What Causes Oak Galls?
Gall-making insects and mites produce chemicals that interact with plant hormones, causing the tree’s tissue to deform and swell. There are more than 1,500 species of these gall producers, most of which are insects and mites.
Galls may also result from fungal, bacterial, or viral infections. The presence of these growths can be alarming for homeowners, especially when they appear suddenly or in large numbers. Early detection and proper oak gall treatment can significantly reduce long-term damage..
Most Destructive Oak Galls in Missouri
Two types of non-stinging wasps cause the most damaging galls to oaks in Missouri: the gouty oak gall wasp and the horned oak gall wasp.
These wasps lay their eggs inside tree tissue, which reacts to the wasps’ chemicals, causing it to grow abnormally and enclose the developing wasp larvae. Gouty and horned galls form on leaves, twigs, and branches. Over time, these swellings can compromise branch structure, restrict nutrient flow, and leave your oak vulnerable to pests, tree diseases, and internal decay.
The gouty oak gall wasp attacks the following oak types:
The horned oak gall wasp targets:
- Pin
- Scrub
- Black
- Blackjack
- Water oaks
A few small galls on a single branch one year can spread to dozens across the canopy the next. If you suspect your tree is infected, contact an ISA-certified arborist in St. Louis County for a professional diagnosis and treatment plan. Omni Tree Service inspects and treats oak gall infestations throughout Wildwood, Chesterfield, and Ballwin.
What Do Gouty and Horned Oak Galls Look Like?
Oak galls are not aesthetically pleasing, appearing as unsightly brown balls that grow from a branch or look like hanging fruit or nodules. Here’s how to tell the two most common types apart:
- Gouty oak galls are smooth and round.
- Horned oak galls feature small spikes or “horns.”
- Young galls will look like tumors on twigs, with a greenish-brown bark color.
- Mature galls may reach up to 2 inches in diameter and become hard, woody, and dark brown.
Over time, a single tree can accumulate hundreds of galls, especially if the infestation goes untreated. This is why it’s so important to have your tree inspected and treated by a professional as soon as you notice any signs of galls. Early intervention can limit structural damage and improve long-term outcomes.
Are Oak Galls Dangerous to Humans or Pets?
Oak galls can look strange and concerning, but they are not harmful to people or animals. However, falling galls or weakened branches can pose safety risks in yards, especially during storms or windy days. If your oak tree is heavily infested, it’s a good idea to have a professional arborist evaluate the risk and provide guidance on next steps.
If you’re unsure about the condition of your tree, reach out to Omni Tree Service for a detailed inspection and professional oak gall treatment in St. Louis County.
Oak Gall Treatment and Management
Wasps responsible for oak galls can overwinter in fallen leaves and twigs, reinfesting your trees the following season. That’s why the most effective oak gall treatment involves both prevention and removal.
Working with a qualified tree service in St. Louis County ensures you’re not just removing visible damage, but also protecting surrounding trees and future growth.
What to Do if Your Oak Tree is Infected
Removing and destroying the small galls when they are developing on twigs and branches before the wasps emerge may help to reduce the infestation. To do this, you should:
- Prune and destroy gall-infested twigs and branches.
- Burn or step on the galls to kill the developing larvae.
- Place gall remains in a tightly sealed baggie or trash bag and discard immediately.
- Rake and destroy gall-infested fallen leaves.
Keep in mind that while DIY gall removal may be helpful in minor cases, larger infestations often require expert care to prevent recurring problems.
How to Prevent Oak Galls
Unhealthy or stressed oak trees are more likely to suffer from gall damage than healthy trees. Being proactive is the best way to keep your oak trees healthy.
To keep your oak tree strong:
- Water deeply during St. Louis’s hot summer stretches, when weeks without rain can put serious drought stress on mature oaks.
- Mulch and fertilize properly to support root health.
- Schedule regular inspections with a tree service in St. Louis County to catch gall activity early.
- Avoid unnecessary pruning during active wasp seasons, which may encourage gall formation.
Preventive maintenance is often more cost-effective than major treatment or removal down the line.
Professional Oak Gall Treatment in St. Louis County
When you notice galls on your oak trees, don’t wait for the problem to spread. Omni Tree Service provides professional oak gall treatment and year-round tree care to keep your oaks healthy season after season. Our certified arborists in St. Louis County are trained to identify, manage, and prevent gall infestations using safe, effective methods.
Don’t let oak galls take down a tree you’ve had for decades. Call today for a free estimate or request a visit online.
