Horned Oak Gall Wasp

The horned oak gall is a golf ball-sized growth caused by a small wasp insect (Callirhytis cornigera) which can cause extreme disfiguring and dieback of certain oak species. They are often found on young twigs and have protruding horns through which young females emerge to continue the cycle. Another gall maker is the gouty oak gall (C. quercuspunctata) which looks similar but lacks the horns.

The horned oak gall wasp occurs from southern Canada, through Pennsylvania, south to Georgia, and throughout the Midwest. It attacks the twigs of pin, scrub, black, blackjack, and water oaks. Infestations on young trees can be the most devastating, but older trees will be dangerously affected as well. Galls prevent nutrients and water from moving into new growth, resulting in severe dieback.

  • Common Symptoms
    • Twig galls take up to three years to fully develop mature females that emerge in the spring during bud break. Females lay eggs along undersides of new leaf margins where they form an inconspicuous swelling. Young wasps exit these leaf galls in late May through early June in Kentucky, and mid-July in Illinois and Missouri, where they then insert eggs into young oak twigs. The twig gall then increases in size for about 24 months and develops “horns” when young wasps emerge in the spring. Several stages of generations are always present on susceptible oak trees.

    Treatments
    • Trunk injection with TREE-äge® (Emamectin benzoate) or TREE-äge® G4 will provide both preventative and curative control depending on the time of year treatment is applied. We recommend treatment in September of this year, or early spring next year. Treatment in September will reduce early pest damage next spring. TREE-äge will provide control for a minimum of a year, and potentially up to two years. Treatment with TREE-äge or TREE-äge G4 is effective in managing stem and twig insects when applied as a dilute injection prior to spring insect emergence. The most effective treatments are applied to trees at risk of infestation. Treatment will prevent new galls but will not eliminate old ones. TREE-äge and TREE-äge G4 will additionally protect your trees from up to 51 other pests.

    References And Photo Credits
    • Main Photo taken by Lorraine Graney, Bartlett Tree Experts, Bugwood.org
      Larva taken by Jim Baker, North Carolina State University
      Horn exposed taken by Lorraine Graney, Bartlett Tree Experts, Bugwood.org
      Eggs and Eggs in Oak taken by Jason Sharman, Vitalitree, Bugwood.org

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