[Annual Termite Control Issue] Termite Tendencies

Editor’s note: The following was adapted by Dr. Michael Potter from his chapter in the upcoming 9th Edition of the Mallis Handbook of Pest Control, which will be available from the PCT Media Group next month.

To pest is more complicated to control than subterranean termites. Even with today’s "war chest" of management tools, termites remain a high-stakes challenge. Lately there has been revitalization in the study of the creatures themselves — an effort that was neglected during the "ho-hum" era of chlordane. At the most recent meeting of the Entomological Society of America, a whopping 87% of all urban research presentations involved termites. (In comparison, 35% of all presentations 10 years ago involved termites and almost none did in 1983). Understanding the habits of termites is helpful from a control standpoint, but is also important in dealings with customers. Consumers today have many questions and are more inclined to trust firms that are knowledgeable. This article relates some of what is known currently about subterranean termites’ tendencies.

WHAT’S A COLONY? Subterranean termites live in cooperative, intermingling groups known as colonies. They divide their work among specialized members whose activities include nest and tunnel construction, egg laying, foraging for resources, and attending to nestmates. The intricacy of their social structure is remarkable, considering that the brain of a termite is no bigger than a grain of sand.

The former belief that termite colonies are compact units confined under buildings is incorrect. Mature colonies of most subterranean species tend to be decentralized entities occupying multiple nesting and feeding sites, interconnected by underground tunnels. The centers or "hubs" of activity are usually located near stumps, buried debris or other food resources. According to Dr. Barbara Thorne at the University of Maryland, "Some of the connections between the hubs are heavily traveled highways, some are rarely traveled paths, and others may be obsolete passageways that are out of use." (See Thorne’s story on page 44 of this month’s issue.) Subterranean termite colonies are mobile entities, with a distribution pattern in the soil that changes over time. Termites from different colony populations generally do not intermingle, although it appears they may occasionally occupy common feeding sites.

COLONY DIMENSIONS. Painstaking efforts have gone into studying the dimensions of termite colonies. An overriding conclusion has been that colony population sizes and foraging ranges are highly variable. Estimates of some colonies suggest hundreds of thousands to millions of individuals, occupying areas of up to half an acre, and foraging distances approaching the length of a football field. Other termite colonies apparently are much smaller, containing as few as 10,000 individuals traveling less than 20 feet, with a foraging "footprint" no bigger than an average size room. Yet where buildings are close together, even a "smallish" termite colony could concurrently invade more than one structure. In some cases, larger but fewer colonies may be present; in others, individual colonies may be smaller and more numerous.

Like the movements of an amoeba, a subterranean termite colony’s foraging area changes size and shape, sometimes within weeks or months. The termites may sometimes forage throughout their entire network of tunnels, then at other times forage in only a small portion of their territory. For these reasons, it’s hard to make meaningful generalizations about "average" or "typical" colony dimensions.

SEARCH FOR FOOD. Subterranean termites are persistent foragers. In laboratory tests, they construct branching tunnels in a way that results in an area being efficiently searched. When termites forage outward from food or nesting sites, their tunneling pattern has been described as radial, like the spokes of a wheel. The resultant, wide-ranging search pattern helps in finding available resources — and is central to the belief that bait or monitoring stations installed around buildings will eventually be discovered.

Subterranean termites probably do not detect food from appreciable distances in the soil. In essence, they wander into the resource in the course of their foraging activities. Foraging termites do not "sense" that there is a woodpile "over there" on a person’s property, nor are they aware of bait or monitoring stations until virtually upon them. However, termites do respond to various environmental cues in soil. These include temperature and moisture gradients, odors emitted by decaying wood and plants, soil compaction layers, and even thermal shadows cast by potential food items such as a bush, log or building. Foraging termites also tend to tunnel along roots, stones and other below-ground objects. Structural guidelines afforded by pipes, cracks, mortar and expansion joints, and edges of pavement, are routes often traveled by termites into buildings. In this respect, foraging and food discovery by termites underground is anything but coincidental.

Subterranean termites move up or down in soil in response to environmental conditions. During winter, they may go below the frost line, sometimes deeper than a meter. In times of soil dryness, they may also go deeper seeking moisture. Although subterranean termites do, at times, tunnel deeply in soil, the majority of foraging occurs in the upper few feet since this is where most of the oxygen and food resources, such as plant roots, are located. Termite bait stations typically extend downward about 7 to 9 inches from the soil surface. Liquid termiticides on the other hand — including those which are non-repellent — must be applied to a minimum depth of 4 feet alongside deep foundations. Presumably any termites tunneling deeply around foundations would also encounter the non-repellent treated zones nearer the surface. To lessen the risk of contamination and pesticide "overloading" around buildings, a somewhat shallower treatment depth (e.g., 2 feet) might be warranted with non-repellent applications in the future.

MOISTURE NEEDS. Subterranean termites are soft-bodied creatures, vulnerable to desiccation. Moisture is crucial for their survival and a continuous supply is needed. Deep within their subterranean workings, relative humidity is nearly 100% and water loss through the cuticle (skin) is minimal. Near the surface, however, soil moisture tends to be more variable and influences the location and degree of foraging activity.

Subterranean termites ingest and transport soil moisture to their aboveground feeding sites. Moist soil and feces deposited by the termites helps maintain high moisture content throughout the gallery system — which is why moisture meters are so useful for pinpointing termite activity. It is customary for subterranean termites to return periodically to the soil for moisture. The frequency with which this occurs varies, however, and there is no truth to the belief that termites foraging above ground must return to the soil every 24 hours. Moreover, if the moisture content of wood above ground level is high enough, termites can survive and multiply for an indefinite period with no soil contact.

How much moisture is needed to sustain subterranean termites above ground for extended periods? One might assume there would be a definitive answer to this question, supported by an abundance of research — not so, at least for now. Dr. Brian Forschler at the University of Georgia has estimated that wood moisture readings above 15% could indicate conditions sufficient to sustain subterranean termite activity, but cautions that moisture meter readings in this range do not necessarily indicate termite presence. "Not much" is often all that can be said about the minimum moisture requirements of aboveground infestations. Certainly no more than that from rain seepage, plumbing leaks, condensation or humid conditions arising from lack of ventilation. Correcting existing moisture problems is often a requisite for eliminating subterranean termite populations persisting above ground.

FOOD PREFERENCE. Subterranean termites can consume a wide assortment of foods. The main nutritional ingredient in all of these is cellulose, the hard, structural component of wood and other plant tissues. Termites eat wood by tearing off tiny fragments with their mandibles. Unlike ants, worker termites do not carry away food in their mouthparts; rather, the tiny wood fragments are ingested and stored in the gut for digestion and subsequent sharing in liquid form with nestmates.

Subterranean termites prefer to feed on the softer, more readily digestible springwood, leaving the harder summerwood along the grain intact. They also tend to prefer fungus-decayed wood to sound wood and moist wood to wood that is dry. Wood partially decomposed by certain types of fungi may be of higher nutritional value and easier to digest. Although moist, fungus-damaged wood is generally preferred, termites are capable of digesting sound, dry wood, and decay is not a prerequisite for attack. Although certain types of wood (e.g., cedar, cypress, redwood) are less preferred, no wood or plant-based material is immune to termite attack, especially if there are limited alternatives.

Termites also penetrate and damage non-cellulosic items of no nutritional value. Although termites are fragile, soft-bodied insects, their mandibles are hard, enabling them to chew through tough, seemingly impenetrable materials. They commonly penetrate drywall, plaster and stucco — although field reports of termites chewing through concrete usually involve exploitation of small, pre-existing cracks or holes. Termites are able to squeeze through very tiny openings (or more precisely, about the width of a termites’ head capsule, which for most species is less than 2 millimeters). Termites can chew their way through rubber, vinyl and some plastics, softer metals (lead, copper, aluminum), asphalt, carpet, and, of course, foam board insulation. Inedible materials are not ingested, and the fragments bitten off are either discarded or incorporated into shelter tubes.

DAMAGE POTENTIAL. How much damage can termites do in a certain amount of time? This question often arises during litigation when attempting to prove how long subterranean termites have infested a structure. The rate and extent of structural damage depends on many factors. For example — the number and size of colonies, the species of termite involved, and the degree to which feeding has been focused on the building. The amount of wood consumed by colonies may be distributed over many different feeding sites on a property other than the building (decaying tree roots, stumps, woodpiles, the neighbor’s house, etc.). The intensity of termite attack can also be influenced by seasonal variation, and the moisture content and nutritional quality of the structural members. Consequently, it’s extremely difficult to predict with confidence how long an infestation has been present. Efforts to date termite infestations based on damage are fraught with uncertainty. Such opinions are usually unreliable, except in the most general sense, e.g., "the termites appear to have been there for quite a while."

It is similarly hard to predict the pattern of termite damage within a building. For reasons that are not well understood, subterranean termites will often damage some wood members in a structure, and not others. It is not uncommon, for example, to observe intense feeding near the point of termite entry such as along the base of a wall, with adjoining areas fed upon intermittently or ignored. Accessing suspected areas by the removal of sections of baseboard or wall covering, or perhaps through use of a borescope, is the usual way to determine the extent of damage. It is not the norm to remove all wall coverings inside a building when prospecting for termite damage; oftentimes a determination can be made by selectively removing lower portions first and continuing to remove until there ceases to be damage.

Author’s Acknowledgement

The work of several researchers is referenced in this article. Among them, Drs. Brian Forschler, Univ. Georgia; Roger Gold, Texas A&M; Ken Grace, Univ. Hawaii; Mike Haverty, USDA Forest Service; Greg Henderson, Louisiana State Univ.; Susan Jones, Ohio State Univ.; Shripat Kamble, Univ. Nebraska; Brad Kard, Oklahoma State Univ.; Mike Lenz, CSIRO, Australia; Faith Oi, Univ. Florida; Mike Rust, Univ. California; Nan-Yao Su & Rudy Scheffrahn, Univ. Florida; Barbara Thorne, Univ. Maryland; Pat Zungoli, Clemson Univ., and others. Their research is helping the industry "think" like a termite, as it has learned, over time, to "think" like a cockroach.

The author is a professor and urban extension entomologist at the University of Kentucky.