Carpenter Ants in Trees

Carpenter ants nesting inside trees often expel large quantities of coarse sawdust
Carpenter ants nesting inside trees often expel large quantities of coarse sawdust found around the base of the tree. 
Photo by Suzette Striegel, Mahaska County Extension

By Donald Lewis
Extension Entomologist
Iowa State University

Big, black ants always attract attention, whether they are on the peony blossoms, digging in the ground, crawling on your kitchen floor or trundling up and down the old tree trunk next to the house.  The description of "big, black ant" nearly always refers to carpenter ants, even though other species of ants are nearly as large and nearly as black.  For example, the big, black ants that nest in the ground making ant mounds in your lawn are not carpenter ants but an unrelated species called field ants.
Carpenter ants serve a useful purpose in the real world.  Their nests consist of tunnels and chambers chewed into soft wood.  The openings and hollow spaces they create contribute to the natural decay of stumps, logs and dead limbs.  Without recyclers such as carpenter ants we would be up to our necks in dead, un-decomposed organic matter in the landscape and woodlands.
In addition to logs and stumps another common place to find carpenter ants is nesting inside trees, especially older trees that are hollow or have a significant amount of dead, decayed wood.  The nests are made in wood that has already started to decay because of excess moisture let in through an opening to the outside such as mechanical injuries, knots, cracks, holes and old insect tunnels.  The ants are only taking advantage of an existing situation of soft, weak wood in which to establish their colony. Stress, mechanical injury, environmental conditions, disease or other insects are responsible for killing limbs or sections of the trees in which the ants are able to nest. Once injury has occurred, wood decay can set in if moisture is present; it is the wood decay that gives the carpenter ants the opportunity to colonize the tree. 
Carpenter ants are opportunists that take advantage of an existing situation. The opening to the outside, the moisture, and the soft, decayed wood already existed. The ants came second.   The ants may extend the damage into sound wood and an injured tree may be further losing structural strength.  Still, ant control is not essential for the tree's health.
Carpenter ants make nests in wood by chewing sandpaper-smooth tunnels and chambers in wood. They cannot eat the wood. The wood is discarded as shredded fragments of coarse sawdust ejected from the nests. The sawdust may contain dead ants and bits of dead insects that the carpenter ants have eaten.
Control of carpenter ants inside a tree may not make any difference to the tree but it may be warranted as one way to reduce invasion of the ants into adjacent structures. Insecticides labeled for use on trees in the landscape can be applied (dusted or sprayed) directly into the nest cavity. An alternative is to treat the trunk and ground around the tree in hopes of reducing the population of foragers (workers). Treatment is not likely to permanently rid a tree of carpenter ants; retreatment every year or so may be necessary. 
Plugging or sealing tree cavities or treating tree wounds with wound dressings is not advised. Such treatments are unnecessary and will not eliminate nor prevent decay or carpenter ant activity.  
Cutting down an otherwise viable landscape tree in the hopes of avoiding ant problems in the home is an extreme response and is not recommended. If someone suggests removing trees from your landscape to control ants, remember that complex problems rarely have such simple solutions. Check and double-check with qualified arborists before making drastic and irreversible assaults on your landscape plantings.
On occasion, carpenter ants do infest the house and ruin window frames, door sills, deck boards and other things made of lumber.  However, in almost all cases of severe damage there is also a persistent and long-standing moisture problem. Too much moisture in lumber leads to both wood decay and carpenter ants.  It's hard to say which lets you down faster. For ant control indoors, consult your local pest control professional for sprays, dusts or baits to reduce the ant population.


Contacts :
Donald Lewis, Entomology, (515) 294-1101,
Del Marks, Extension Communications, (515) 294-9807,

One high resolution photo is available for use with this column: Carpenterantsawdust.jpg [1.5 Mb]
Suggested Caption: Carpenter ants nesting inside trees often expel large quantities of coarse sawdust found around the base of the tree.  Photo by Suzette Striegel, Mahaska County Extension.