Short Bait Exposure Controls Asian Subterranean Termite Colonies
Termites cause huge economic costs to society—as much as $40 billion per year worldwide. In the southeastern United States, two invasive species, the Formosan subterranean termite (Coptotermes formosanus) and the Asian subterranean termite (Coptotermes gestroi) are causing increasing economic losses.
Successful management of subterranean termites depends in part on eliminating colonies, because colony elimination reduces the number of termites locally and also reduces the production and dispersion of winged termites that form new colonies in surrounding areas. An effective tool for eliminating colonies is a bait containing a type of insecticide called a chitin synthesis inhibitor (CSI). A key question in the optimization of termite control with CSI baits: How long do termites have to feed on the bait to acquire a lethal dose that will eliminate a colony? The answer could increase the effectiveness of termite control, but determining the lethal dose is difficult.
Testing lethal CSI doses in the field is challenging because the size of termite colonies is unknown, and lab studies have limited applicability to colonies in the wild if they use small numbers of termites. University of Florida entomologists Thomas Chouvenc, Ph.D., and Nan-Yao Su, Ph.D., overcame these limitations by studying the effect of a CSI on large colonies but in controlled laboratory conditions where the number of individuals was known. Their results were published this month in the Journal of Economic Entomology.
Understanding the effectiveness of CSIs is important to termite management for several reasons. CSIs affect termites by inhibiting the synthesis of chitin, the main component of insect exoskeletons. This disrupts termite molting, leading to death. CSIs are used in a dry bait, as opposed to the wet formulations used with some other termite insecticides. The dry baits offer the advantage of being less damaging to the environment, because only the termites that feed on baits are affected. And the effectiveness of CSIs is heightened because, when termites are killed by the insecticide, other termites consume the dead termites, exposing them to the CSI for secondary toxicity. Their effect is further increased because CSIs are spread when termites share regurgitated fluids. Also, CSIs offer the advantage of not causing something called secondary repellency, when termites avoid the immediate area around a bait because dead termites surround the bait. With CSIs, mortality is delayed, so termites leave the immediate vicinity of the bait and go back to the central nest to molt. An additional advantage is that CSIs reduce the need for liquid insecticides, which can be more damaging to the environment.
In their study, Chouvenc and Su created more than 100 termite colonies from single mating pairs. Then they chose nine colonies at random to serve as manipulated treatment colonies and chose five colonies at random to serve as unmanipulated controls. Of the nine treatment colonies, three were exposed to CSI insecticide baits for one day, three were exposed to baits for three days, and three were exposed to baits for 10 days. The investigators added blocks of wood to each control colony instead of CSI baits. They left all the colonies alone for 80 days, and then opened each and counted the termites. They counted the termites again at 90 days.
The results were compelling. After 80 days, one colony in the one-day group was eliminated, two colonies in the three-day group were eliminated, and all three colonies in the 10-day group were eliminated. By 90 days, all termites in all treatment groups were eliminated. That is, the 90-day effectiveness of the baits was 100 percent, even in colonies only exposed for one day. In contrast, all control colonies survived. And Chouvenc points out that the amount of damage the termites would be causing was reduced long before 90 days. “Within a month after feeding on the bait,” Chouvenc says, “termites become sluggish and slowed down their feeding activity. Therefore, even if the termites were still alive, they would not be causing much damage at that point.”
Results from Chouvenc and Su’s study has applications to control of Asian subterranean termite colonies in the field, and to the control of other termite species, including Formosan subterranean termites. “For two reasons, this study supports the use of baits for control of subterranean termite colonies, especially for aggressive species such as the Formosan subterranean termite and the Asian subterranean termite,” Chouvenc says. “First, the time frame for colony elimination is much shorter that we previously thought. Just one day after starting feeding on baits, the termite colony is doomed. Second, we proved that the amount of active ingredient required to kill a colony is ridiculously small.”
The bait affects the colony in several ways:
- It eliminates individuals.
- It decreases the amount of food gathered by the colony.
- It stops the life cycle of the termite colony and prevents further production of winged termites.
Three temporal considerations are also related to the effectiveness of baits: how long it takes for the colony to find the bait, how long for the colony to take in a lethal dose, and how long for individuals to be affected by the bait. Because CSIs disrupt the molting process, the amount of time for individuals to be affected by the bait is determined by the time of molting.
The present study determined that the amount of time for the colony to take in a lethal dose is one day or less. Chouvenc and Su suggest that future research for the application of remedial treatment should focus on the amount of time it takes for the colony to find the bait and the amount of time for individuals to be affected by the bait. Chouvenc says, “A couple of students in our lab are currently looking into ways to speed up the molting process, so that the colony would collapse faster.” Their research will help shape control efforts for these economically damaging invasive species.
Journal of Economic Entomology
John P. Roche, Ph.D., is an author, biologist, and science writer dedicated to making rigorous science clear and accessible. He has a Ph.D. in biology, has published more than 180 articles, and has written and taught extensively about science. For more information, visit http://authorjohnproche.com.