Asian Chestnut Gall Wasp
Dryocosmus kuriphilus
A tiny wasp that is considered the most serious insect pest of chestnut worldwide. Its larvae force the tree to grow green or rose-colored galls on buds, shoots, and leaves in spring, which stunt growth and sharply cut nut production.
🔎 How to spot it
The adult is a small shiny black wasp only about two and a half to three millimeters long, with translucent wings and orange to brown legs; there are no males, as the species reproduces with all-female parthenogenesis. The damage is far easier to see than the wasp: green to rose-colored galls, from a few millimeters up to about four centimeters, swell on new buds, shoots, leaf midribs, and petioles in spring. After the adults leave, the galls dry to brown, woody knots that persist on the tree.
🥀 Damage it causes
Larvae feeding inside the buds make the tree form galls instead of normal shoots, leaves, and flowers. This stops shoot elongation, aborts flowers, and can cut wood and nut yield by half or more, with severe, repeated infestation causing twig dieback and sometimes tree death. The dried old galls also remain on the tree and clutter the canopy.
🛡️ Prevent it
Inspect new spring growth in May and June for the fresh green or pink galls, and on small or young trees prune out and destroy the galls before the adults emerge in early summer. Plant resistant cultivars where available (the hybrid Bouche de Betizac is noted as fully resistant), and avoid moving infested nursery stock, which is how the wasp reaches new areas.
🧯 If it is already here
Insecticides are generally ineffective, because the larvae are protected deep inside the plant tissue of the gall. The most effective control is biological: the introduced parasitoid wasp Torymus sinensis attacks the gall wasp and has brought it under control in many regions. For a backyard tree, pruning out galls before emergence and choosing resistant varieties are the practical tools.
💡 Good to know
Native to China, the wasp was found in the United States in Georgia in 1974 and has spread across much of the chestnut-growing East. It has one generation a year: females lay more than one hundred eggs in the buds in summer, the tiny larvae overwinter inside the dormant buds, galls form as the buds break in spring, and new adults emerge to start the cycle again.
For educational and informational purposes only. Pest control advice is general guidance drawn from university cooperative extension sources; always identify a pest positively and read and follow the label on any product before use, especially around food crops, children, and pets.