Do you really know how termite baits work? Read our summary of the work from leading US termite researcher Thomas Chouvenc.
The first commercial termite baits based on chitin synthesis inhibitors (CSIs) were launched in 1995. However, their acceptance by the industry as a mainstream termite control method took time and even now, termite baits are not utilised by a number of pest control companies. Part of the reason for this is a lack of understanding as to how termite baits actually work, which has led to misconceptions about their performance and uncertainty as to how to best utilise this unique management tool.
Leading termite researcher Assoc. Prof. Thomas Chouvenc from the University of Florida has written a comprehensive overview summarising the latest research on termite baits. His review ‘How Do Termite Baits Work?’, published in the Journal of Economic Entomology in October 2024, highlights how CSI baits utilise termites’ inherent colony demography, behaviour, and physiology, which gives rise to colony collapse through a characteristic succession of events, leading to colony elimination. Here we have a look at the key learnings from this work.
Termite baits are particularly effective on the pest species in the Heterotermitidae, which includes the Coptotermes, Reticulitermes, and Heterotermes genera, for reasons which will be explained later. The paper therefore uses Coptotermes as the primary model for explaining the mode of action of termite baits.
How do baits affect the individual termite?
After the ingestion of termite bait, there is no immediate impact on the termite’s physiology or behaviour. Its lethal time effect is dose-independent – if the termite eats more bait, it doesn’t die any quicker. The time to death is related to the moulting cycle of the individual termite. Workers within the Heterotermitidae moult periodically throughout their life and CSIs impact the moulting process by preventing the production of chitin, an essential component of the termite’s cuticle (Figure 1). As a result, a termite that has ingested termite bait dies when it tries to moult. Without a replacement cuticle, it dies a rapid death through loss of haemolymph during the attempted moult, often dying in a “jack-knifed” position.
In contrast, the workers of termitids (‘higher termites’) go through a series of moults until they reach a final state after which they no longer moult for the remainder of their life. As such, CSI baits do not impact these mature workers and this is the key reason why termite baits have limited impact or take longer to eliminate nests of termitid termites (which includes Nasutitermes, Microcerotermes and Macrotermes).
An important recent learning about the moulting process is that the termite returns to the central part of the nest to moult. This behaviour is integral to the colony elimination process.
How do baits lead to colony death?
The foragers (older workers) feed on the termite bait (Figure 2). Although they form typically only around 5% of workers, within a couple of days of feeding, the foragers have ingested enough bait to deliver colony elimination. This occurs from the CSI being spread through the colony via trophallaxis.
Within the first couple of weeks, the first affected workers return to the central nest and die during the moulting process. These dead workers are cannibalised, spreading the CSI further (secondary spread).
At this point the queen will have been impacted by the CSI, whether through trophallaxis or the nutrient recycling from the dead workers. Although the queen doesn’t die (as she doesn’t moult), the CSI does impact egg development. With the queen unable to lay viable eggs, population recovery is now no longer possible.
Around the same time, there is mass death of the brood, due to their fast moulting cycle. This occurs around 3-4 weeks after first feeding on the bait. At this point, the colony is doomed.
The youngest cohorts of workers return to the central nest and start dying around the 5-6 week mark. Dealing with this mass death is beyond the cannibalistic processing capacity of the colony and the rapid build-up of cadavers causes the abandonment of the central nest by the royal pair.
As the older cohorts of workers die, the worker to soldier ratio changes (as the soldiers don’t moult). At this point, around the 7-8 week mark, most feeding activity has ceased. Within three months, the colony is dead. Without workers to provide food, the soldiers and royal pair die of starvation.
Essentially the ingestion of CSI bait speeds up the colony ageing process. Typically, as queens age and their reproductive capacity declines, the population within the colony ages as the number of eggs produced by the queen and therefore the number of new workers declines over time. This period of senescence can take up to three years before the colony eventually dies. With colonies affected by CSI baits, it takes around three months.
Factors affecting the speed of colony elimination by baits
Although the Coptotermes model indicates a typical timeline of three months, from initial ingestion to colony elimination, a number of factors are proposed that impact the speed of action. The key factors being seasonality/weather, type of CSI, colony size and colony structure.
Seasonality can impact level of feeding and rate of moulting. For example, in geographies which experience colder winters, feeding levels and moulting rates decrease, lengthening the time taken to achieve colony elimination. Reduced levels of precipitation can also reduce hit rates of bait discovery, as well as decrease levels of feeding.
It shouldn’t be assumed that a particular CSI bait performs the same on all species nor that all CSIs deliver the same level of performance. For example, it has been suggested that some CSIs, such as bistrifluron, could also impact the peritrophic membrane in the gut of affected individuals. As a result, the termites would die of starvation rather than through moulting, which could deliver colony control in as little as one month. However, the data set of different CSIs on the wide number of termite species is very limited.
Colony size and health will also impact the time to colony death. Colonies that are small or unhealthy are likely to succumb quicker than larger colonies.
Colony structure can also impact speed, or apparent speed, to colony elimination. CSI baits are also effective on Rhinotermitidae, such as Schedorhinotermes species. However, unlike Coptotermes which have a single central nest, Schedorhinotermes species often have connected satellite nests as part of the same colony. As a result, when Schedorhinotermes feed on a CSI bait, although it will eliminate the nest that has fed on the bait in much the same timescale as Coptotermes, it is always possible that a related Schedorhinotermes satellite nest may move in to feed on the same bait station, giving the impression that the bait hasn’t worked or is taking longer than expected to have an impact. A similar observation may occur in areas of high termite colony density when termites from an unrelated colony can move into the workings of the deceased colony.
With this increased understanding of how termite baits work, this recently published review provides a clear explanation as to why termite baits are the proven method to deliver reliable colony elimination. This understanding not only allows pest managers to utilise this effective tool for the appropriate species and situation, but allows them to provide suitable explanations to manage customer expectations.
Further reading, original paper available in open access: Chouvenc, T (2024). How do termite baits work? Implication of subterranean termite colony demography on the successful implementation of baits. Journal of Economic Entomology, toae243, https://doi.org/10.1093/jee/toae243
Xing, L., Chouvenc, T. and Su, N.Y. (2014). Behavioral and histological changes in the Formosan subterranean termite (Isoptera: Rhinotermitidae) induced by the chitin synthesis inhibitor noviflumuron. Journal of Economic Entomology, 107(2), pp.741-747.
