Parasitic wasp release planned to rescue sweet chestnut trees

The government has approved wasps as a "natural pest control" measure in an effort to save sweet chestnut trees from an invasive species.

Around 150 Torymus sinensis wasps will be released in 10 unknown locations in the southeast of England by Fera Science, a research organisation backed by the government, in order to tackle the Oriental chestnut gall wasp. 

The gall wasps have been attacking the UK’s sweet chestnut trees. First discovered in a Kent wood in 2015, they are named after the galls their larvae create on the buds and leaves of sweet chestnuts. Large numbers of gall wasps can weaken the trees and make them more vulnerable to other pests and diseases, including Sweet Chestnut Blight.

The threat they pose has increased in recent years. James Heal wrote on Twitter: “In 2017, I found the first record of Oriental Chestnut Gall Wasp (Dryocsumus kuriphilus) in Epping Forest - Bush Wood. In 2018, I found several galls. In 2019, most Sweet Chestnuts were affected. In 2020, every Sweet Chestnut is infested. Extraordinary expansion!”

To tackle this, the government plans to release the parasitic wasps, which lay eggs in chestnut tree galls and this larvae feed on gall wasp larvae. The process is already present in England but in small numbers.

Chief plant health officer Nicola Spence said: “Threats to sweet chestnut trees have increased as a result of tree pests and diseases such as Oriental chestnut gall wasp and Sweet Chestnut Blight. The release of this biological control agent represents a huge step towards protecting the health of sweet chestnut trees and will further enhance the resilience of our treescape.”

A risk analysis conducted by the DEFRA found the release would be irreversible, with "no practical measures available for the eradication or containment" of the parasite. 

The same pest has been successfully released to control the same pest in France, Japan, the US and other countries.

Conservationists are concerned by the release of a nonnative species to target another, arguing that it could have long term consequences for wildlife including native wasps.

Matt Shardlow, head of the insect charity Buglife, told The Times that “unlike in previous releases of nonnative species to control a pest, the parasitic wasp could also harm native species, including at least eight native species of UK gall wasp that create oak apples and other types of gall that support insects and are food for birds.”

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