Dr Thomas Chouvenc, Assistant Professor at the Ft Lauderdale Research and Education Center of the University of Florida, shares his insights into the role of CSI baits in the battle against Coptotermes globally.
Subterranean termites present particularly difficult challenges for the pest control industry around the world owing to their ability to forage long distances underground and also to their capacity to establish large colonies with high destructive potential.
A handful of subterranean termite genera represent the bulk of the estimated US$40B annual cost of termites worldwide (Rust and Su, 2012), and among them, species within the Coptotermes-Reticulitermes-Heterotermes complex are the primary actors responsible for structural damage (Chouvenc et al., 2021), among a few other notable termite pest genera.
Termite pest control research over the past seven decades reflects the major pest status of these three termite genera, as more than 80% of the extensive scientific literature published on subterranean termites focused on them. Also, without neglecting many other important termite pests, it can be argued that many species within Coptotermes are among the most problematic and most difficult termite pests to control.
Coptotermes, a global pest
The genus Coptotermes has an estimated 21 valid described species around the world and most of them — if not all — have pest status in their respective geographical distributions (Chouvenc et al., 2016). Suspected native distribution of these species includes three in Africa, six in Australia, one in central-south America, and 11 in East Asia, Southeast Asia and Oceania. Remarkably, Coptotermes is internationally known as an important invader (Evans, 2021), but only two species are actually known to have major invasive capabilities: Coptotermes formosanus (the Formosan subterranean termite) and Coptotermes gestroi (the Asian subterranean termite). Other Coptotermes species are also known to have spread beyond their initial native range, but not at the scale that C. formosanus and C. gestroi have.
Owing to their ability to be inconspicuously transported by human activity, many urban localities in tropical and subtropical areas around the world are increasingly at risk of potential damage. As a result, Coptotermes has received many nicknames depending on location, as it is known for its unique ability to infest the centre of live trees (tree-piping termites), to infest structures with extensive damage potential (house termites, super termites) and to produce defensive secretions from its soldiers’ frontal gland (milk termites). With such a large impact around the world, termite control companies remain a fundamental element of the global pest management industry and unambiguously, Coptotermes has forced termite control practices to change and adapt over the past few decades.
The evolution of chemical control methods
In a not-so-distant past, subterranean termite control primarily relied on organochlorine chemicals such as chlordane, heptachlor and DDT as soil termiticides, but all were eventually phased out in most countries because of their bioaccumulation in the environment and their long-lasting ecological impact. By the mid 1990s, newer chemicals with relatively short half-lives were implemented as soil termiticide solutions to create a temporary chemical barrier around structures. However, alternative termite control strategies also emerged in this time frame, one of them being chitin synthesis inhibitor (CSI) bait formulations. In the past 30 years, subterranean termite baits have become a critical tool against Coptotermes infestations throughout the world, and several active ingredients and formulations have emerged on various international markets.
Despite the established success of CSI baits against Coptotermes, their use is still not universal because of several factors. First, CSI baits are not always accessible or affordable in some parts of the world. Second, their use is not always accepted by pest control companies who prefer to rely on traditional liquid termiticide protocols. Third, the efficacy of different formulations and active ingredients can vary across manufactured products and target species, resulting in confusion in the industry. As a result, the use of liquid termiticides to fight Coptotermes infestations has remained dominant in many regions of the world.
Nowadays, many liquid termiticides products are using non-repellent formulations where their implementation results in a chemical treated zone around structures, to protect them from termite attack. It was initially thought that the non-repellency of these liquid termiticide formulations would allow for the transfer of the active ingredient to the rest of all individuals within a termite colony.
However, it has since been demonstrated that their lethal-time dose dependency properties (the more termites are exposed to it, the faster they die) result in rapid accumulation of dead individuals within three metres from the treatment, leading to the emergence of secondary repellency (Su, 2005). Such properties ultimately prevent further exposure for the rest of colony that may then continue to forage in non-treated areas. Therefore, these liquid termiticide treatments often fail to fully eliminate colonies, as demonstrated in C. formosanus and C. gestroi (Chouvenc, 2018). Instead, they provide a temporary chemical exclusion around a treated structure, until time breaks down the barrier, or until Coptotermes colonies can eventually find their way around it.
Colony elimination using CSI baits
The use of CSI termite baits presents a fundamentally different approach to subterranean termite control. The goal of CSI baits is colony elimination, and it relies on two termite features: their inherent moulting physiology and their trophallactic behaviour (food sharing). Let’s here review some basic aspects of Coptotermes biology to provide some context (Chouvenc and Su, 2014).
A subterranean termite colony represents a social family unit with a primary pair of reproductives (queen and king), a brood, (eggs and larvae) and sterile helper castes (workers and soldiers) which are relatively long-lived individuals but are maintained in a juvenile morphology throughout their lives.
As a result, termite workers must moult regularly in order to renew their ageing cuticle and replace their dull mandibles with a new pair of sharp ones (Xing et al., 2013). In addition, workers are the only individuals in the colony that are able to feed on wood, while all other castes rely on workers to be fed and to be taken care of. Termite workers continue to periodically moult throughout their lives until they die of senescence (old age). However, it was observed that workers do not moult at foraging sites due to lack of care and protection, but instead return to the central part of the nest, the location of the royal pair and the brood (Kakkar et al., 2018) to be cared for by young workers during the moulting process. (Su et al., 2017).
The formulation inside CSI bait stations can be consumed by foraging workers, which are part of the older foraging demography (Su et al., 2017). It was determined that less than 5% of the foraging workers within a large colony are required to ingest sufficient active ingredient from a single bait station to kill the colony (Gordon et al., 2022). This is because the consumed bait is shared with the rest of the colony through trophallactic exchanges. CSI baits interfere with the production of chitin, which is a structural compound that is part of the insects’ cuticle. Therefore, bait consumption does not directly affect termites, but only becomes activated when each individual termite engages in their periodic moult. As a result, CSI baits possess lethal-time dose properties, which result in a delayed mortality, allowing the active ingredient to spread to all individuals prior to the onset of mortality from asynchronous moulting. As the larvae, the queen and king are fed by such active workers, they are therefore also rapidly exposed to the CSI bait.
Once CSI-exposed workers engage in their periodic moulting event near the central part of the nest, the CSI interferes with the formation of the new cuticle, leading such individuals to bleed to death (Xing et al., 2014), and be rapidly consumed by nestmates. Such cannibalism results in secondary toxicity among the brood, young workers, the queen and king. Interestingly, because it only takes a couple of weeks for larvae to moult to the next instar compared to up to several months for workers, larvae die early within the baiting process, even before most workers start being affected by the bait. In addition, subterranean termite queens of baited colonies rapidly lose their ability to lay viable eggs, preventing colonies rebounding once exposed to CSI baits (Chouvenc and Lee, 2021). As all workers progressively die, the soldiers, queen and king ultimately die from starvation, inevitably leading to colony elimination within three months, even with a minute amount of active ingredient having been ingested (Chouvenc and Su, 2017, Chouvenc, 2018, Gordon et al., 2022).
Therefore, some of the recent termite research about CSI baits allows for the reconstitution of the mechanistic processes chronologically involved in subterranean termite colony elimination in the field with a CSI bait approach.
- Foragers (relatively old workers within the colony) feed on the bait formulation.
- They bring it back to the central part of the nest and share it with the rest of the colony by trophallaxis, resulting in the queen, king and larvae being exposed to the active ingredient.
- Early worker mortality further contributes to the secondary spread of the active ingredient at the central part of the nest.
- The queen becomes unable to lay viable eggs, and all larvae die in the process of moulting. At this point, very few workers are impacted by the CSI, as moulting has yet to occur for them. This implies that although foraging activity is maintained by workers, the colony has already lost its entire brood.
- After 45 days, workers cease feeding altogether and progressively start dying in mass numbers around the central part of the nest, which often forces the queen and king to relocate several times to different areas, as dying termites keep accumulating in their direct surroundings. By day 80, only old workers remain, unhealthy and about to die.
- Finally, all soldiers then die from starvation between 80 days and 90 days, at which point the queen, who has lost more than half of her biomass, and the king, eventually starve to death, as the colony ultimately collapses.
It is now known that CSI baits work at the colony level for Coptotermes. Even after a fraction of the colony has fed for a short time on a CSI bait formulation, the colony has already reached a point of no return toward colony elimination. Within a month, it has lost its ability to recover the upcoming loss of its workforce because the brood has already been wiped out and population replacement functions have already been terminated. What is remarkable is that as long as a subterranean termite colony is able to feed just a few milligrams of active ingredient from a small fraction of foragers, colony elimination is ensured, confirming that subterranean termite CSI bait technologies, which use a minimal amount of highly target-specific active ingredients, remain a cost-effective pest control solution with a virtual absence of unintended impact on the environment.
Strategic bait placement
With termite bait technology improvements over the past three decades, in-ground (IG) bait stations can now be used both for preventive and remedial protocols, as the stations with active formulations can be placed around a structure to be protected. However, many Coptotermes species often start colonies in trees and only reach structures after many years of colony growth within these trees. While inspections of structures for active Coptotermes infestation can be a routine protocol, inspection for signs of termite activity in surrounding trees can reveal the presence of Coptotermes in the vicinity of a structure even before any structural damage may occur. Expanding inspections to surrounding trees can therefore provide a unique opportunity to install above-ground (AG) baits directly on active infestations.
While in-ground bait stations depend on the termite foraging behaviour to find them, above-ground bait stations (only used a remedial treatment) can allow for immediate feeding on a CSI formulation by the colony. The combination of routine inspection of trees and the use of above-ground baits on a detected infestation can therefore have major practical consequence in sustainable structural protection, beyond what the use of in-ground stations can provide.
- Save the tree from an accumulation of catastrophic damage,
- Eliminate the colony early to prevent potential structural damage,
- Prevent the colony from participating in further dispersal flights to make more colonies in the direct surrounding environment.
The future of CSI baits
As new formulations, new protocols, and maybe new CSI active ingredients will continue to reach various markets in the decades to come, it is widely expected that termite CSI baits will continue to become more accessible and versatile in their use against various Coptotermes pest species. Although CSI baits have yet to be used universally, it is expected that the industry will progressively reduce its reliance on liquid termiticides as bait technologies continue to improve. With the shift toward the use of baits, the pest control industry also needs to adapt protocols to such new tools, where inspection for active infestations can turn into a golden opportunity to apply an above-ground bait formulation for an immediate initiation of the colony elimination process.
Ironically, while baits have been criticised in the past as very technical products to use, such views no longer apply. Once familiar with the protocols, a trained technician can visually detect an active Coptotermes infestation (in trees or structures) and install an aboveground bait station on the infestation in less than five minutes to initiate immediate feeding toward colony elimination. The initial labour-intensive hurdles that first came with the very first bait products no longer apply when taking into account the series of innovations that happened since their first introduction in the market.
Arguably, many now see CSI baits as the most cost-effective solution to protect against Coptotermes damage, and it may only be a matter of time before their use become a universal norm.
Assistant Professor in Urban Entomology at the Ft Lauderdale Research and Education Center of the University of Florida Institute of Food and Agricultural Sciences, Florida, USA
Dr Thomas Chouvenc is an Assistant Professor in Urban Entomology at the Fort Lauderdale Research and Education Center of the University of Florida in the US. His research focuses on subterranean termite biology, with a particular interest in the field of ecology, symbiosis, evolution, behaviour and control of termites.
Among some of his recent work, Dr Chouvenc investigated the impact of chitin synthesis inhibitor baits on subterranean termites at the colony level, documented the important role of access to dietary nitrogen from soils in Coptotermes, and characterised the complex demographic dynamics of termite colonies over several decades. Dr Chouvenc is also the coordinator of the University of Florida School of Structural Fumigation, and the organiser of the International Termite Course for Academics and the Termite Course for Professionals.
All images courtesy of Thomas Chouvenc/UF-IFAS.
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Chouvenc T, Su NY. 2017. Subterranean termites feeding on CSI baits for a short duration still results in colony elimination. Journal of Economic Entomology 110: 2534-2538.
Chouvenc T, Lee SB. 2021. Queen egg laying and egg hatching abilities are hindered in subterranean termite colonies when exposed to a chitin synthesis inhibitor bait formulation. Journal of Economic Entomology 114: 2466-2472.
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