Pest management professionals often use an insecticide known as a chitin synthesis inhibitor (CSI) to eliminate colonies of the Formosan subterranean termite (Coptotermes formosanus). A recent study at the University of Florida found that CSI baits have no effect on the termites’ natural compulsion to return to their central nest to molt, an important factor in the efficacy of such baits. (Photo credit: Gerald J. Lenhard, Louisiana State University, Bugwood.org)

Last fall, as shared here on Entomology Today, researchers at the University of Florida reported their discovery that worker-caste Formosan subterranean termites (Coptotermes formosanus) have a strong urge to return to their central nest when it’s time to molt, near the colony’s reproductives (king and queen) and their eggs.

This was a promising finding for the prospects of termite pest management, because it suggested that termites baited with an insecticide known as a chitin synthesis inhibitor (CSI) would not fall dead right at the bait station, which would then deter other termites from coming near it. But the entomologists behind this research knew they needed to study the termites’ behavior further to confirm this hypothesis.

In January, results of that next study were published in Scientific Reports, and the work confirmed that the CSI bait has no effect on the termites’ “molting side fidelity.” In other words, termites baited with a CSI return to their central nest to molt just as they do otherwise. And that’s where the baited termites die, as the CSI interferes with their molting process.

Garima Kakkar, Ph.D., (lead author on the study), University of Florida colleague Nan-Yao Su, Ph.D., and Weste Osbrink, Ph.D., of the U.S. Department of Agriculture’s Agricultural Research Service, conducted the experiment, in which they observed six colonies of C. formosanus each housed in nest habitat that included both a central nesting area as well as foraging sites connected to the central nest by 25 meters of coiled tubes. Three colonies were baited with cellulose pellets containing the CSI noviflumuron and three were baited with untreated cellulose. Both treatments, however, were dyed blue so that the researchers could observe the baited termites, which took on a bluish color after consuming the bait, and compare their activity.

To test whether Formosan subterranean termites (Coptotermes formosanus) baited with a chitin synthesis inhibitor would return to their central nest molt, researchers at the University of Florida built an extended foraging arena composed of six planar arenas (24 x 24 x 1.4 centimeters) filled with moistened sand and connected to each other by a 5 meter-long coiled Tygon tubing to form a linear distance of 25 meters. In the figure, “RC” (at right) signifies release chamber and “TC” (left) signifies treatment chamber. (Image originally published in Kakkar et al 2018, Scientific Reports, CC BY 4.0)

In the untreated colonies, all molting happened in the central nest, just as in the 2017 study. In the CSI-treated colonies, the baited termites also returned to the central nest to attempt to molt, but the termites exhibited an interesting additional behavior: As CSI-affected termites died in their attempts to molt near the colonies’ eggs and reproductives, the workers that tend to the king and queen and brood picked them all up and moved to a different location. And they did this over and over.

“The reproductives were chased by dead termites and at the same time distributed the corpses at various locations in the gallery system as they moved,” the researchers write in their report.

Another recent study at the University of Florida found a C. formosanus colony could be eliminated within 90 days with just one day of exposure to a CSI bait. When combined with a molt-accelerating compound, colony elimination could be sped up further yet, which Kakkar and colleagues say is supported by the consistency of termites’ compulsion to molt in their central nest.

“The ability of workers with an acquired lethal dose of noviflumuron reaching the nest before molting incidence implies that upon adding molt inducing chemicals to induce early molting, regardless of how fast it is, there is a scope to reduce the lethal time for colony elimination without any bait station aversion,” they write.