A new study shows that a colony of Formosan subterranean termites (Coptotermes formosanus) wiped out with insecticide bait is likely to be invaded by a a neighboring colony, which is then also exposed to the bait. (Photo credit: Scott Bauer, bugwood.org)

By John P. Roche

Termites pose huge economic costs because they consume wood in buildings. Worldwide, termite control and termite damage cost $40 billion annually. The Formosan subterranean termite, Coptotermes formosanus, is the most economically damaging species of all. It is estimated to cause $300 million in damage each year in the city of New Orleans alone. A new study in the Journal of Economic Entomology by Sarah Bernard and Nan-Yao Su of the University of Florida and Weste Osbrink of the USDA Agricultural Research Service has discovered behavioral patterns that could help with control of this devastating insect.

This species’ name derives from “Ilha Formosa” the historical Portuguese name for the Island of Taiwan, where the species was first discovered, though it is believed to have originated in southern China. It is an invasive species in North America. In the U.S., its range spans the south from Florida and the Carolinas to Tennessee and Texas. It is also present in Hawaii.

Colonies of Formosan subterranean termites have two characteristics that make them particularly difficult to control. Their colonies contain millions of individuals, and they have a wide foraging range of up to a radius of 300 feet. For many termite species, insecticides can be applied to soil beneath buildings and this controls the insects. But, because of their large numbers and wide range, soil insecticides are not effective for control of Formosan subterranean termites. Instead, Su developed a strategy of using insecticide-laden food baits to knock out C. formosanus colonies. This Sentricon bait system, which Su commercialized in 1995, uses a chemical that inhibits synthesis of chitin, a polysaccharide present in insect exoskeletons. By preventing synthesis of chitin, chitin synthesis inhibitors (CSIs) kill termites and eliminate colonies. And because baits target colonies directly, they can reduce the use of insecticides. Su estimates that Sentricon has eliminated the need for 9,000 metric tons of insecticides.

One process that could influence the effectiveness of bait systems is invasion by neighboring colonies. When a colony is knocked out, if other colonies of C. formosanus invade the tunnels of the eliminated colony and eat the bait, members of the invading colony could also be eliminated by the same bait, and thus it would make sense to maintain baits in their original location. Su and his colleagues have observed that such re-invasion of neighboring colonies sometimes occurs in nature. Studies have found that some eliminated colonies in the wild remain empty for months, but others can be reinvaded within days. Understanding the process of re-invasion has clear implications to optimizing control strategies, and this understanding relies in part on an understanding of termite behavior.

In the present study, Bernard, Osbrink, and Su wanted to observe how established colonies of C. formosanus would react behaviorally to nearby colonies that had been baited. They wanted to test if termites from a nearby colony would invade a baited colony when they discover that termites from the baited colony were weakened by a CSI and whether they would avoid tunnels filled with dead termites.

In their study, the researchers used arenas made of parallel sheets of plastic with sand between them where termites in different populations could build tunnels and interact with each other in conditions approximating those in the wild. They used four pairs of control arenas and four pairs of treatment arenas. Each pair of arenas was connected by a tube, and one arena in each pair of treatment arenas was baited with a CSI called noviflumruon.

In their study, Bernard et al observed Formosan subterranean termites from one colony (individuals dyed dark) biting individuals from another colony (individuals that are not dyed dark) in a tunnel.

Bernard and her colleagues observed that in pairs of control (i.e., un-baited) populations, termites from the two populations fought when they encountered each other, then blocked connecting tunnels to form stable inter-colony boundaries. In pairs of treatment populations, termites from baited populations were weakened and were not successful at blocking tunnels, which were then invaded by members of the un-baited populations. Within one month following connection of the treatment populations, un-baited populations had completely taken over baited populations. Within 67 days after connection, noviflumruon baits had killed all the invading termites.

“This confirms my initial notion that baits placed in a colony can be consumed by an invading colony and end up killing multiple colonies,” Su says. This is good news for designing effective control measures for Formosan subterranean termites.

In terms of invaders avoiding dead termites, Bernard says, “Occasionally, invading termites would dig new tunnels to avoid dead, but I was surprised that the number of new tunnels was insignificant compared with how many tunnels were unblocked and used freely by the invaders.”

This study provided other insights into inter-colony behavior. Colonies of this species can co-exist in soil, and they don’t usually invade other colonies. “What surprised me,” Su says, “was that when one colony is weakened by baits, and even before they are killed, it looks like the neighboring colony can sense this and begin to invade. How do they sense it? By odors, or by behavioral responses? We simply don’t know.”

Su adds, “We used to think that each colony has its own colony-recognition scent, but the result seems to suggest that, if they do, it does not deter the invading colony if the other is weakened by baits. Whatever colony odors that might exist did very little to discourage the invaders.”

Perhaps future investigation can determine how un-baited colonies detect when another colony is weakened. If this cue is a chemical, it might provide an additional way to heighten the inter-colony effectiveness of CSI baits.

John P. Roche is a science writer and author with a Ph.D. in the biological sciences. He has served as a senior scientist and adjunct professor at Boston College, as an editor-in-chief of periodicals at Indiana University and Boston College, and as a science writer at Indiana University and the University of Massachusetts Medical School. He has published more than 160 articles and has written and taught extensively about science. For more information, visit http://authorjohnproche.com/.