Callirhytis quercuscornigera

The horned oak gall is distinguished from other oak galls by its large size, woody texture, twig location, and the presence of multiple horn-like projections. Similar bullet galls (Disholcaspis spp.) lack horns and have smooth surfaces. Oak apple galls are larger, spongy, and leaf-based. Gouty oak galls are irregular woody swellings without organized horn structures. The horns are diagnostic—no other North American oak gall wasp produces this structure.

Associated with oak trees, particularly pin oak (Quercus palustris), but also other red oak group species. Found in deciduous forests, urban plantings, suburban landscapes, and riparian corridors where host oaks occur.

Eastern United States, with records from the Mid-Atlantic region (Maryland, Virginia), Midwest (Missouri), and extending through the oak-dominated forests of the eastern deciduous biome. Specific range boundaries not fully documented.

Asexual females emerge from twig galls in spring shortly after oak bud-break. Sexual generation develops in leaf galls during summer, with mating and egg-laying into twigs occurring late summer. Twig galls require approximately 30 months to mature before the next emergence.

Larvae feed exclusively on nutritive plant tissue induced within the gall—abnormally proliferating cells rich in proteins and carbohydrates that the wasp stimulates the oak to produce. Adults do not feed; they rely on larval reserves.

• Quercus palustris - primary hostPin oak; documented site of heavy infestations causing tree decline
• Quercus rubra group - hostRed oak section species

Complex heterogonic cycle: (1) Asexual females emerge from woody twig galls in spring, fly short distances, and oviposit into developing leaf buds, inducing small, inconspicuous leaf galls; (2) Both sexes emerge from leaf galls in summer, mate, and females oviposit into tender green twigs; (3) Eggs hatch and induce woody twig galls that enlarge over 2–3 growing seasons; (4) Fully developed asexual females emerge after approximately 30 months to complete the cycle.

Asexual females are poor fliers and move only short distances from emergence sites to oviposition locations on the same or nearby trees. The larva remains sedentary within the gall, feeding on induced nutritive tissue. Gall development is continuous while attached to the living twig.

Gall formation creates a specialized microhabitat that supports a community of inquilines, parasitoids, and hyperparasitoids. The gall's sweet secretions attract predatory wasps including yellow jackets and paper wasps, which may incidentally protect the gall from other herbivores. Heavy infestations can cause branch dieback and tree stress, potentially influencing oak population dynamics in localized areas.

Documented as a significant pest of ornamental pin oaks, with heavy infestations contributing to tree decline and mortality—notably observed at Dulles Airport. The 30-month gall development period makes management difficult. Galls are sometimes collected for educational purposes to demonstrate insect-plant interactions.

• Disholcaspis spp.Form smooth, hornless bullet galls on oak twigs; lack the diagnostic horn projections of C. quercuscornigera.
• Callirhytis quercuspunctataForms gouty oak galls—irregular, knobby woody swellings without organized horn structures; galls are more amorphous and often clustered.
• Andricus spp.Oak apple gall formers produce larger, spongy, often detachable galls on leaves or twigs with entirely different architecture.

• gall wasp
• cynipid
• oak pest
• heterogony
• parthenogenesis
• forest entomology
• urban pest

• BugGuide
• iNaturalist taxon
• NCBI Taxonomy
• What’s that on an oak leaf? Animal? Plant? Fungus? Nah, gall insect – Galls wasps, Cynipidae — Bug of the Week
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