How temperature and moisture impact termite behavior.
Editor’s Note: In this excerpt from the 10th Edition of the Mallis Handbook of Pest Control, one of the industry’s leading termite authorities discusses how this destructive pest responds to its environment. To order a copy of the book, visit www.pctonline.com/store.
As true of all cold-blooded creatures, temperature strongly influences termite activity. Studies have shown that subterranean termites will not forage in areas where upper level soil temperatures are either too hot or too cold. Optimum temperatures for termites range from 75°F to 95°F (24°C to 35°C). At temperatures above 100°F or below 25°F, termites may die in a matter of minutes. Through their movements, termites are able to avoid such extremes and exploit areas where temperatures are more suitable. During the year, for example, the center of colony activity may shift from the outer to inner portion of a log, or from the soil surface to areas deep underground where temperatures are more moderate.
Research on subterranean termites in Arizona and California indicate that termites seek out cooler, shaded areas when temperatures at the soil surface get too hot. It also has been suggested that subterranean termites can detect temperature gradients in the soil and use “thermal shadows” cast by vegetation (or presumably a house), to help locate above ground food sources. Termites may vary their foraging activities throughout the day and from season to season in response to sun exposure and heating of the soil surface.
Less is known about what termites do during winter. In areas where winters are mild, termites may be active year round. In colder climates, such as the Midwest and northeastern United States, subterranean termites retreat from the soil surface in late fall and move deeper into the ground where temperatures are more moderate. Various studies have reported downward movement of the eastern subterranean termite, R. flavipes, to depths greater than 40 inches (100 centimeters) during winter. By retreating to such depths, R. flavipes colonies are able to pass the winter in an inactive state in the colder regions of its range. Tree stumps, logs and other large wooden objects above ground also can serve as overwintering sites for termites and provide refuge against otherwise lethal temperatures. When termites are discovered in such places during cold weather, they may appear frozen and immobile; however, they often soon recover when held at warmer temperatures.
In cold climates, field experience indicates that subterranean termites can remain active year-round in centrally heated buildings. Various authors have recorded termite feeding throughout the winter in northern and midwestern states using an acoustic emission detector.
Role of Moisture. Termites are soft-bodied insects and are vulnerable to desiccation. The amount of moisture required, however, varies among species. Drywood termites obtain all the moisture they need from the wood they consume, much of which is seemingly dry. In the case of subterranean termites, moisture is usually the most crucial requirement for survival and a continuous supply is needed. Deep within their subterranean workings, relative humidity is nearly 100 percent and water loss through the cuticle (skin) is minimal. Near the surface, however, soil moisture tends to be more variable and is likely to influence foraging activity. This has been found to be true of western subterranean termites foraging in California. In urban areas, they suggested that high soil moisture from sprinkler irrigation might create ideal habitats for termites around buildings.
In laboratory feeding studies, subterranean termites have shown a distinct preference for moister wood. Subterranean termites also ingest and store soil moisture in water sacs (salivary reservoirs) in their bodies for transport to their above ground feeding sites. Moist soil and fecal material deposited by the termites aids in tunnel construction and helps maintain high moisture content throughout the gallery system. This is why dry, brittle shelter tubes seldom contain live termites; active tubes in use by termites often have a moister, less “crumbly” consistency, especially while under construction.
Termite Survival Above Ground
If the moisture content of wood above ground level is high enough, subterranean termites can survive and multiply for an indefinite period with no soil contact. Field experience offers many examples of termites living exclusively on above-ground sources of moisture, such as roof leaks, condensation or plumbing problems.
So, how much moisture is needed to sustain subterranean termites above ground for extended periods? Considering the importance of this question, one might think there would be a definitive answer, supported by an abundance of research — not so, at least, as of this writing. It is estimated that accurate wood moisture readings above 15 percent could indicate conditions sufficient to support subterranean termite activity, but 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 above ground infestations. Correcting existing moisture problems is often a requisite for eliminating subterranean termite populations persisting above ground.
Too much water, however, can be lethal to termites. Reductions in foraging populations of Reticulitermes following prolonged periods of rainfall and saturated soil conditions in Georgia have been observed. Subterranean termites escape drowning not by abandoning their galleries but by entering a state of quiescence for up to several hours. Flooding associated with hurricanes also can diminish termite numbers. Significant reductions in Formosan termite populations occurred in flooded areas of New Orleans after Hurricane Katrina, although some populations subsequently recovered possibly by persisting above ground in tree cavities. Under normal rainfall conditions, enough time usually occurs for excessive amounts of water to move through the subterranean galleries. The dampwood termites are extremely tolerant to moisture and have been found in water-logged wood, water tanks and in harbor structures directly over water.
The author is a professor and urban extension entomologist at the University of Kentucky. For a list of references for this article, visit www.pctonline.com and click on “online extras.”