Beetle Boom at Shinjuku Gyoen: Exciting for Visitors but Troubling for Trees

Shinjuku Gyoen, located between Tokyo's Shinjuku and Shibuya wards, has recently become a hotspot for rhinoceros beetle sightings. Since July, social media posts have increasingly described these beetles as "swarming" and "impressive."

The park is a popular urban oasis, drawing many families eager to create summer vacation memories. However, the presence of these beetles suggests a more concerning issue with the city's trees.

Connection to Tree Mortality

Rhinoceros beetle sightings are closely linked to tree mortality, a growing concern in Tokyo's parks. This trend is thought to be related to an increase in oak die-back.

When trees are attacked, they produce sap that attracts rhinoceros beetles. A representative from the Ministry of the Environment's Shinjuku Gyoen Management Office noted, "Various threats could be responsible, including the oak ambrosia beetle."

The oak ambrosia beetle, a weevil species about 5 millimeters long, specifically targets oak and sawtooth oak trees. It creates horizontal holes about 2 millimeters in diameter to lay its eggs. During this process, the weevil introduces fungi that cause the oak trees to wilt and ultimately die.

These dead trees pose significant risks, including branch breakage and tree falls, and often require removal to prevent infections from spreading. In Shinjuku Gyoen, horizontal holes caused by the weevil have been observed at the base of the trees.

In Tokyo's metropolitan parks, oak die-back was first confirmed in approximately 380 cases in 2019. By 2023, this number had surged to around 29,400 cases. Shinjuku Gyoen has also been affected, with about 10 trees dying annually since the phenomenon was first identified there in 2019.

Timely Interventions

To address this issue, Shinjuku Gyoen is taking proactive measures, such as injecting chemicals into affected trees. However, if the condition of trees attracting rhinoceros beetles worsens, they may eventually need to be removed.

The representative from the Shinjuku Gyoen Management Office explained, "As trees weaken, they become more susceptible to infestations. To maintain a healthy environment, it's sometimes necessary to promptly remove these weakened trees. Our goal is to protect historic trees through timely interventions."

Additionally, the collection of plants and animals is prohibited at Shinjuku Gyoen. Visitors are encouraged to observe the natural environment but should exercise caution due to the presence of hornets, which are also attracted to tree sap.

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(Read the article in Japanese.)

Author:  Shota Ohaka, The Sankei Shimbun