Wood is a biological material. If protected from moisture and insect attack, it can last for centuries. When wood is not properly protected, however, it will succumb to biological processes that decompose wood: insects that eat the wood or fungi that cause rot and decay. The most damaging insects that attack structural wood are termites. Their activity results in damage and control costs that exceed $1.5 billion per year nationally. Beetles are the next important group of insects that attack wood, while bees, wasps and ants are third in importance, depending on geographical location. Wood-inhabiting fungi are another group of organisms that occasionally cause problems.

The WDO inspection is intended to report any visible evidence of reportable insect infestations in or on the structures to be inspected. Live insects do not need to be present on or in the structure for the inspector to report the structure as infested. Evidence of termite activity may include mud shelter tubes, shed swarmer wings, or excavated wood. Carpenter ant activity may be determined from frass or excavated wood. Frass at exit holes in wood and loose frass may be evidence of powderpost beetles. Carpenter bee activity may be evident from round holes in fascia boards or deck rails and fecal material and dislodged pollen on the uppermost siding.

In addition to inspecting for wood-destroying insects, the inspector must look for signs of previous inspections or treatments. Inspection companies and pest management companies often place their printed stickers in areas that should be conspicuous to another inspector. Look for these stickers on electrical panels, furnaces, water heaters, stairway stringers, etc.