Horned and gouty oak galls on bare oak twigs

Why this time of year

Once most oak leaves drop, the woody twig galls that were hidden all summer stand out against the sky on pin, black, red, and scarlet oaks.

How to notice it

1. Pick an oak that has dropped its leaves and scan the outer twigs against the sky for round, woody lumps up to about golf-ball size. Some oaks, especially young pin oaks, hold dry brown leaves well into winter, so skip those.
2. Look closely at one lump with binoculars or at a low branch: small horns sticking out all around the surface point to horned oak gall; a lump with no horns may be gouty oak gall or a horned gall too young to have grown its horns.
3. Look for tiny holes at the horn tips, where adult wasps came out; those galls are old ones the wasps have left.
4. Leave the galls on the tree and count how many you find on one branch, then compare with a neighboring oak.

Sources

• umass.eduUMass Extension (primary, state extension). Horned oak gall wasp (Callirhytis quercuscornigera) hosts are black oak, pin oak, and scrub (bear) oak; its golf-ball sized woody stem galls are 'easily noticed after leaf drop'; horns 'may be visible approximately 22-24 months after the initiation of the gall'; 'In the springtime, mature adult wasps emerge from the tip of each horn'; the species 'can alternate between agamic (all female) and sexual (male and female) generations', with a leaf-galling stage of blister-like galls in leaf veins; adult females emerge 'in May and June from their fully developed twig galls'; heavy populations disfigure the tree and cause crown dieback.
• umass.eduUMass Extension (primary). Gouty oak gall wasp (Callirhytis quercuspunctata) hosts are black, pin, red, and scarlet oak; 'The galls themselves are solid and woody'; 'Galls of either species can be up to 2 inches in diameter' and may form large masses on branch sections; 'These galls can disfigure the tree or cause crown dieback'; 'Historically, stem galls on oaks have been a serious problem in Plymouth County, MA and Long Island, NY' (Massachusetts occurrence of oak stem galls in general); its life cycle 'may not be fully understood'.
• entomology.mgcafe.uky.eduUniversity of Kentucky Entomology (primary, extension fact sheet). Horned oak galls are 'golf ball-size woody growths on oak twigs'; dried galls are 'especially noticeable after autumn leaf fall' and 'remain aesthetically disfiguring for years after the wasps have emerged and abandoned them'; 'Lightly infested pin oaks can live for many years without apparent harm', but outbreaks cause 'extensive branch dieback and even tree death'; the twig-gall generation takes 'about 33 months from egg hatch to adult emergence' (Kentucky study); gouty oak gall is a different wasp that 'lacks the horns'.
• extension.psu.eduPenn State Extension (primary). Horned oak gall has 'small horns that protrude from around the circumference of the gall' and 'One adult gall wasp emerges from each of these horns'; horned oak gall wasp 'attacks the twigs of pin, scrub, black, blackjack, and water oaks'; gouty oak gall 'develops in the twigs of pin, scarlet, red, and black oaks'; 'Both of these woody twigs galls on oak look similar'; 'Two or more years are required for gall wasps that develop in woody twig galls to reach maturity'.
• yardandgarden.extension.iastate.eduIowa State Extension (primary). Horned oak galls on pin oak branches are caused by a tiny wasp (Callirhytis cornigera) with a two-part life cycle: spring females lay eggs on developing leaves forming small vein galls, adults from those lay eggs on twigs, and the twig galls take almost three years to develop, with horn-like projections tied to adult emergence (paraphrase; page limited verbatim quoting).
• ipm.missouri.eduUniversity of Missouri IPM (primary extension, refute pass): 'Quercus (pin, red, and white oaks)' 'retain many of their dried leaves until late winter'; marcescence 'is often a juvenile trait'. Basis for skipping leaf-holding oaks.

Checked against these sources on October 23, 2026.

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