Oak leaves turning brown? Could be the two-horned gall wasp...
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Steven Swain, Horticulture Advisor, UCCE Marin
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Check along the lateral veins to see if you can find galls forming. Photo: Bob Mauceli
The galls are the work of the two-horned gall wasp, Dryocosmus dubiosus, a tiny honey-brown wasp the size of a gnat. These tiny wasps are not dangerous to humans in any way, and they do not have horns; they’re named for the shape of the leaf galls they cause to form on the undersides of oak leaves. Female gall wasps use their stinger to deposit an egg in the leaf veins of a coast live oak leaf. The egg releases hormones into the leaf, causing it to form a small tumor (gall) around the egg. A tiny wasp larvae then hatches out of the egg, and eats the gall from the inside. When the gall is almost completely eaten, it turns brown and falls off the tree into the leaf-litter below. The tiny wasp pupates inside the gall, and then chews a hole in the gall casing, emerging as a member of the next generation of tiny gall wasps.
Coast live oak leaves with brown, scorched margins due to feeding on the leaf underside by two-horned oak gall wasp. Photo: Jack Kelly ClarkWhen the galls of the two-horned gall wasp form on a leaf vein, it cuts off the flow of nutrients to the outer portion of the leaf. The leaf edges then turn brown after a week or two, giving the leaf a mottled appearance. If there are a lot of galls on a single leaf, it may turn entirely brown. Fortunately, this isn’t as bad for the oak as it may appear. Even if most of the leaves on an oak tree are affected, the wasps don’t damage the leaves until the fall. By then, the oak has often recovered its investment in the leaves in terms of the amount of sugar it gets back from them. Thus, while the tree may not look as good as we’d like, the damage isn’t as bad as it may seem.
Galls, and necrosis where galls dropped from the leaf, caused by two-horned oak gall wasp infesting coast live oak. Photo: Jack Kelly ClarkWe can all hope that’s the case, as treatment of gall wasp infestations is not well-researched. No effective control methods are currently known, or likely required. We expect afflicted oaks will do much better in 2024, even if we do nothing.