Moulting is the trigger for activating chitin synthesis inhibitor baits. But what do we know about the moulting process itself?
Little is known about moulting in subterranean termites, yet it is an important aspect of the control of termites through the use of chitin synthesis inhibitor baits e.g. chlorfluazuron, novaluron, and hexaflumuron. What we do know is that the moulting process involves the formation of a new exoskeleton and the shedding of the old outer cuticle (known as ecdysis), a process that is triggered by the release of certain hormones.
Termites produce a number of important developmental hormones, of which hydroxyecdysone is the primary moulting hormone. An increase of this hormone leads to moulting, while juvenile hormones maintain insects in their existing form.
In most insects, the first visible sign of moulting occurs when the old exoskeleton starts to separate from the underlying epidermis (known as apolysis). In termites, the first indication appears when the workers stop foraging, which is around ten days before moulting. A few days after they stop foraging, they will void their gut contents. This voiding of the gut fauna is likely stimulated by the high levels of juvenile hormone (which in Coptotermes formosanus has been recorded about six days before moulting).
About one day before moulting, the workers become listless, and their head capsules may take on a speckled look. When they finally shed their exoskeleton, they usually need help from other workers. The ‘reward’ for providing this help is that the workers get to eat the shed tissues. Observations with Coptotermes formosanus indicate that around 1% of the worker termites in a colony moult each day.1
Wood-feeding lower termites greatly depend on symbiotic gut protozoa and bacteria for the digestion of cellulose, meaning that once they have moulted, they need to reacquire their gut fauna from other termites. This happens about three days after moulting. It occurs through proctodeal feeding (feeding on excrement or anal fluids) on non-moulting workers. The usually large flagellate protozoa are consumed in their smaller encysted format, which then grow into the normal size and form. It is likely that transfer in this manner evolved to allow the protozoa to avoid the termite’s foregut, where wood is broken down into small particles, and where they might also be damaged. In Coptotermes formosanus it has been shown to take over a week for the gut fauna levels to return to normal.
From an evolutionary perspective, the wood cockroach Cryptocercus punctulatus, which is one of the earliest Blattodae ancestors of termites, displays a similar process. After moulting, once the cockroaches start to feed again, these protozoa are transferred as encysted cells by proctodeal feeding on other cockroaches in the community.
With only 1% of termite workers actually moulting each day, it would seem that it would take around three months for the entire colony to moult. This potentially impacts the effectiveness of the chitin synthesis inhibitor termite baits previously noted, since they only prevent new exoskeleton production.
However, it is likely with younger workers present that the moulting frequency will be greater than 1%. Studies with Requiem Termite Bait performed by Dr Michael Lenz (CSIRO Australia, unpublished) have shown that a key impact of the termite baiting process is that once a large number of termites are impacted by the bait, bacterial disease can rapidly spread through the colony leading to its demise.
Work by Rojas and Morales-Ramos (US Dept of Agriculture) showed that chlorfluazuron also impacts on termite health in several ways. Firstly by preventing egg-hatching, which in turn reduces colony viability; secondly by breaking down the peritrophic membrane, which makes it impossible for the termites to process their food, leading to sickness and death; and thirdly by breaking down their malpighian tubules, required for excreting wastes, again placing great stress on the termites. All of these aspects, combined with the inhibition of chitin production, advance the colony elimination process.
Steve Broadbent, Regional Director, Ensystex Australasia
1 A Rainaa, YI Parka, D Gelman. 2008. Journal of Insect Physiology 54:155–16, Molting in workers of the Formosan subterranean termite Coptotermes formosanus.