Asian chestnut gall wasp (Dryocosmus kuriphilus)

Common Names: Oriental chestnut gall wasp, chestnut gall wasp

Description:

Asian chestnut gall wasp (Dryocosmus kuriphilus) is a tiny wasp from China that attacks chestnut trees [1]. It was accidentally brought to the United States in 1974 on imported chestnut plants and was first found in Georgia, and it has since spread through chestnut-growing areas [1]. MISIN tracks it because it is considered the most damaging chestnut pest in the world: its grub-filled galls deform new growth and can cut a tree's nut crop and wood production by half or more [1][3].

Identification:

The wasp itself is rarely seen ? the adult is a tiny black insect only about 1/8 inch (2.5?3 mm) long [1]. What you notice instead are the galls: rounded, swollen growths it causes on the new buds, leaves, and shoots of chestnut trees [1]. Fresh galls are green or rose-pink, roughly pea- to marble-sized (about 5?20 mm, sometimes up to 1? inches), and they later dry into hard, brown, woody knobs that can hang on the twigs for a long time [1]. It is easiest to spot the bright green-to-pink galls in late spring and early summer (roughly April to June) on the season's new growth [1]. Remember that this gall appears only on chestnut (Castanea) trees [1].

At-a-glance cues:

1. Rounded green-to-rose galls on new chestnut buds, leaves, and shoots in late spring and early summer [1]
2. Galls about pea- to marble-sized (5?20 mm, sometimes larger), later drying to hard brown woody knobs [1]
3. Found only on chestnut (Castanea) trees [1]
4. The adult is a tiny (about 1/8 inch) black wasp, rarely seen [1]

Hosts:

The Asian chestnut gall wasp lives only on chestnut trees (genus Castanea) and attacks all of them ? Chinese, Japanese, European, and the native American chestnut, as well as hybrid varieties [1]. It turns up wherever chestnuts grow: backyards, orchards, nurseries, and forests [1]. It is native to China and has spread around the world, reaching the United States in 1974 and later Europe and other chestnut-growing regions [1][2][3]. Because it depends entirely on chestnut, its range follows its host trees [1].

Life Cycle:

This wasp spreads in two main ways [1]. Most long-distance spread happens when people move infested chestnut plants ? nursery stock, cuttings, or scionwood ? that carry the wasp's tiny overwintering larvae hidden in the buds; the adults can also fly to nearby trees on their own [1]. So the simplest way to keep it off your property is to avoid bringing in chestnut planting material from infested areas.

The wasp has an unusual life cycle [1]. There are no males ? females reproduce on their own from unfertilized eggs, and each lays a few hundred eggs [1]. There is one generation a year: adults emerge in early summer and lay eggs inside chestnut buds; the tiny larvae stay hidden and dormant in the buds over winter; then as the buds open in spring, the larvae trigger the swollen galls, feed inside for about a month or two, and emerge as new adults in early summer [1]. For control, timing is everything ? galled shoots must be pruned out before the new adults emerge in early summer [1].

Impact and Damage:

The Asian chestnut gall wasp is harmless to people and pets ? it does not sting or bite, and it attacks only chestnut trees [1]. On those trees, though, it is severe. The galls form on the new growth and choke it off: they suppress shoot growth, reduce flowering, and can cut a tree's nut yield and wood production by 50% to 75%, while weakening the tree and killing twigs [1]. It is regarded as the most important pest of chestnut trees worldwide [1][3]. That matters for backyard and orchard chestnuts and for the long effort to restore the native American chestnut, which the wasp also attacks [1]. Where chestnuts are grown commercially, heavy infestations cause major economic losses [3].

Similar species:

The gall on chestnut is distinctive ? no other insect makes galls like this on chestnut trees in our region ? so misidentification is mostly with galls on other trees.

Oak gall wasps (various species in the family Cynipidae) make rounded galls that look broadly similar, but they form on oaks (Quercus), not chestnut. The Asian chestnut gall wasp's galls appear only on chestnut (Castanea), so the host tree is the simplest way to tell them apart [1].

Monitoring and rapid response:

Signs to watch for:

1. Rounded green-to-rose galls on the new buds, leaves, and shoots of chestnut trees in late spring and early summer [1]
2. Old, hard, brown, woody galls left on the twigs from previous years [1]
3. Stunted or deformed new growth and reduced nut set on chestnut [1]
4. Galls only on chestnut (Castanea) ? not on oaks or other trees [1]

What you can do:

1. Check backyard chestnut trees in spring (about April?June) for the fresh green galls [1]
2. Prune out and destroy galled shoots before the new adults emerge in early summer ? bag and trash them, or burn where it is allowed ? to reduce next year's wasps [1]
3. Do not move chestnut nursery stock, cuttings, or scionwood from infested areas; this is the main way the wasp travels [1]
4. Where available, plant resistant chestnut varieties ? some hybrids are highly resistant [1]
5. Orchard growers and natural area managers can prune across a planting, plant resistant cultivars, and support agency-led biological control using the parasitoid wasp Torymus sinensis, which has sharply reduced infestations in other regions [1][3]

When to call a professional:

1. Orchard-scale or forest infestations that are beyond hand-pruning [1][3]
2. Biological-control releases of Torymus sinensis, which are organized by researchers and agencies rather than individuals [3]
3. For chemical control options, contact a licensed applicator or your local cooperative extension office.

Report sightings to MISIN. Help track this species' spread by submitting observations through the MISIN web site or the MISIN mobile app (Android, iOS, Windows, and macOS). Photos at multiple angles paired with a precise GPS location are the most valuable data for confirming new occurrences and triggering rapid-response review.

Credits:

1. University of Florida IFAS, Featured Creatures (EENY-710). Asian Chestnut Gall Wasp, Dryocosmus kuriphilus Yasumatsu. ask.ifas.ufl.edu
2. Forest Research (UK). Oriental chestnut gall wasp (Dryocosmus kuriphilus). forestresearch.gov.uk
3. Avtzis DN, Melika G, Matosevic D, Coyle DR (2019). "The Asian chestnut gall wasp Dryocosmus kuriphilus: a global invader and a successful case of classical biological control." Journal of Pest Science 92:107?115. semanticscholar.org

---