Research from Taiwan shows that hybrid colonies of Coptotermes gestroi and Coptotermes formosanus termites have successfully established in the field. Dr Phil Ridley discusses the implications of a potential ‘super termite’ hybrid.
The vast majority of invasive pests have been introduced through human activity, mainly through accidental introduction along shipping routes. As bad as an individual incursion can be on the endemic ecosystem, there is always the potential that the invasive pest could interbreed with a closely related native species, with unknown consequences. But what about the possibility of interbreeding between two invasive species?
Although the probability of such an event occurring is small, researchers have now established that hybrid populations exist in the wild, created through interbreeding of two of the most significant invasive termite species, Coptotermes formosanus and Coptotermes gestroi. Work is underway to understand the nature of these hybrid populations and their potential impact on termite management in the affected areas. This is the story so far…
Key Terms
Species – a group of individuals that can interbreed in nature and produce fertile offspring
Hybrid – the offspring of two distinct lineages (varieties or species)
F1 hybrid – offspring resulting from the cross-mating of distinct parental types (in this case C. gestroi v C. formosanus)
F2 hybrid – offspring resulting from the cross-mating of hybrid individuals (from F1)
Backcross mating – pairing between hybrid individuals and one of the original pure bred parental genotypes
Coptotermes gestroi and Coptotermes formosanus are two of the world’s most destructive termites. The natural range of C. gestroi is in Southeast Asia whilst C. formosanus is thought to have originated in southern China. However, as a result of human mediated transport over the last few hundred years, they have both significantly expanded their range.
The two species have different ecological requirements, with C. gestroi being a more tropical species and C. formosanus preferring temperate/subtropical climates. As a result, the number of locations where conditions allow both species to thrive is quite limited. Nevertheless, four locations have been identified where both species co-exist: southeastern Florida, southern Taiwan, Oahu (Hawaii) and Hainan (China). With the swarming season for these species having been observed to overlap in these locations (except Hainan), the question has been asked whether the two species could interbreed and if so, would fertile hybrids be the result?
Could they interbreed?
During field work in 2013 and 2014 pairings of combinations of C. formosanus and C. gestroi were commonly observed, suggesting that interbreeding (heterospecific tandem behaviour) was at least attempted by alates of the two Coptotermes lineages. Interestingly, males of C. gestroi would readily engage in tandem behaviour with females of C. formosanus, coupling in the field. This observation was confirmed in laboratory trials1 with the males of both species preferring C. formosanus females over C. gestroi females. It was demonstrated that the two species share the same sex pheromone, but female C. formosanus females produce tenfold the amount of pheromone compared to C. gestroi females, resulting in mating confusion for C. gestroi males.2
with C. formosanus females both in the laboratory and in the field (photo credit: Thomas Chouvenc/UF-IFAS)
This is an important factor as C. formosanus males may not be as motivated to tandem with C. gestroi females, for a lack of “sex-appeal”.3 However, if given the opportunity, C. formosanus males would still mate with C. gestroi females in the laboratory to provide viable colonies. The data also demonstrated that there was no difference in colony survival rate in the laboratory between hybrid colonies and pure-bred colonies after one year. So yes, C. formosanus and C. gestroi can successfully interbreed. Such putative hybridization would only be possible if alates of both species could disperse at the same place and at the same time, which does appear to be the case for localities where both species are established.4
Do they exhibit hybrid vigour?
One characteristic often discussed in the conversation about hybrid organisms is ‘hybrid vigour’. Hybrid vigour is an often-used term in agriculture and is the aim of most breeding programs – to produce a new genotype that exhibits increased fitness and yield. In relation to termite hybrids, this raises several questions. Could they exhibit increased rates of growth (both individually or as a colony), increased reproductive success, more tolerance to a wider range of environmental conditions, or some other characteristic that increases their competitiveness?
Initial laboratory data1 not only showed that hybrid colony survival was the same as pure-bred colonies, but that the hybrid colonies had twice as many individuals as the pure-bred colonies at the one-year mark. However, later research,5 which measured the growth of laboratory-reared colonies over an extended four-year period, showed there was no significant difference in terms of colony growth, numbers of individuals and wood consumption. Whilst these studies showed no evidence for hybrid vigour, they equally demonstrated that hybrid colonies had some similar properties to the pure-bred colonies, which clearly have the potential to be very damaging. It can therefore be assumed that hybrid colonies could also be a significant issue in the field.
Although there was no evidence of hybrid vigour from these colony growth observations, several of the hybrid colonies (female C. formosanus, male C. gestroi) contained secondary reproductives even though the primary reproductives were still present. This would be a very rare occurrence in pure-bred colonies. Should these secondary reproductives prove to be fertile, this reproductive plasticity could confer competitive advantage on these hybrids. But such findings would need to be demonstrated and evaluated in the field. The interaction between two distinct genetic lineages with hybrid individuals may also lead to developmental alterations and levels of infertility.
A preliminary study on hybrid workers’ mobility and moulting behaviour suggests some level of developmental alteration in F1 Coptotermes hybrid colonies,6 as genetic combination can lead to novelty phenotypes, both beneficial and detrimental. One other manifestation of potential hybrid vigour that was also assessed was temperature tolerance. In a clever experimental design using a thermal bridge7 researchers determined the lowest temperature at which workers remained active. Workers of C. formosanus remained active as low as 11°C, whereas workers of C. gestroi only remained active at temperatures above 22°C. These results are perhaps unsurprising given it is a tropical species. What was surprising was that for both hybrid variations – whether formed from a female C. gestroi or female C. formosanus – the cold boundary was warmer than for either parent, at 25°C. It was apparent that the Coptotermes hybrid species were far less tolerant of cold conditions than the parent species and were more in line with the tropical C. gestroi.
However, in a second experiment, hybrid colonies were able to survive at temperatures combining the potential climate range of both parental species.8 Therefore, while hybrid termite colonies may thrive differently under different climatic conditions, they may still be able to survive and establish in a wide range of localities, if fertile. The takeout from these laboratory trials is that although colony growth is in line with the parent species, there is little evidence of hybrid vigour (improved fitness). Indeed, the temperature tolerance trials indicated that the hybrids may only thrive in the more tropical environments. The presence of the secondary reproductives in the laboratory hybrid colonies was an interesting observation and certainly differences in colony reproduction could make or break the success of hybrids. Field data is essential to understand the real-world potential for these hybrids to establish.
Do hybrid colonies exist in the field and do they produce fertile offspring?
With most of the research on Coptotermes hybrids having taken place in the laboratory, it was important to understand whether hybrid colonies existed in the field and if so, whether they had produced fertile offspring. The best place to start looking was Taiwan, as both species had been living in the same areas for a significant period of time; C. formosanus is native to the area and C. gestroi was introduced more than 100 years ago.9
Researchers carried out a three-year field survey10 collecting alates from successive swarm seasons and undertook detailed identification (looking at morphology and genetic characteristics). Out of the 27,601 alates collected over the three-year period, 4.4% were confirmed hybrid alates, thereby confirming that field-established hybrid colonies existed and produced alates. However, approximately 90% of these hybrid alates were female. The authors suggested possible reasons for this, commenting, “Sex ratio of the eggs laid may be skewed towards females in hybrid colonies, male alates may have less viability compared to female alates or male alates may be poor flyers.”
The researchers then set up mating combinations with the field-collected alates to see whether the results of previous laboratory studies were substantiated and whether hybrid alates could produce viable colonies. The results from Taiwan demonstrated that C. gestroi and C. formosanus combinations can indeed produce viable colonies, supporting the earlier Florida laboratory results. The researchers also carried out backcross studies, pairing hybrid alates with pure-bred alates. They discovered that female hybrid alates had poor egg-laying capabilities and did not successfully establish incipient colonies. In contrast, despite the low numbers of male hybrid alates collected, they did successfully establish incipient colonies when paired with C. formosanus females. Despite the poor fertility of hybrid females and low numbers of hybrid males, genetic evidence from four alates collected suggested that hybrids had still been successful in establishing successful backcross or F2 colonies in the field.
This Taiwanese study was critical, as it provided a template of the morphology of hybrid alates, providing researchers with the tools to identify any further evidence of F1 hybrid colonies being established in multiple parts of the world.11 One other interesting observation from the field data was that although laboratory data indicated both male C. gestroi and male C. formsanus termites strongly prefer female C. formosanus termites, the hybrid alates collected in the field demonstrated both C. gestroi and C. formosanus maternal lineage, demonstrating that C. formosanus males did still mate successfully with C. gestroi females.
The key learnings from this Taiwanese study are that the hybrid colonies have indeed been successfully established in the field and that they produce alates. However, it appears that the hybrid alates have a low level of reproductive success. So, although there was evidence that some field colonies had been established, the success of hybrid colonies beyond the first F1 generation may be relatively low, but sufficient for some degree of gene flow between the two parental species.
Termite hybrids – should we be concerned?
Based on the laboratory and field data collected to date, C. formosanus x C. gestroi hybrid colonies are here to stay, more so in areas where reasonable numbers of both parental colonies co-exist. However, while the number of locations that are suitable for both parent species to thrive appears to be restricted, based on their temperature tolerances, it is most likely that human-mediated movement of these termite pest species will continue to increase their geographical distribution overlap.12
The reproductive fitness of the hybrid colonies appears to be relatively poor, which may further limit the potential for the hybrid variations to outcompete the pure-bred species. This would be supported by the observation that hybrid Coptotermes are yet to be detected in the south of Taiwan, where both species have existed for the longest period of time. This may indicate that the hybrids were unfit from an evolutionary point of view and were outcompeted by the dominant C. gestroi populations. In Taiwan, the hybrid populations were found towards the northern limit of the C. gestroi distribution, where high populations of C. formosanus are found. Alternatively, as C. gestroi populations are establishing northward in Taiwan (possibly the result of climate change), it is also possible that historical gene exchanges between the two species in the south of the island could have resulted in some C. gestroi being more tolerant of relatively cooler climates. There is still much unknown about the long-term consequences of hybridization in Coptotermes, and future research on this front will be necessary to determine what is speculative and what is factual.
So, based on the information to date, it doesn’t appear that the hybrid termites are ‘super termites’ that will dominate an area and expand their distribution. However, it doesn’t mean that they won’t create issues in the locations in which they are found, from a termite management point of view.
Hybrid colonies – impact on termite management
Both laboratory and field data demonstrate that F1 hybrid colonies can successfully establish and develop to a stage where they can produce alates. Therefore, individual colonies have the potential to be at least as damaging as their pure-bred parent. The challenge with the hybrid colonies is that we know very little about their behaviour. With C. gestroi and C. formosanus having very different foraging behaviours,13 further research is required to understand the foraging behaviour of the hybrid colonies, so suitable control methods can be recommended. At least the hybrid soldiers have been characterised, so pest managers in areas where hybrids may exist should be able to determine if they are dealing with a hybrid colony.14
Dr Phil Ridley, Director, Professional Pest Manager
Dr Phil Ridley would like to acknowledge Assoc. Prof Thomas Chouvenc of the University of Florida for taking the time to review this article prior to publication. All images credited to Thomas Chouvenc/UF-IFAS
More information on termites.
1 Chouvenc, T et al. (2015). Hybridization of Two Major Termite Invaders as a Consequence of Human Activity. PLOS ONE 10(3). DOI:10.1371/journal. pone.0120745
2 Chouvenc, T., Sillam-Dussès, D. and Robert, A., 2020. Courtship behavior confusion in two subterranean termite species that evolved in allopatry (Blattodea, Rhinotermitidae, Coptotermes). Journal of Chemical Ecology, 46(5), pp.461-474.
3 Mizumoto, N., Lee, S.B., Valentini, G., Chouvenc, T. and Pratt, S.C., 2021. Coordination of movement via complementary interactions of leaders and followers in termite mating pairs. Proceedings of the Royal Society B, 288(1954), p.20210998.
4 Chouvenc, T., Scheffrahn, R.H., Mullins, A.J. and Su, N.Y., 2017. Flight phenology of two Coptotermes species (Isoptera: Rhinotermitidae) in southeastern Florida. Journal of Economic Entomology, 110(4), pp.1693-1704.
5 Patel, Jayshree et al. (2023). Equivalent Colony Growth of Hybrids of Two Invasive Coptotermes Species Can Threaten Urban Areas. Journal of Economic Entomology. 116. 10.1093/jee/toad020.
6 Lee, S.B., Chouvenc, T., Patel, J. and Su, N.Y., 2020. Altered mobility and accumulation of inefficient workers in juvenile hybrid termite colonies. Frontiers in Ecology and Evolution, 8, p.589762.
7 Patel, J., Chouvenc, T. and Su, N-Y (2019). Temperature Preference of Two Invasive Subterranean Termite Species and Their Hybrids (Blattodea: Rhinotermitidae: Coptotermes). Journal of Economic Entomology. 10.1093/jee/toz210.
8 Patel, J.S., Tong, R.L., Chouvenc, T. and Su, N.Y., 2019. Comparison of temperature-dependent survivorship and wood-consumption rate among two invasive subterranean termite species (Blattodea: Rhinotermitidae: Coptotermes) and their hybrids. Journal of economic entomology, 112(1), pp.300-304.
9 Su, N.Y., Chouvenc, T. and Li, H.F., 2017. Potential hybridization between two invasive termite species, Coptotermes formosanus and C. gestroi (Isoptera: Rhinotermitidae), and its biological and economic implications. Insects, 8(1), p.14.
10 Li, H-F et al. (2024). Hybrids of two destructive subterranean termites established in the field, revealing a potential for gene flow between species. Heredity 132: 257-266.
11 Chouvenc, T. and Li, H.F., 2023. Hybridization between Coptotermes formosanus and Coptotermes gestroi. In Biology and Management of the Formosan Subterranean Termite and Related Species (pp. 353-364). GB: CABI.
12 Duquesne, E. and Fournier, D., 2024. Connectivity and climate change drive the global distribution of highly invasive termites. NeoBiota, 92, pp.281-314.
13 Su, N-Y and Lee, S-B (2023). A comparison of tunnel geometry between the Formosan subterranean termite and the Asian subterranean termite (Blattodea: Rhinotermitidae). Pest Management Science 79 (10): 3999- 4003. https://doi.org/10.1002/ps.7594
14 Patel, J.S et al. (2023). Soldier phenotypic differences among 2 invasive and destructive Coptotermes species and their hybrids (Blattodea: Isoptera: Rhinotermitidae). J Insect Sci. 2023 Nov 1;23(6):2 https://doi. org/10.1093/jisesa/iead095
