This past summer, the Buckeye Environmental Horticulture Team inspected a tree where a pile of sawdust had accumulated around the base of the trunk. Soon, we spotted chips actively falling from the tree and traced the debris path back to a tiny hole where the culprit was “taking out the trash”. It was carpenter ants, (Camponotus sp.) excavating wood out of a small hole within an old pruning wound showing signs of rot due to exposure.

OH NO!? Are the carpenter ants eating the tree?

Unlike termites, carpenter ants do not eat wood. These ants are opportunistic lodgers. They excavate nest tunnels from soft rotting wood with their large mandibles. They rarely (never say never) nest in healthy trees or in-tact wood. That wood has to already be wet, rotting or otherwise compromised. The sawdust we found is from the ants cleaning up after themselves. They throw the soft, chewed wood out. They do not eat it. This excavation leaves behind smooth galleries for their nest. And you may not always SEE the evidence of their labors. It was dry and hot that week, but a good rain could have washed the sawdust evidence away!

In nature, this excavating behavior helps to decompose large logs and tree trunks by speeding up the breakdown and recycling of nutrients in the forest. The tunnels they create become entry points for more moisture, fungus and bacteria to enter and further breakdown the substrate into nutrient-rich humus.

How did my tree get rot?

Entry wounds in the tree let in moisture that creates the perfect conditions for fungus to thrive. Moisture and fungus both can easily enter through pruning cuts or broken branches, man-made holes, woodpecker feeding, deer rubbing their antlers, lawnmowers or weed-wacker damage, lightning strike etc. Once the fungus gets to work, this can cause heart-rot, a condition where the center of the trunk or the "heartwood" decays and/or becomes hollow. The voids this creates can become homes for owls and raccoons, birds, and bugs. As wood is softened, the carpenter ants can take advantage of it.

Will this kill my tree?

Not necessarily! Trees can survive with heart-rot (and carpenter ants for that matter) and still grow and have healthy canopies. As long as the tree's cambium is healthy (the outer ring of tissue that transports water and nutrients) it can go on living. Carpenter ants working within that compromised wood will not kill the tree either. This tree, which is the subject of this article, still had a full canopy. However, it clearly has some hollow voids in it. There was even a wild honey-bee hive in one of the large lateral branches, another sign of a hollow in the tree.

However, there is a point at which the rot and excavated space within the tree can compromise its structural integrity. Years of internal rot can cause even the healthiest looking tree to fall in a storm. Depending on a tree’s location, a safety inspection with a certified International Society of Arboriculture (ISA) TRAQ-trained arborist may be warranted to determine if the tree is a fall risk to nearby structures or vehicles. Can you accept the risk posed by a tree with possible rot? You can find certified arborists listed and their specializations at treesaregood.org.

Action Items? What Next?

Carpenter ants can be a good indicator of soft wood and rot inside and out. Finding them is a sign that you should take a look at your tree and consider any needs for pruning or removal of risky trees. Likewise when you find carpenter ants in your home, it may be time to double check for leaks around pipes and window wells and door frames.

Mature carpenter ant colonies can split off into 1 to 10 satellite nests attached to the original. This means that the nest you find may have additional pods nearby in other decaying logs or even in damaged wood in your home or other structures. Foraging ants can travel over 300 feet from the nest to find food. They may travel over branches and vines touching rooftops or utility lines to get into homes or other wood. The ants may be attracted to plant seeds or to sap-feeding insects such as aphids to collect the sugary honeydew waste they excrete.

Controlling the ants will not correct or reverse internal damage that exists within a tree. If you do choose to control the colony, you must get to the queen to eliminate them completely. This can take time if you cannot find access to the internal nest. Using granules or dust formulated insecticides that can be carried by the worker ants into the heart of the nest can help, but always follow all label instructions of any product you use as the label is the law for use. More on control and inspection can be found in the OSU Factsheet on carpenter ants here. But remember, even after controlling the ants, any rot or hollow spaces will persist and a safety inspection with a certified arborist may be a necessary next step.

Removing tree limbs with obvious rot may reduce the potential for colonization by carpenter ants. Selecting a certified arborist to properly prune a tree is a must. Never allow a "tree pruner" to "top a tree." It does more damage than good by creating an open wound exposed straight up to rain.

You may have seen old trees filled with cement but this is NOT a recommended practice. Do not fill tree voids with rocks, bricks, or cement. These items will cause more damage when the tree moves and sways with the wind. It does not correct nor improve the structure of a hollow tree. Additionally, any filler materials would also be a future hazard for anyone that needs to remove or prune the tree.

What you can help with is preventing or diverting water from entering into tree cavities and furthering rot. Small openings or large holes on a tree will become entry points for water and rot. You want to give the tree time to heal itself, but in cases where the tree cannot heal itself and an entry hole persists, you can intervene to help reduce water entry. Sheet metal has been used to produce a roof over upward facing holes in trees to divert water away from an opening. There may be some value in using water-resistant foam to seal off an entry hole (but not used to fill the cavity entirely) if water cannot be diverted any other way. However, these foams can also act to trap moisture and humidity. Spray foams may also break down with UV exposure and weathering. Therefore it is not a long-term nor full-proof solution. Do not attempt to "clean out" a hollow void or cavity either as this risks breaking into healthy wood and spreading fungus. Do not drill a hole to empty water either. It is thought this will just provide a new wound to further spread rot.

Ron Wilson