Wood destroying organisms (WDOs) fall into two broad categories: insects and fungi. Together, they cause billions of dollars in structural damage each year across the United States, and fungi alone account for roughly as much destruction as termites. Understanding which organisms attack wood, how to spot the evidence, and what keeps them at bay is essential for any homeowner or property buyer.
The Main Types of Wood Destroying Organisms
Several groups of insects and fungi feed on or break down structural wood. The major insect culprits are termites, powderpost beetles, deathwatch beetles, false powderpost beetles, carpenter ants, and the old house borer. On the fungal side, wood decay fungi (commonly called wood rot) are the primary concern. These organisms can hollow out timbers completely, sometimes leaving only a thin outer shell that looks perfectly intact from the surface.
Termites: Subterranean vs. Drywood
Termites are the most well-known WDOs, and the two types homeowners encounter most often behave very differently.
Subterranean termites live in the soil and build mud shelter tubes to travel between their underground colonies and the wood they feed on. They tend to eat only the softer springwood in lumber, leaving alternating layers of damaged and intact wood. Their tunnels typically contain soil. Because they need ground contact and consistent moisture, finding mud tubes on a foundation wall is one of the clearest warning signs.
Drywood termites, by contrast, live entirely inside the wood above ground level and need no contact with soil at all. The wood they digest provides enough moisture to survive. They prefer to nest under wooden roof shingles, eaves, or siding. Unlike subterranean species, drywood termites consume both the soft springwood and the harder summerwood, creating smooth, sculpted galleries. Instead of soil in the tunnels, you’ll find small fecal pellets, which they push out of tiny “kick holes” in the wood surface.
How to Spot Termite Activity
Termite swarmers (the winged reproductive adults) are often the first visible sign of a problem. Finding large numbers of swarmers emerging inside a building means a sizable colony is actively feeding on the structure. Finding just a few swarmers indoors, particularly in neighborhoods with heavy Formosan termite populations, doesn’t necessarily indicate an active infestation in your building. But hundreds of them inside is a definite problem regardless of any previous treatment the structure has received.
Swarmers emerging from a stump, log, or woodpile more than 25 feet from the foundation are rarely cause for concern.
Mud shelter tubes on foundation walls, interior walls, ceilings, or exterior soffits deserve professional inspection even if you don’t see live termites. Tubes found higher up on a building, on walls or fascia boards, often indicate a Formosan termite infestation specifically. If you break open a tube and find it repaired later, that confirms active termites. Old, inactive tubes should be scraped away completely so they don’t cause confusion during future inspections.
When you find damaged wood, look inside the galleries. If the galleries contain dried mud but no live termites, the damage could be from a previous infestation that was already controlled. Live termites in the wood confirm an active infestation, though they’ll vacate quickly once the area is disturbed.
Wood Boring Beetles
Several beetle families attack dried, seasoned wood in homes. Unlike termites, beetles do their damage during the larval stage: adult females lay eggs in wood pores or cracks, and the larvae tunnel through the wood as they feed and grow, sometimes for years before emerging as adults.
The telltale sign is small round exit holes on the wood surface, typically 1/32 to 1/16 inch in diameter. These holes appear when adult beetles chew their way out after maturing. The texture of the powdery residue (frass) left behind helps identify which beetle you’re dealing with. Lyctid powderpost beetles produce frass that looks and feels like talcum powder. Anobiid deathwatch beetles leave frass that’s also powdery but has a distinctly gritty texture when rubbed between your fingers.
The real danger with wood boring beetles is reinfestation. Adults emerging from infested wood can lay eggs right back into the same timbers, and successive generations can continue the cycle until the wood is completely hollowed out. Fresh frass spilling from exit holes or new holes appearing in previously clear wood indicates an active, ongoing infestation rather than old damage.
Carpenter Ants
Carpenter ants don’t eat wood. They excavate it to create nesting galleries, pushing out piles of coarse sawdust-like debris called frass. They prefer wood that’s already softened by moisture or decay, which is why carpenter ant problems frequently overlap with moisture problems. Large black ants seen regularly indoors, especially at night when they’re most active, combined with small piles of wood shavings near baseboards or window frames, are the classic indicators.
Wood Decay Fungi
Fungi cause a level of structural damage comparable to termites, yet they get far less attention. Wood decay fungi break down the cellulose, lignin, or both in wood, progressively weakening it until it crumbles or fractures under load.
Moisture is the controlling factor. Wood decays when its moisture content rises above 30%. Below 20% moisture content, decay fungi simply cannot grow. The range between 20% and 30% is a gray area where conditions may or may not support fungal activity depending on temperature, wood species, and how long the moisture persists. This is why keeping wood dry is the single most effective defense against rot. Leaky roofs, poor drainage, plumbing leaks, and ground contact all create the sustained moisture that fungi need.
The three main types of wood rot are brown rot, white rot, and soft rot. Brown rot breaks down cellulose and leaves the wood dark, shrunken, and cracked in a characteristic cubical pattern. White rot degrades both cellulose and lignin, leaving the wood lighter in color, spongy, and stringy. Soft rot develops in extremely wet conditions and tends to affect the wood surface gradually.
Prevention Strategies
Preventing WDO damage starts with two principles: keep wood dry and create barriers between vulnerable wood and the organisms that attack it.
For termites, the standard approach for new construction is a two-step liquid treatment. First, a horizontal barrier of treated soil is applied beneath the slab or foundation before it’s poured. Second, a vertical barrier is applied around the exterior foundation perimeter after the building is completed and final grading is established. This creates a continuous zone of treated soil that kills or repels termites attempting to tunnel through it. The best soil-applied liquid treatments provide protection for 8 to 12 years or longer.
Borate treatments applied directly to wood framing during construction protect against both termites and wood decay fungi. Buildings that receive borate treatment still need an exterior perimeter liquid treatment, but the borate replaces the under-slab portion of the barrier. Termite bait stations, which use small monitoring devices placed in the ground around the building, are also now approved for preconstruction use in some states.
Foam treatments serve as a supplement to other methods. The insecticide is mixed with a foaming agent to create a shaving-cream-like consistency that can be injected into wall voids and foundation gaps, reaching areas a liquid pour can’t access. The foam breaks down quickly, depositing insecticide residue on the inner surfaces of the treated space.
Any wood element in direct ground contact, such as posts or landscape timbers, needs chemical treatment to resist both termites and decay fungi. Beyond chemical barriers, good building practices matter enormously: proper grading to direct water away from foundations, adequate ventilation in crawlspaces, prompt repair of plumbing leaks, and keeping firewood and wood debris well away from the structure. These simple moisture-control measures address fungi, carpenter ants, and termites simultaneously, because all of them thrive where wood stays damp.