What mulch does to help plants is what also attracts insects. Mulch -- either organic or inorganic -- does a great job of keeping the soil beneath it moist. This moisture, along with cover from predators, makes mulch an ideal environment for insects. Dig into a pile of mulch and you’ll find insects scurrying everywhere. The closer you get to the moist soil, the more insects you’ll likely find. Termites, in particular, have bodies that dry out quickly. They require moisture from soil or wood to keep their bodies hydrated.
Most termite species in the United States are subterranean. They wander about in narrow, underground tunnels seeking moisture and food supplies. Some species travel as far as 250 to 300 feet from their colonies in a quest for food and water. University studies, such as those conducted at the University of Maryland, demonstrate that mulch itself does not attract termites and organic mulches are not a good food source for termites. Coarser mulches, such as bark and bark nuggets, tend to dry out more quickly than finer materials. That makes the environment less hospitable to termites and other insects.
As organic mulches break down, they add important nutrients to the soil. The rich, black soil that results is a type of compost that holds the minerals plants need. The decomposition process also attracts insects, such as sowbugs and millipedes, that consume decaying matter. Most of these nutrients are of little interest to termites, which thrive on cellulose fibers. Bark used as mulch breaks down, but over a longer period of ,as do most coarser mulches.
Mulch depth should never exceed 3 inches, even for bark. More than that, and moisture levels stay consistently high. The mulch, especially at the bottom where it contacts soil, then has little opportunity to dry out. That high moisture level could retain termite colonies, which is of particular concern with thick mulch mats close to the home. Some gardeners recommend maintaining a foot of bare ground between your home’s foundation and the mulch. Mulch should never touch any part of the house that's made of wood, such as shingles or framing timbers. That makes it too easy for termites to enter.