I have a mature Norway maple on the boulevard of my property (city-owned). At the top of the trunk, just below where the tree branches out, there is a hole. For the past few years I’ve noticed ants around the hole and sawdust at the base of the trunk in the summer. This year, I’ve noticed larger debris at the base of the trunk. Should I leave it or try to plug the hole? I want to preserve the tree for as long as possible as a number of larger trees have been removed from our street.
The sawdust at the base of your Norway maple (Acer platanoides) suggests that you have carpenter ants nesting in your tree. The ants themselves are not a problem for your tree but are an indicator of decaying wood in your tree. The ants seek out wood that has already started to decay. The decay can be caused by a number of conditions such as knots, cracks, mechanical injuries or old insect tunnels which allow excess moisture to penetrate the tree. Once moisture is present, wood decay can set in and provide the opportunity for carpenter ants to move in.
Plugging the hole is not advised as it will not eliminate either the ants or any wood decay. However, you’ve indicated that you have a mature tree and that you’ve noticed the ants and sawdust for a few years now so I suggest you’ll want to check out whether the decay is extensive and threatening the structural integrity of the tree. This assessment needs to be done by a certified arborist. As this is a city-owned tree, a service request (see link below) can be placed to request a city arborist investigate the issue. I know you are keen to preserve the tree but if the decay is extensive, the tree will be removed and a new tree planted at no charge.
Note that Norway maples are no longer being planted in Toronto. The Norway maples ability to tolerate difficult urban conditions as well as its dense, shade-producing crown has made it an attractive option for street plantings for many years. However, it has become invasive in natural habitats such as Toronto ravines and many parks due to its prolific seed production and ability to thrive in dense shade. The canopy is so dense that groundcovers and shrubs cannot grow, leaving bare soil susceptible to erosion. When a Norway maple must be removed, it is replaced with a native tree – possibly one of our native maples.
If you’d like more information about carpenter ants in trees, I’ve copied a link below to a good article from Iowa State University Extension.