A second study into termite behaviour by the University of Florida looks at the interesting moulting preferences and behaviours of termites affected by IGR insecticides.
Researchers from the University of Florida have been investigating why chitin synthesis inhibitors (CSIs) are so effective in eliminating termite colonies. Little was known about the behaviour of bait-affected termites and the site of mortality of these workers within the colony – both key to understanding bait performance.
The first step in their investigation was to establish the moulting behaviour of termites under normal foraging conditions. It was observed that workers preparing to moult returned to the central nest to moult, preferentially moulting in the presence of eggs and reproductives (Kakkar et al., 2017). If this behaviour was to also be observed in bait-affected termites, it would certainly contribute to the efficacy of termite baits, as termites would return to the nest before dying.
Their follow up study (Kakkar et al., 2018) investigated whether termites that had ingested bait containing CSIs did indeed show the same behaviour.
The researchers set up an elaborate network of laboratory arenas to observe the behaviour. The study assessed the behaviour of six colonies of Coptotermes formosanus, each housed in a nest habitat that included a central nesting area as well as foraging sites connected to the central nest by 25 metres of coiled tubes. Three colonies were baited with cellulose pellets containing the CSI noviflumuron and three were baited with untreated cellulose. Both treatment and control baits were dyed blue so that the researchers could observe the termites that had eaten the bait.
In the untreated colonies, all moulting happened in the central nest, just as in the previous study. Workers travelled from the foraging site to the central nest for moulting and stayed inside, or in the close vicinity, until at least 36 hours after moulting.
In the CSI-treated colonies, the baited termites also returned to the central nest to attempt to moult, but the unaffected termites in the colony exhibited an interesting additional behaviour: as CSI-affected termites died in their attempts to moult near the colonies’ eggs and reproductives, the workers tending to the king, queen and brood continually relocated from one site to another, to escape the dead and dying termites. Presumably this was an attempt to keep the reproductives and developing termites healthy to ensure colony survival. Ultimately it was a futile exercise, with the three treated colonies collapsing after 60 days.
So what does the research mean for pest managers? It means that unless the central nest resides close to the bait station, death and the accumulation of dead termites will always happen away from the bait station. (The accumulation of dead termites near a bait station could result in avoidance of that feeding site). The mode of action of CSIs and the moulting behaviour of termites therefore ensures affected colonies will continue to feed, ensuring a lethal dose is consumed and spread within the colony.
With the affected termites dying in the central nest, it creates significant hygiene problems for the colony. Mass death of significant numbers of workers would give rise to microbial growth, potentially responsible for the rapid colony collapse seen during some baiting treatments.
The researchers posed another question: could adding chemicals to the bait matrix to reduce the time to the next moult – meaning the termites would return to the nest more quickly, resulting in even quicker elimination of the colony – without causing termites to die near the bait stations?
Initial studies have indicated that the combination of moult-accelerating compounds (MACs) with CSIs in baits deliver faster control than baits using only CSIs or MACs. However, although significantly faster, the termites would still have time to return to the central nest to moult.