Dr Theodore Evans, Associate Professor at the University of Western Australia in Perth, shares an insightful overview of how today’s pest termite species came to dominate the urban environment.
Summary
There are some similarities and differences between the pest termites found in cities in Asia and Australia. A similarity: Coptotermes species are the most common urban pests. A difference: Coptotermes are not pests in tropical Australian cities, instead Mastotermes are.
However idiosyncratic, these differences arise from the same ecological and evolutionary patterns that arise when natural forests are cleared for urban development, and the dominant termite species disappear with them. This allows subordinate termite species to thrive, as they can survive the disturbed, hotter and drier conditions found in cites. However, Coptotermes may not remain the major pests in cities. Baiting appears to be most effective against central site nesting species, including Coptotermes; and there is an emerging pattern of fewer Coptotermes infestations in some parts of Australia. Multi-site nesting species, such as Schedorhinotermes and Heterotermes, may become the most common pest termites in cities in the future in Australia, and perhaps in Asia as well.
Pest termite species in Asian and Australian cities
In the cities of Asia and Australia, the most common pest termites are various Coptotermes species native to the area. Indeed they are more than common; Coptotermes species are usually responsible for 90% or more of infestations in urban buildings across Southeast Asia and Australia.
The other 10% or so of infestations are of other pest termite species. In tropical south and southeast Asian cities, there are pest species of Schedorhinotermes, Microtermes, Odontotermes, and Microcerotermes; these are usually found in the outer suburban and semi-rural areas away from the urban core. In temperate Australian cities, there are pest species of Schedorhinotermes, Heterotermes, and Nasutitermes; the latter more common away from the urban core.
There is a perception that Coptotermes is the ubiquitous pest termite genus in cities. Perhaps because Coptotermes species are so common in cities and often cause higher damage than other pest termite species (perhaps also because Coptotermes formosanus has received so much attention as it invades cities in the US). However, this is not always the case. In direct contrast to the previous examples, in tropical Australian cities, Coptotermes are not pest termites. Instead Mastotermes and Schedorhinotermes are the most common pest species, with some Heterotermes and Microcerotermes also posing a threat.
Although these similarities and differences between pest termite species in different areas seem idiosyncratic, there are consistent factors that explain them. These factors have arisen from evolutionary history and ecology of termite communities, and how urbanisation has changed those communities.
Natural termite communities in forests
In the forests of south and southeast Asia, wood-eating termite communities in natural forests are dominated by the ‘higher’ termites (i.e. those without symbiotic gut protozoa), especially the fungus-growing subfamily Macrotermitinae (Macrotermes, Odontotermes and Microtermes), also the glue-gun soldier subfamily Nasutiterminae (Bulbitermes), and the broad subfamily Termitinae (Prohamitermes, Globitermes and Microcerotermes).
The dominant wood-eating termites in Asian forests are the fungus-growing termites, especially species of Macrotermes, then Odontotermes. Species of Macrotermes are large bodied, often double or triple the size of other termites. Their colonies are populous as well, with estimates of them being two to ten times higher than other species. Odontotermes are smaller bodied and less populous than Macrotermes, but larger and more numerous than other termites. These termites are able to suppress their competitors. Coptotermes, Heterotermes and Schedorhinotermes are found in forests, but rarely (5% of samples), usually living opportunistically between their larger rivals.
In the forests of Australia, wood-eating termite communities are dominated by Coptotermes. This is due to Australia’s isolation as an island continent. For millions of years only primitive termites lived in Australia, such as the dinosaur termite Mastotermes darwiniensis. Then around 15 million years ago Australia had drifted close enough to Asia for termites to colonise, rafting across the ocean likely in floating tree trunks; Coptotermes was among the first group to arrive. They had escaped their old rivals, the fungus-growing Macrotermitinae, because these termites, with their fungal symbionts, are not able to raft. Without Macrotermes, Coptotermes spread and diversified into various ecological niches across the island continent.
Some Australian Coptotermes species evolved the ability to build mound nests, the only ‘lower’ termites to do so. All other mound-building termites are higher termites, typified by the mounds of various Macrotermes in Asia. Mound building is a behaviour of ecologically dominant termites; only with populous colonies can they monopolise food resources and so have the capacity to invest in these enormous and expensive structures. Other termite species are restricted to areas between the Coptotermes colonies (ca. 20% of samples), and usually target less preferred foods.
Changes to forests means changes to termite communities
Cutting trees and clearing forests is enormously disturbing, as it changes the habitat, both for the tree species and every organism that depends on them. Resources change, including food, soil and nesting sites, as does the climate. These changes affect termites as much as other groups of animals.
Although most people think of termites as voracious and indiscriminate eaters, this is not the case; termites can be fussy. Each termite species has preferred wood species, dependent on hardness and density of the wood fibres, and natural protective chemicals. Other factors influence preferences, such as position (under, on or above ground), moisture levels, and attack by fungi and other microorganisms. Some termites prefer their wood to be above ground, dry and microbe free (e.g. Cryptotermes drywood termites); others prefer wet, rotted and below or on-ground (e.g. Porotermes, Zootermopsis). There are many ecological niches along the continuum between these extremes; almost all disappear with the clearing of the forest.
The soil changes with clearing. There is physical disturbance of the soil: roots can be upturned; humus and microbe-rich topsoil buried, and mineral-rich subsoil exposed. Heavy machinery and the movement of tree trunks compact the soil. These changes are usually increased with urban construction: roads and buildings need firm foundations.
The loss of tree canopy changes the climate. Without the canopy there is more variation in temperature. During the day there is greater sun exposure to heat the soil, whereas at night the heat radiates to the sky faster. Overall, the average temperature is higher. Rainfall may decrease, as trees release volatile chemicals into the atmosphere that help to seed rain. When rain does fall, it runs off quickly due to the lack of roots and soil compaction. Thus soils are drier.
Consequently there is a large drop in the number of termite species. The general pattern is that soil-feeding termites disappear, even with relatively little tree felling and disturbance. Termites that use highly rotted wood or leaf litter decline, disappearing as tree felling and disturbance increases. Termite species with large colonies vanish, as the food and water resources are no longer sufficient to support their needs.
Thus the weedy, subordinate species survive: in Asia this means mostly Coptotermes and Schedorhinotermes, and in Australia Mastotermes (in the tropical north), Heterotermes and Schedorhinotermes. Without the ecologically dominant termites to limit these weedy species, their populations grow.
Pest termites are survivors in urban conditions
Urban environments are harsh, from the perspective of most termite species. They are hot and dry, with low wood quantity per hectare, and low tree diversity (street trees compared with forest trees). Most termites, including the ecologically dominant termites have insufficient resources to survive. Even if they could scrape together enough food, as Macrotermes sometimes do in semi-rural areas, their mounds are easy to see and destroy.
Coptotermes can survive well in urban conditions. They evolved to live inside tree trunks, likely so they avoid their superior rivals, Macrotermes and Odonotermes. Consequently, Coptotermes mostly eat sound heartwood. They like damp and slightly rotted heartwood, because this makes chewing and digesting the heartwood easier, however they can live on sound heartwood. They can survive hotter and drier conditions as well. These conditions are found in agricultural systems, such as oil palm plantations in Southeast Asia, semi-rural and suburban areas, and most especially in urban centres.
History demonstrates the rise of Coptotermes gestroi and Mastotermes darwiniensis
There are several examples from history that demonstrate the rise of pest termite species as a consequence of the demise of ecologically dominant termite species. There two clear examples in the region: one from Southeast Asia and the other from tropical Australia.
Coptotermes gestroi is the most common and destructive pest termite species in urban Southeast Asia. It was not always so important. Indeed historically, Coptotermes gestroi was not considered common or a pest species. Historically, the small kampung (villages) of Southeast Asia were less disturbing to the forests and soils. The dominant Macrotermes and Odontotermes remained abundant, so Coptotermes were rare, as is true for rural villages today. However, when European colonists cleared large areas of native forest to create large plantations of introduced Hevea brasiliensis (para rubber) and other exotic tree species, then various Coptotermes species, including C. gestroi, became common and pestiferous. The sudden appearance of Coptotermes in rubber plantations gave the European colonists the false impression that introduced trees were vulnerable whereas native plants were resistant. This is not true; many native trees are vulnerable to Coptotermes. And this same pattern of forest clearing, plantation, and Coptotermes population explosion, is observed today.
Mastotermes darwiniensis is the most common and destructive pest termite species in human-disturbed areas of tropical Australia, in urban areas and plantations. Coptotermes acinaciformis (a mound-building species) dominates the natural forested areas, particularly those with older, larger Eucalyptus trees. Mastotermes darwiniensis are rare, and are typically found in areas regenerating after fires, eating fast-growing Acacia and Grevillea shrubs. The Coptotermes acinaciformis mound-nests are destroyed when forests are cleared for urban development and agriculture, but the underground nests of Mastotermes darwiniensis survive. The potential of the Central American and Caribbean species Pinus caribbea as a forestry species was destroyed in tropical Australia due to ravages of Mastotermes darwiniensis. Mango, Mangifera indica, has thrived, but farmers must monitor and act against these termites every year.
Predicting future termite pest species
Termite management methods may be driving a potential change in urban pest termite species. If so, the current pest termite species may become less important in the future, and alternative termite species will become the new major pests.
For 50 years (ca. 1940s to 1990s) arsenic dusts and chemical barriers in soil were the major management methods of termite control in Asia and Australia. Arsenic dusts were dispersed into voids in walls and timber to kill termites in buildings, but were less commonly used. Insecticidal chemicals were mixed with soil around the perimeter of buildings to create a repellent barrier. This barrier protected buildings by preventing termites from accessing the building. Dusts and chemical soil barriers killed some foraging termites, however colonies were relatively unaffected.
Baiting became more common around 2000. Baiting differed from previous management methods because the objective was to eliminate the termite colony. Colony elimination was defined as killing all termites in the colony, including the reproductives and brood, so that the colony could not revive. Baiting can achieve colony elimination. There can be challenges, which require skill, experience and persistence to overcome – sometimes with extra help with spot treatments of insecticidal liquids or dusts.
Baiting is more likely to eliminate colonies with a central nest. A central nest will contain one queen and king in a royal chamber, one nursery and one moulting and growth chamber; all adjacent in the centre of the nest. Colony elimination is more likely with a central nest because the bait active ingredients usually act together with entomopathogenic fungi. Most bait actives are chitin synthesis inhibitors, so the termites die as they attempt to moult. When many termites die attempting to moult simultaneously, there are too many corpses for the surviving termites to clean away. Entomopathogenic fungi invade the nest to infest the corpses, and then spread to the living termites, including the reproductives and young in the adjacent chambers.
Coptotermes species are usually central site nesting. Consequently, baiting works well against them in most situations. When baiting is used consistently against Coptotermes infestations in urban areas, most colonies will be eliminated. In this way baiting empties the urban environment of the dominant termite species, and so creates a vacuum for other termite species.
There is anecdotal evidence that other species of termites that live in urban areas are expanding to fill the void left by Coptotermes. This is particularly true in temperate Australia, from Brisbane to Sydney. Coptotermes species did comprise the majority of infestations, however after 20 years of baiting, this is no longer the case. It appears Coptotermes are slowly being eradicated from at least some urban areas, and that Schedorhinotermes species are becoming increasingly common pests in those same urban areas.
Schedorhinotermes species, plus Heterotermes, Microcerotermes and other termites, may be more difficult to manage than Coptotermes, particularly using baiting. These termites are multi-site nesting species. These species have colonies with multiple nests, each with (secondary) reproductives, nurseries and moulting and growth chambers. Food, including bait, is returned mostly to the closest nest, with little moved onto distant nests. As bait is not moved to all nests, it is unlikely to affect the entire colony, and so colony elimination is less likely.
Conclusion
Humans have helped to create Coptotermes (and Mastotermes) as urban pests. They did so by clearing natural forests, removing ecologically dominant termite species, and thereby allowing the subordinate Coptotermes termite species to thrive. Baiting has proved effective against central nesting Coptotermes species, and this appears to allow multi-nesting Schedorhinotermes species to replace them – at least in temperate Australia. It is interesting to note that Mastotermes is a multi-nesting species, and baiting is ineffective against them. There are many Schedorhinotermes species in Southeast Asia, and they are usually the most resilient termites in the disturbed lands of cleared forests (at least until Coptotermes species expand and replace them). It seems plausible that Schedorhinotermes may become more common in Southeast Asia and Australia, if baiting continues to eliminate Coptotermes colonies.
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