Hokkaido officials mark a dead Japanese oak during a field survey in Hakodate, October.
Oak wilt, a disease caused by a fungus spread by insects, is moving northward and rapidly expanding across Hokkaido. Experts say global warming has allowed the carrier species to survive the winter.
In addition to reducing timber resources, the outbreak raises concerns about its impact on wildlife. With on-site eradication struggling to keep pace with the disease's spread, efforts are now underway to harvest and make use of affected trees before they succumb to oak wilt.
In mid-October, as the autumn foliage season began, Hokkaido government officials conducted a field survey in the mountains near Hakodate. Drone footage from above revealed patches of forest stripped of greenery amid the dense woodland. "Leaves are falling unusually early for this time of year. These trees might be dead already," an official speculated.
Telltale Signs
Carrying bear-repellent spray on their belts, the officials climbed steep, pathless slopes for about 15 minutes to reach the affected trees. Amid the lush forest, they came upon a solitary Japanese oak that had lost all its leaves.
At its base was frass, a mix of wood shavings and insect droppings — a telltale sign of oak wilt. Tiny, one-millimeter holes marked where insects had bored into the trunk. "We need to deal with this tree before summer, when the insects disperse," the official said.
The presence of the oak ambrosia beetle, carriers of the oak wilt pathogen, was first confirmed in southern Hokkaido in 2020. Takuya Kobayashi, chief researcher at the Hokkaido branch of the Forestry and Forest Products Research Institute, explained, "Thanks to global warming, this species may now survive the winter and mature into adults."
Challenges in Containment
A 2025 survey found 1,637 affected trees across two cities and nine towns in southern Hokkaido. By comparison, fiscal 2023 recorded just 15 trees in two towns, and last year there were 213 trees in three towns.
"It's difficult to contain. The more it spreads, the more budget and manpower are required, and eradication efforts simply can't keep up," Kobayashi admitted.
The work in unmaintained forests, coupled with the risk of encountering brown bears, further adds to the challenge.
Native oak species in Hokkaido account for about 7% of the total forest biomass. Among them, the Japanese oak is widely used in furniture-making around Asahikawa, but discolored, affected trees cannot be utilized.
Kobayashi also noted, "Many wild animals, including brown bears and insects, rely on the leaves and acorns of oak species for food, so there are potential impacts on both industry and ecosystems."
Cautious Optimism
In Hokkaido's Mori Town, oak wilt damage was confirmed for the first time in 2025. In July, it signed an agreement with a company to utilize the timber before the outbreak spreads. They have begun producing prototype barrels using Japanese oak from town-owned forests.
A town official expressed cautious optimism: "With the construction of whiskey distilleries advancing across Hokkaido, there is demand for barrels. We hope this initiative will lead to a viable business."
But they also conveyed a sense of urgency: "Oak wilt spreads rapidly, so we cannot afford to be complacent. We want to make use of the timber before its value is lost."
Related:
- The One Million Roaches Powering Japan's Pest Control Giant
- Beetle Boom at Shinjuku Gyoen: Exciting for Visitors but Troubling for Trees
- Hokkaido's Bears: Harmonious Coexistence Wanted
(Read the article in Japanese.)
Author: The Sankei Shimbun
