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The city of Matsumoto in Nagano Prefecture has been struggling to cope with serious damage to pine trees in its mountain forests caused by pine wilt, the disease seen with infestation by the pinewood nematode parasite that can kill affected trees within a few weeks.

Some local residents have called on the municipal government to continue a program of aerial chemical spraying using unmanned helicopters, but others have questioned its effectiveness and safety.

Yoshinao Gaun, who became the city’s mayor in March, said Tuesday that the municipal government has decided not to conduct the aerial spraying that had been scheduled for fiscal 2020.

The city had allocated ¥6.25 million in its initial budget for the fiscal year to spray chemicals over 29.2 hectares of land, at the request of five residents associations. But one citizens group has been asking the municipal government to abandon the project altogether.

The spraying of chemicals “has a limited effect, only delaying by a year or two the progression from ‘slight damage,’ which can be subject to preventive measures, to ‘expanded damage’ which requires felling,” Gaun said.

“Although the health hazards (of chemical spraying) are not yet scientifically proven, it is necessary to take precautions,” he added.

Gaun said the municipal government would strengthen other measures to protect pine trees, including inserting pesticide into their trunks, removing damaged trees and replacing pines with other varieties.

“Japanese red pine forests are the symbol of our hometown. We will allocate funds to preserve and restore them,” he said.

In mid-May, damage was seen at a pine forest on city-owned land in the Sorimachi area within Matsumoto’s Shiga district. Many trees were left withered and collapsed, covered with foliage from broad-leaved trees, such as acorn trees, near their roots.

The damage was caused by the pinewood nematode, a worm-like creature about 1 millimeter long that invades the stems and branches of pines and is carried from tree to tree inside the bodies of pine sawyer beetles.

The nematode was brought to Japan in the mid-Meiji Era (1868-1912) along with lumber imported from the United States.

The first case of pine wilt disease in the nation was confirmed in Nagasaki Prefecture. Damage due to the disease peaked in fiscal 1979, by log volume affecting 2.43 million cubic meters nationwide. Since then it has been on a declining trend, down to 350,000 cubic meters in fiscal 2018.

In Nagano Prefecture, the disease was first found in 1981. The first case in Matsumoto was confirmed in the Shimauchi district around 2004, before it started to spread rapidly into the Shiga district around 2011. In fiscal 2019, the damage reached 16,763 cubic meters by log volume within the city — a more than tenfold increase over the past decade.

In fiscal 2013, the city began aerial spraying of chemicals in response to requests from a residents council in the Shiga district that was tackling the pine wilt.

According to a survey by city authorities on the mortality rate of pine trees, the rate seen in the Fujiike area of Shiga district in the fall of 2014 was 4.76 percent in sprayed zones and 6.86 percent in unsprayed zones.

But in the fall of 2018, the rate rose to 59.05 percent in sprayed locations and 91.18 percent in places that hadn’t been sprayed. By 2018, the rate in the Sorimachi area, which had been 4.17 percent in sprayed zones and 12.62 percent in unsprayed zones in 2014, jumped to 61.67 percent and 83.50 percent, respectively.

While the pine mortality rate was comparatively low in places where chemicals were sprayed, the damage had continued to expand. Aerial spraying was suspended in the areas of Fujiike and Sorimachi last fiscal year after it was determined that the measure would not bring about further benefits.

Gaun also admitted that the spraying “has not been effective at preventing damage.”

Protecting pine trees

According to the Forestry Agency’s Forest Protection Promotion Office, the aerial spraying of chemicals started in 1973, mainly in western Japan, and “showed a remarkable effect.”

While noting that chemicals are not necessary if all damaged trees are felled and removed, an agency official acknowledged that “if the withered trees are not felled in a timely manner, the damage will spread quickly. The slower the measures, the more the damage will expand.”

The city of Shiojiri began to see damage in 2015 in an area bordering Matsumoto. It has been felling pine trees, including those that have not yet withered, and replacing them with broad-leaved trees instead of spraying chemicals.

However, in the Shiga district, where matsutake mushrooms have been produced as a local specialty, some residents are opposed to replacing pines with other kinds of trees.

“We want to protect our pine trees and bring back production of matsutake mushrooms,” said Fumihiko Kaneko, 68, head of the residents council for Shiga that is tackling the pine wilt. “If spraying (pesticide) can reduce damage, we should continue doing it.”

On the other hand, Shiga farmer Minekazu Aoki, 70, has called for the chemical spraying to be suspended. “Trees in mountain forests change along with changes in the environment, including pollution,” he said. “Even if we could protect pine trees and harvest matsutake mushrooms, we might not be able to sell them because of harmful rumors connected to chemical spraying.”

Concerns over chemical spraying

Some residents have also urged the city to halt the spraying of chemicals, with a citizen group voicing concerns that it could pose a health hazard.

The city has been measuring atmospheric concentration levels at seven locations in residential areas some distance from the pine forests, both before and after spraying. In most places the level was negligible.

The chemical used is a neonicotinoid pesticide that includes acetamiprid, commonly used in farming. “It is included in the Agricultural Chemicals Regulation Act and it is safe if used properly,” a Forestry Agency official said.

But regulations on the use of acetamiprid differ according to country, with France banning the agricultural use of pesticides that contain it. Some researchers in Japan have warned that there are risks involved with using the chemical.

A research paper released last year by a team headed by Dr. Go Ichikawa, 40, of Dokkyo Medical University in Tochigi Prefecture, stated that acetamiprid metabolite, believed to have been absorbed by a pregnant woman from agricultural produce, was detected in the urine of her low-birth-weight newborn baby. It was not clear whether the substance had any effect on the growth of the baby.

Other past research showed that a mouse exposed to acetamiprid during its development was found to have a behavioral disorder once it reached maturity.

City residents are divided over the measure. According to a questionnaire about aerial spraying of chemicals presented to residents by the city, 49.3 percent supported spraying and 45.3 percent were opposed. Among those who responded with neither view, one commented that spraying could be conducted if proper risk management measures were taken to protect the ecosystems and people’s health. But another respondent expressed caution that uncertainty remained over the safety of the chemicals involved.

This section features topics and issues from the Chubu region covered by the Chunichi Shimbun. The original articles were published May 17 and 20.

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