaka Prefecture, has now spread across nearly the entire country, according to the National Institute for Environmental Studies and other sources.

The redback spider is found in regions such as Australia and Southeast Asia. Males are 4 to 5 millimeters long, and females are 7 to 10 millimeters long. The female is venomous, and its bite can cause severe pain and, in some cases, sweating, palpitations and other symptoms.

The redback spider was first found in Takaishi, Osaka Prefecture, in September 1995. The poisonous arachnid is believed to have attached itself to containers shipped from overseas. According to the Osaka prefectural government and others, there have been more than 100 reports of injury in the prefecture so far.

The spider was initially found mainly in western Japan, according to NIES, but it was discovered in Tokyo in 2014, Hokkaido in 2015 and Nagano in 2019. It has now been confirmed in 45 of 47 prefectures, with only Aomori and Akita prefectures having no reported sightings. No deaths have been confirmed in Japan, according to experts.

The spiders were able to expand their range in part because they are small and often hide in roadside ditches and at the bottom of planters. They are believed to have made their way across Japan by attaching themselves to building materials and other objects.

“It is highly likely that the species has already invaded all prefectures, including ones that have no reports of its discovery,” said Koichi Goka, head of the Ecological Risk Assessment and Control Section at NIES. “Japan is probably a pleasant place for the spiders to live because there are few competing species or natural enemies.”

“Now that they have spread so far, it will be difficult to eradicate them,” said Takahide Kamura, a professor emeritus at Otemon Gakuin University and an expert of arachnid taxonomy. “Keeping in mind that they can be found anywhere, one should take thorough basic measures, such as killing them with insecticide whenever one finds them.”