Greater protection for sweet chestnut trees in England Approval given for the release of a biological control agent to reduce the spread of Oriental Chestnut Gall Wasp. From: Credit: Dr Chris Malumphy, Fera Science Ltd. Scientists are stepping up the fight against an invasive tree pest of sweet chestnut trees - Oriental Chestnut Gall Wasp - the UK’s Chief Plant Health Officer announced today. Approval has been given for the release of a parasitoid wasp called Torymus sinensis, a natural biological control agent, to help reduce the spread of Oriental Chestnut Gall Wasp in England to protect the health of sweet chestnut trees.