Mosquitoes readily lay their eggs in rainwater that has collected in tree holes. In fact, tree holes or other natural cavities are a favorite breeding site for the Asian tiger mosquito, a potential vector of Zika virus.

Tree holes are usually the result of decay that follows injury from breakage, pruning, lightning, trunk damage, limb dieback, etc.  In the old days, folks routinely filled tree holes with concrete. Now we know that filling the hole often does more damage to the tree. Recent expert advice is to leave a hole or pruning wound alone, the tree will form a protective barrier around the opening.

When Should You Fill Tree Holes?

As we encourage people to look for and eliminate mosquito breeding sites, the question arises: Should you fill in tree holes?

• The answer should depend on whether that tree hole is holding water, or whether it could. Some tree hole openings are angled so that they don’t easily collect water.
• The answer will depend on whether the tree hole has become a nest site for squirrels, woodpeckers, or other birds. When animals are nesting in a tree hole, you can assume that it will not be a mosquito site. Nest holes are kept dry by their occupants.
• Finally, the answer will also depend on how accessible the hole is and the level of mosquito pressure on your property.

Here’s a Recommended Approach

Fill the hole if it’s holding water, is accessible and is of a reasonable size, and is not occupied by animals. If you’d rather not have animals nesting in the hole, wait until any babies have left the nest before filling the hole.

Don’t use concrete, gravel, or sand as fill. Hard fill materials don’t flex when the tree flexes, causing abrasion to the healed edges of the hole and allowing decay to spread. Gravel and sand can also hold water, allowing decay. Hard fillers add weight to the tree and can also cause serious problems when the tree is taken down, especially if chain saws are used.

Soft, flexible fill materials like spray insulation or expanding foam are the best choices for filling a tree hole. Remove water and let the hole dry but don’t dig out soft wood as you can cause injury during that process. Inject the foam slowly and don’t overfill. Shave off any unsightly excess foam after it dries or consider using a black-tinted foam. These fillers don’t provide a perfect solution for the tree either, but in areas with mosquito-transmitted diseases, the benefits probably outweigh any risks to the tree.

If you have any questions about the health or stability of a tree, especially if the hole or decay is large, contact a certified arborist or tree expert before you attempt to fill any holes. Be aware that many tree holes will be up high in a tree and can’t be reasonably filled under any circumstances.

Photo Credit : “Dendrothelm” by StevoH – Own work. Licensed under CC BY-SA 3.0 via Wikimedia Commons.