Twohorned oak gall wasp—Dryocosmus dubiosus
Twohorned oak gall wasp (Cynipidae) is a tiny wasp that forms a brown, oblong or round, seedlike gall on evergreen oak leaves. High populations of the gall wasp cause extensive leaf scorching, and some infested leaves drop prematurely.
Each cynipid wasp larva during the second generation feeds inside a brown, oblong swelling that is 1/10 inch long. Each gall has two protuberances, hence the name "twohorned." Galls occur on the underside of coast live oak and interior live oak leaves, usually on a leaf vein.
The first-generation wasp larvae develop during late winter or early spring from eggs laid inside oak catkins. Affected flower parts turn into small reddish or brown galls, that when cut reveal pale larvae feeding inside.
Second-generation larvae form galls after hatching from eggs laid in the veins of leaves. The wasps overwinter as pupae in galls, which often drop from the leaf.
Gall wasp abundance varies naturally from year to year, although these population fluctuations may not be obvious because live oaks can retain galled and scorched foliage for several years.
Damage from the leaf-galling generation of twohorned oak gall wasps is often confused with damage from fungi that cause oak twig blight and certain beetles e.g., oak twig girdlers. However, these other pests cause the entire leaf to die, and they kill many adjacent leaves in a group.
Twohorned oak gall wasps kill only portions of each leaf; leaf margins beyond each gall often discolor and die. Each leaf with galls remains partly green, and discolored leaves will be on the same twig as unaffected leaves.
Gall wasps are naturally killed by a complex of fungi, parasites, predators, and competing insects (primarily moth larvae and other wasps) that live within galls. Although aesthetically undesirable, these oak gall wasps do not seriously threaten tree health. No control is known or recommended.