Recognizing and Treating Oak Gall Wasps

Right now is the time of year when all the leaves have dropped and you step out into your front yard; look at your oak tree (in Cody it's Burr Oak) and think to yourself, something doesn't quite look right. Or in my case I drive around Cody and constantly take photos of trees. These weird growths on this tree in Cody that I took photos of are Oak gall wasps. Oak gall wasps are tiny insects that can cause noticeable changes on oak trees. I have had several calls and inquiries lately about what to do and how to treat galls. While they rarely threaten the tree’s health, their presence can be alarming to homeowners.

What Are Oak Gall Wasps?

Oak gall wasps are small insects that lay their eggs on oak trees. When the eggs hatch, the larvae release chemicals that cause the tree to form growths called galls. These galls look like round or irregular bumps on leaves, twigs, or branches. Each gall provides food and shelter for the developing larvae.

How to Recognize Oak Gall Wasps and Their Galls

Identifying oak gall wasps starts with spotting the galls on your oak tree. Here are some key signs:

• Appearance of galls: Galls can be round, spiky, or irregular. They often look like small balls or lumps attached to leaves, twigs, or branches.

• Location on the tree: Galls commonly appear on oak leaves, buds, or twigs. Some species target specific parts of the tree.

• Timing: Galls usually form in spring or early summer when wasps lay eggs.

• Tree health: Despite the galls, the tree often looks healthy. Galls rarely cause serious damage.

  
  

If you inspect the galls closely, you might see tiny holes where adult wasps have emerged. These holes are a sign that the wasps have completed their life cycle.

Why Do Oak Gall Wasps Matter?

Most oak gall wasps do not harm the tree significantly. The galls are more of a cosmetic issue than a health threat. However, heavy infestations can cause:

• Branch weakening: Large numbers of galls on twigs may weaken them.

• Reduced growth: Severe galling can slow new growth.

• Aesthetic concerns: Galls can make trees look unhealthy or damaged.

  
  

How to Treat Oak Gall Wasps

For homeowners dealing with oak gall wasps, it's important to understand that there really isn't a simple pesticide that reliably kills the wasp inside its gall because the larvae are protected inside the plant tissue once the gall forms. Treatment focuses on managing the galls rather than eliminating the wasps completely. Here are practical steps:

1. Prune Affected Branches

Removing twigs or branches with heavy gall infestations can reduce wasp populations. Prune in late winter or early spring before new growth starts. Dispose of pruned material away from the tree to prevent wasps from emerging nearby.

2. Horticultural Oils (Dormant Oil Sprays)

Horticultural oils sprayed in late winter to early spring can smother overwintering eggs or larvae in buds and bark crevices before the growing season begins. Apply before buds break-typically late winter or early spring.

3. Neem Oil Spray

Neem oil is a plant-derived product; spray neem in early spring around the time adult wasps are emerging. It can prevent egg-laying and reduce adult activity.

4. Encourage Natural Predators

Birds, parasitic wasps, and other insects prey on gall wasps. Creating a garden environment that supports these predators helps keep wasp numbers in check naturally. Plant flowers that attract beneficial insects. Hang bird feeders-birds help eat adult wasps and larvae in galls.

When to Seek Professional Help

If gall infestations are severe and affecting tree health, consulting an arborist or tree care professional is wise. They can assess the situation and recommend treatments tailored to your tree and local conditions.

Professionals may suggest targeted pruning, soil treatments, or other methods to support tree recovery. They also help identify if other pests or diseases are involved.

Preventing Oak Gall Wasps in Your Garden

While you cannot completely prevent oak gall wasps, these tips reduce their impact:

• Inspect oak trees regularly for early signs of galls.

• Remove and destroy galls before adult wasps emerge.

• Keep trees healthy with proper watering and fertilizing.

  
  

By staying vigilant, you can enjoy your oak trees without worrying about gall wasps.