|
Getting your Trinity Audio player ready...
|
| Welcome to our tree column, “Ask your local arborist.” Each month, arborists from the Davey Tree Expert Company in Menlo Park discuss a tree-related topic from general care and planting tips to how trees can impact your property. If you have a tree-related question or an idea for a topic to discuss, email lifestyle@embarcaderopublishing.com or post in the comments section at the end of this column. |
If you’ve noticed unusual bumps that look like tiny “horns” or brown spots on the leaves of your oak tree this winter, you’re not imagining it. Many residents in the region are seeing a higher population of tiny gall wasps, most notably the two-horned gall wasp (Dryocosmus dubiosus), affecting their oak trees.
These insects are tiny, unassuming wasps that lay eggs in oak catkins (the tree’s flowers) and along the veins of leaves. When a female wasp deposits an egg on a leaf vein, the tree reacts by forming a gall: a small, plant-made structure that looks like a bump or blob on the leaf. The characteristic “two horns” give this species its common name.
What you’re seeing
The life cycle of the gall wasp includes multiple generations each year. One generation develops in late winter to early spring on catkins, while the next forms on the leaf veins. As larvae feed inside these galls, the tree’s tissue grows around them, creating the strange growths you’re seeing.
While these galls can be plentiful, sometimes dropping like tiny pellets onto patios or driveways, the good news is that galls themselves rarely threaten the overall health of a mature oak tree. They usually affect only parts of a leaf, while the rest of the leaves on the same twig remain healthy.
A natural part of oak ecology
Oak trees are famous for hosting a wide variety of gall-forming insects. In California alone, hundreds of gall wasp species produce diverse gall shapes and sizes on oaks and other native species. Many of these interactions occur naturally and are part of a larger ecological web that includes predators and parasites that help keep gall wasp populations in check.
Oak galls are a type of abnormal plant growth produced by small gall wasps, and while they can be unsightly, they usually do not cause significant long-term damage. Some leaves with galls may fall prematurely, but overall tree decline from this alone is uncommon.
When to think about tree health
Even though the two-horned gall wasp itself isn’t typically a serious health threat, large infestations can be a sign a tree is stressed by other factors (drought, competing pests, drainage issues, etc.).
For homeowners who are curious or uneasy about the bumps on oak leaves, here’s what to consider:
- Recognize typical symptoms: “Horned” galls on leaf undersides and catkins, mild leaf discoloration, and occasional leaf drop are all common.
- Don’t panic: Most gall wasp infestations are managed by natural controls, such as fungi, parasites and other insects that limit wasp numbers.
- Consider overall tree care: Healthy, well-watered and structurally sound oaks are better able to withstand pest pressures in general. Consulting a professional for seasonal check ups helps ensure your oak’s vitality.
What to watch for this spring
- Tiny galls on new leaves or catkins
- Areas of mild leaf browning or spotting
- Increased leaf drop compared with previous years
None of these alone indicates a dying tree, but they are good reminders to keep an eye on overall tree health.
George Reno is a local arborist at The Davey Tree Expert Company in Menlo Park, which serves communities on the Peninsula, including Menlo Park, Atherton, Palo Alto and Redwood City. Reno can be reached at 866-923-5658.
