Insects, such as the Formosan subterranean termite, are cold-blooded animals that depend on temperature and weather conditions for normal functioning. Temperature, humidity, rainfall, wind and even electrical charging in the air can affect termite behavior. Understanding how the weather affects Formosan subterranean termites may lead to improvements in ways to manage them.
During cold months, social insects like subterranean termites and ants go into a quiescent state, huddling together until warmer weather allows them to search again for food. Foraging by Formosan subterranean termites is strongly influenced by temperature. Louisiana’s steamy, subtropical climate is a big reason such a massive population of this invasive insect pest lives here. Termites can forage for food most of the year in Louisiana.
The annual swarm season of the winged reproductives (alates) also is influenced by temperature and moisture. In Louisiana, May is warm, humid and the air is electrically charged because of frequent late-afternoon thunderstorms. These conditions allow the Formosan subterranean termites to start their mating flights around Mother’s Day each year.
Termites exert some control over their own environment, especially in regulating nest temperatures. The termite nest is a well-constructed labyrinth of galleries and pathways surrounded by a hardened shell made from their own excrement, glandular secretions and transported clay particles. Nest temperatures in a Formosan subterranean termite colony stay fairly constant (91 degrees F) for all but the coldest months as a result of heat generated by the metabolism of millions of tiny termite bodies, aided by the even tinier dung-loving fungi that cohabit the nest.
Formosan subterranean termites, unlike most subterranean termite species, will build nests above and below ground. In Louisiana, aboveground nests are often found in attics and walls near doorways and window frames and in the center of living trees that the termites have consumed. Again, weather may play a role in this “small-minded” (a termite brain is the size of a grain of sand) but wise decision.
Subterranean termites often are exposed for long periods to inundation from rainfall that can accumulate into puddles on top of the soil for days. To study how subterranean termites react to and are affected by rising water levels, we took Formosan subterranean termites, along with their more commonly found cousins, the eastern subterranean termite and the southern subterranean termite, and completely submerged them in water in a laboratory setting. Besides giving us clues as to how heavy rainfall might affect a ground-nesting termite colony, the results of this study gave us insight on termite movement along waterways. We wanted to determine, for example, how likely a new infestation might occur as a result of a termite-infested tree limb breaking during a storm and traveling across a lake or down a river to fertile, cellulose-rich environs.
Our study indicated that subterranean termites escape drowning when confronted with excessive amounts of water not by seeking higher ground, as commonly observed in red imported fire ants, but by entering an immobile state that conserves oxygen. In this suspended state, it took 30 hours of submergence for 90 percent of the eastern subterranean termites to drown, 23 hours for southern subterranean termites and 16 hours for Formosan subterranean termites. That is a long time for any animal to hold its breath! Of course, a natural soil profile or a log floating down a river would have air pockets. The time before drowning would likely take even longer. Such a natural event has been documented. In 1992, an areawide flood in west central Georgia that saturated the soil for more than a month resulted in a 77 percent reduction in subterranean termite populations.
Because Formosan subterranean termites are the most susceptible subterranean species to drowning, that suggests they have a possible survival strategy for building aboveground nests that has evolved over millions of years. Termites have been on this earth for more than 200 million years. Unwittingly, this nest construction behavior also helps Formosan subterranean termites bypass traditional liquid ground barrier treatments applied commonly by pest control operators today, since this treatment does little to affect an aboveground colony.
Although too much water can kill a termite colony, water is necessary for their survival and termites will actively seek it. Formosan subterranean termites will even transport water from below ground to aboveground sites. If water is not constantly available to the colony, the soft bodies of subterranean termites will quickly dry out. For this reason, a leaking roof that causes moisture buildup in the walls of a house will make a pest control operator hesitant to provide a treatment warranty. Pest control professionals know of the termite’s attraction to these moist areas.
Such aboveground nest construction has led to the misconception that Formosan subterranean termites will start a new colony in an aboveground location, like the walls of a house. This is not likely. A study we conducted evaluated the likelihood of a colony getting its start in a normally dry wall. Given the choice between a moist location and a dry location to start a new colony, the new queens and kings almost always chose the moist site. The termites choosing the dry site would have soon died. The drastic drop in termite numbers observed in our 2000 census may have had something to do with the drought of 1999, making it harder for aboveground colonies to survive. See Figure 1 on page 9.
Knowing the moisture-loving habits of Formosan subterranean termites can help in their management. To find what termites consider the most favorable conditions in which to live, we conducted a 13-month study around a termite-infested apartment complex in New Orleans. Since moist soil conditions are believed to be one of the most important conditions, wooden stakes were placed near locations like dripping faucets, air conditioning units, areas having improper drainage and wood debris that helped retain moisture, with a similar number of wooden stakes placed in areas not appearing so attractive. Four separate inspections of 426 stakes placed around the apartment complex showed that moister sites had an almost 2 to 1 difference in attacks. With the advent of termite baits available in today’s termite control marketplace, this fact can be particularly important. The more attractive to termites the baits can be made, the greater the impact they will have on reducing termite populations.
We tested this idea of “precision targeting” in the baiting of termites. About 1,200 cardboard monitors were placed around 16 public schools in Orleans, Jefferson and St. Bernard parishes and examined each month for more than two years for attacks by termites (hits) before and after treatments. We were particularly interested in knowing how soil temperature and moisture affect hits by termites. Termite attacks on monitors fluctuated with the seasonal changes in temperature, with winter showing the lowest “success rate.” We also found that Formosan subterranean termites and native subterranean termites prefer different sun exposures. Formosan subterranean termites showed a strong preference for east and north exposures, whereas native subterranean termites were not so picky but seemed to like southern exposures the most. Soils on the east and north sides of a building retained more moisture and did not dry out or stay as hot. Monthly soil temperature readings at 4-inch depths verified that sun-exposed locations were warmer. Native subterranean termites may have selected southern exposures more often because they were farther away from the large populations of the more aggressive Formosan termite.
The more we know how the weather affects termite behavior and biology, the more likely we can develop improved methods to control them. Now if we could just understand the weather!
U.S. Department of Agriculture Agricultural Research Service Southern Regional Laboratory-New Orleans and the Louisiana Department of Agriculture and Forestry
Gregg Henderson, Professor, Department of Entomology, LSU AgCenter, Baton Rouge, La.
(This article appeared in the spring 2001 issue of Louisiana Agriculture.)