
Tree holes, with their mystical allure, tempt our imaginations into believing in the possibility of hidden realms and extraordinary adventures. However, in reality, these spaces can also become a breeding ground for bothersome mosquitoes, leaving homeowners pondering: Should I fill tree holes on my property?
What Are Tree Holes?
Also known as tree cavities, tree holes can appear in any part of a tree, come in varying sizes and depths, and may indicate something is happening to the tree. Larger holes can indicate old age, damage, decay, or improper pruning (leading to tree wounds); smaller holes could be woodpeckers or wood-boring insects at work.
Do Mosquitoes Like to Live in Tree Holes?
All mosquitoes are attracted to standing water (or soil near water) to lay their eggs, no matter where that water is found. However, there is at least one species that is often found in tree holes. In California, the Western tree hole mosquito (Aedes sierrensis) is drawn to the standing water in tree holes of laurel, oak, eucalyptus, madrone, and some other local species.
Western tree hole mosquitoes arrive from March through August in residential yards in California. They love human and canine blood and are responsible for canine heartworm disease.
What the Experts Say About Tree Holes

Tree experts don’t recommend filling tree holes to stabilize a tree. Trees deal with the holes themselves in a process called compartmentalization. As long as they are healthy, watered well, and getting nutrients, the tree will seal off the injury from the rest of the tree.
Reasons Not to Fill Tree Holes
- Tree wood naturally expands when hot and contracts when cold. The material you decide to fill the tree hole with might not “move with the tree,” causing gaps and more damage.
(A good example of this — think of your exterior door’s wooden framing in your house. It might be harder to open and close your door when it’s hot and humid because the wood has expanded due to rising temperatures and moist air conditions.)
- Filler materials like concrete can create dangerous scenarios if the tree needs removal. If the tree service pros use a chainsaw to remove the tree, imagine the damage if they hit the concrete inside the hole.
- Most people presume that filling a tree hole will prevent water from entering. (Think again!) Water is sneaky and will find a way even into the smallest crevices. Water can lead to a tree developing wood decay fungus.
Note: In the past, the old thought process was to fill the holes with concrete to prevent water from getting in, making the tree more stable and preventing it from falling over. But the concrete caused further damage due to its immobility to the tree. Water still managed to creep in, and the concrete was too inflexible.
Reasons You May Want to Fill Tree Holes
There are a few reasons why filling tree cavities may be a good idea:
- Disease-carrying mosquitoes can lay eggs in the standing water of the tree holes.
- Wildlife can make homes in these holes, depending on their size and depth.
- Children and pets are curious and could try to stick their limbs into the tree hole.
Note: Before you go full steam ahead and start filling holes, you should contact a good arborist for an initial consultation. They’ll be able to advise you on the condition of the trees, possibly diagnose what’s creating the holes, and educate you on the pros and cons of filling the tree holes.
Wet Tree Holes vs. Dry Tree Holes

The first thing you should do is look in the tree holes to see if they are wet or dry. It’s especially helpful to use a flashlight after heavy rain and look inside for standing water. A wet tree hole will attract female mosquitoes that need water in which to lay their eggs. A dry tree hole can attract wildlife that are looking for a place to call home.
If your tree hole remains dry after heavy rains, female mosquitoes won’t look there to lay eggs. However, if it does collect water, address this issue to prevent mosquitoes from breeding in your tree. In addition, if you’re concerned about children or wildlife getting into the hole, this is another reason to consider filling it.
How to Fill a Tree Hole: Three Options
So, you’ve decided to plunge and fill the tree hole! You’re fed up with worrying about your kids sticking their limbs in the tree hole, your pets investigating the hole, and wildlife like raccoons becoming potential squatters in your tree.
Below are the three best ways to fill (or manage) a tree hole: mosquito dunks, water-absorbing polymer crystals, and expanding foam (also called expansion foam).
Mosquito Dunks
What are Mosquito Dunks?
Mosquito dunks are small, donut-shaped larvicide disks placed into standing water to kill larvae, like the Aedes female mosquito.
How Do Mosquito Dunks Work?
These bacterial larvicide disks contain a natural soil bacterium called Bacillus thuringiensis subsp. israelensis (Bti). Larvae feed on these dunks and die soon after. You can use these in any place with standing water where you wish to kill larvae, like tree holes, rain barrels, birdbaths, and ornamental ponds.
Mosquito dunks are considered a safe, natural, and effective way to control mosquitoes because they do not harm birds or fish.
Water-Absorbing Polymer Crystals
What Are Water-Absorbing Polymer Crystals?
Despite the long, scientific-sounding name, these are tiny pellets (the size of raw sugar crystals) that come in a plastic bag or small bucket with holes that extract all moisture from the air in a small area. The little pellets absorb, puff up with water, and transform into a gel-like substance. They are like mini dehumidifiers, except they are less expensive and don’t require electricity.
Although there isn’t a lot of research on using these crystals to absorb standing water in tree holes, it’s something else to consider to get rid of the standing water.
Where Are Water-Absorbing Polymer Crystals Used?
People frequently use water-absorbing polymer crystals in their RVs, boats, or basements as a short-term, low-budget option for decreasing moisture in the air. Because they are completely biodegradable and non-toxic, they can also be used as part of a soil mix in container plants, so you don’t have to water as much.
Expanding Foam

A filler such as an expandable foam, commonly used in residential insulation projects, is another option for deterring standing water. Spray foam is a type of expansion foam that offers faster application than traditional expandable foam.
Here are some tips when considering a spray foam:
- Always read and follow the directions on the manufacturer’s product label.
- Make sure the product is water-resistant.
- It should be able to fill larger holes/gaps (depending on the size of the tree hole) to deter the accumulation of standing water. This prevents mosquitoes and also prevents wood decay fungus, which will occur eventually if standing water remains in a tree hole.
- Remove standing water before applying spray foam so you don’t encase it.
- Don’t remove any “punky wood“ (a stage of decay in any wood where it feels almost like a spongy, styrofoam-like consistency — light and slightly squishy).
- Use wire mesh over the face of the spray foam to prevent wildlife from burrowing or chewing through.
- Try to use closed-cell foam rather than open-cell foam because it doesn’t absorb water.
Note: The goal of using an expandable spray foam should be to attempt to seal the hole entirely to keep water from collecting in the tree hole and to be flexible enough to move with the tree.
FAQ
No. It is not advised to use gravel or sand because these materials don’t stop water from collecting. Sand is very porous, and gravel can be a safety issue if tree work needs to be done.
No. You’ll just be adding more holes to the tree trunk and defeating the purpose of wanting to fill in the initial tree hole.
The sapwood (new wood) is affected. Sapwood is important because it’s the tree’s pipeline for getting water to the leaves. Depending on the depth of the tree hole, the deadwood, also called heartwood, can be affected.
The average homeowner might need to call a licensed arborist to ensure the tree is still structurally sound with those deep tree holes.
Contact The Pros
Stumped on which option is best for your tree? Pest Gnome connects you to the best pest control experts in your area. They’ll devise an action plan for your yard so you can demystify those magic tree holes and experience a mosquito-free summer.
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