Elimination of tree holes for mosquito breeding

Tree holes near ground level are especially
attractive to the Aedes mosquitoes that may
spread Zika virus.

It seems you never know what interesting places and topics pest control will lead you. This week's rabbit trail for me was a discussion on how best to "fill tree holes" that are a common mosquito breeding site.

With the heightened interest in mosquito control and Zika virus this summer, tree holes are a significant problem. When a limb dies back or fails on a tree, the result is often a pocket in the tree that is capable of holding water. It turns out that such water-filled tree holes are perfect breeding sites for some mosquitoes, including Aedes albopictus and Aedes aegypti, the two potential Zika-carrying mosquitoes.

The standard mosquito control recommendation is to fill these holes to prevent their use for breeding. But how do we best eliminate such holes without hurting the tree?  Here are some practical suggestions on filling tree holes gleaned from colleagues and other reputable resources:

• Filling tree holes with concrete, gravel or sand is a no no. Gravel and sand may help hold water in the tree and promote decay.  Gravel and concrete pose a real safety hazard for arborists or tree owners when the tree eventually has to be cut down--and not good for the chain saws either.  Concrete also adds weight to the tree, and does not flex or bend, causing internal friction and damage to the tree over time.  
• Drilling drain holes to keep water from accumulating is no longer generally recommended, as it may open the tree up to further damage to health tissue, and infection. 
• If a tree hole does not hold water it may be providing wildlife benefits, providing a home for birds or squirrels(!).  On the other hand, tree holes may provide a nesting site for roof rats. 
• Not all tree cavities need to be filled.  However if a tree hole is retaining water or providing a breeding site for unwanted animals, expanding foam may be a good solution. This tool has been largely embraced by the tree care industry because it is lightweight and easy to carry into a tree and is safe for chainsaws.  It also excludes water and will flex and bend with the tree.  Some argue that foam is bad because water often finds it way in anyway and foam retards evaporation, but from a mosquito control perspective the benefits of sealing water-retaining holes probably outweigh the risks.
• As when using expanding foam indoors in a home, don't overfill a hole. Use a foam with a lower expansion ratio, and inject the foam slowly.  Because foams can be unsightly where they emerge from a cavity, you might want to consider a black tinted foam like Pur-black and/or consider smoothing off excessive foam overflow after drying.
• If rats or mice are an issue, using screen or or other pest control excluder materials in the filling may prevent their chewing their way back into the tree.
• It is not necessary to clean out decay from the cavity before filling.  And "tree paint" dressings are no longer recommended by arborists for fresh wounds on a tree.

Thanks to colleagues on the Ornaent Listserve for their interesting and enlightening comments on this question this week.  

BTW, since I've done it here today, referral to a commercial product or website is for informational purposes only and does not imply endorsement by me or by the Texas A&M AgriLife Extension Service.