The latest research looking into the complexities of termite nest structure, swarming behaviour and the roles of the royal pair and reproductives.
Latest termite nest and reproduction research
Life and death of a termite colony
The life cycle of a termite colony is often perceived as a relatively straightforward development pathway: colony founding, growth to maturity, maturity, senescence (old age) and then death. However, recent analysis of a massive field-collected data set, from studies on Coptotermes formosanus over a 25-year period (1985- 2009), suggests there is much greater variability in colony reproduction, demographics and longevity even within a termite species.1
The study describes the changes in the colony dynamics of four colonies over their lifetime. Certainly, one colony followed the ‘standard’ development pathway and was active for a ten-year period, then declined for three years before death. However, other colonies had more variable productivity, responding to the death/fecundity of the primary reproductives in different ways and also potentially being influenced by external factors, such as food availability, climate and competition.
The ability to produce secondary reproductives (which doesn’t always occur) is critical to surviving the loss of the primary reproductives and can allow a colony to thrive for 20-30 years. The second study nest had a pause in reproductive output, which was only rejuvenated following the production of secondary reproductives. The third colony showed a similar pattern although produced a large number of secondary reproductives with the extensive cycles of inbreeding producing smaller individuals across castes. The fourth study nest showed a massive nymph production before senescence, followed by a successful colony takeover by a neighbouring young colony. The ‘reborn’ colony itself was then highly productive for ten years before losing production and then recovering with the production of secondary reproductives. There is no one-size-fits-all when it comes to the life cycle of a termite colony.
Choosing a replacement queen
Many termite species produce secondary reproductives that replace the primary queen should she die. But how does the colony choose which termite workers should develop into reproductives? Some recent research on looked at the behaviours of workers before they differentiated into reproductives. They identified that the workers that developed into reproductives moved less, performed more proctodeal trophallaxis, and were groomed more than others.2 The researchers suggested that the reduced mobility would conserve energy, and the anal feeding and increased grooming would indicate more dominant status. This provides some information on how the colony supports the development of individual workers into secondary reproductives but doesn’t provide information on why they were chosen in the first place.
The same researchers also noticed that when R. labralis workers encountered a closely related species, R. aculabialis, they were either aggressive or ran away. They also noticed that when neotenics of R. labralis were introduced to groups of R. aculabialis they always displayed signs of retreat and avoidance (“elusive” behaviour).3 The researchers went on to demonstrate that “elusive” workers were more likely to develop into reproductives and do so faster than “aggressive” workers. The researchers concluded that with the aggressive trait being a useful behaviour for colony defence, it would make evolutionary sense for the more elusive workers, which show high self-preservation behaviours, to develop into reproductives. They did not investigate whether there was any differences in the sex of the reproductive that developed from so-called elusive or aggressive workers.
Queen primer pheromones
The queens of social insects, in particular ants, wasps and bees, use pheromones – the queen primary pheromones (QPP) – to maintain their reproductive dominance. Although it is believed to be the same in termites, to date the QPP has only been identified in one species of lower termite, Reticulitermes speratus, a volatile blend of 2-methyl-1-butanol and n-butyl n-butyrate. Recent research has now identified the first QPP in a higher termite (Termitidae), Embiratermes neotenicus. In this species, the primary queen is replaced by neotenics produced through parthenogenesis (asexual reproduction).4 Researchers identified the sesquiterpene (3R,6E)-nerolidol as the QPP in E. neotenicus and noted that it supresses the development of parthenogenic neotenic queens i.e. when the primary queen was present and healthy, neotenic production would be suppressed.
‘Termite balls’ and termite eggs
Within some species of subterranean termite (Rhinotermitidae), a fungus is found inside the colony nursery mixed in with the termite eggs. These eggmimicking fungi are referred to as ‘termite balls’. They range in colour from light to dark brown and are between 0.2-0.35 mm in diameter. It is not clear whether this is a parasitic relationship (fungus receiving all the benefit) or a mutualistic relationship; however it has been suggested that the fungus gains protection inside the nest and the termites potentially benefit from using the fungus as a food or as source of cellulase enzymes to aid with wood digestion. These ‘termite balls’ have been reported in seven Reticulitermes species, Coptotermes formosanus and most recently Coptotermes gestroi.5
1 Chouvenc, Thomas & Ban, Paul & Su, Nan-Yao. (2022). Life and Death of Termite Colonies, a Decades-Long Age Demography Perspective. Frontiers in Ecology and Evolution. 10. 911042. 10.3389/fevo.2022.911042.
2 Bai, Zhuang-Dong & Liu, Yibin & Sillam-Dussès, David & Wang, Rui-Wu. (2022). Behavioral differentiation among workers may reduce reproductive conflicts during colony inheritance in the termite Reticulitermes labralis. Insectes Sociaux. 69. 10.1007/s00040-022-00862-8.
3 Bai, Zhuang-Dong & Shi, Chong-Yang & Sillam-Dussès, David & Wang, Rui- Wu. (2022). Elusive workers are more likely to differentiate into replacement reproductives than aggressive workers in a lower termite. Current Zoology. 69. 10.1093/cz/zoac040.
4 Dolejšová, Klára & Křivánek, Jan & Štáfková, Jitka & Horáček, Natan & Havlíčková, Jana & Roy, Virginie & Kalinova, Blanka & Roy, Amit & Kyjaková, Pavlína & Hanus, Robert. (2022). Identification of a queen primer pheromone in higher termites. Communications Biology.5. 10.1038/s42003-022-04163-5.
5 Costa-Leonardo, Ana & Janei, Vanelize & da Silva, Iago. (2022). First Neotropical record of the association between brown sclerotium-forming fungi and termite eggs in a nest of Coptotermes gestroi (Blattaria, Isoptera, Rhinotermitidae). The Science of Nature. 109. 10.1007/s00114-022-01815-8.
Termite Professional Australian edition, 2022
Predicting termite mound structures
Researchers using computer modelling to predict termite mound shape believe that in applying the principles of heat transfer and thermodynamics, computer-generated mound shapes closely match those found in the field.1 Essentially, sun and wind play an important role in the development of mound shape and structure.
Mounds exposed to more sun tend to have a cone-shaped structure that points towards the sun, whereas mounds in shaded areas tend to be vertical domes. Mounds in areas of increased wind are generally shorter in height. The sun and wind factors interact to determine the internal structure. When the need for thermoregulation is more important than the need for gas exchange, the mounds have thicker walls and are built deeper into the soil.
Outside of tropical rainforests, termite alate flights often occur during a specific time of the year, typically during a warmer, more humid period. Whilst most pest managers will tell you this is the case, researchers in Brazil have confirmed the same whilst assessing alate flights in a seasonally dry tropical forest.2 Peak termite swarms, of all species, occurred during February and indeed 97% of all flights occurred during a 20-day period.
The number of flight occurrences and species composition were significantly influenced by the accumulated precipitation over 72 hours and air density. This high level of synchronisation between species suggests that in environments which have variable moisture levels throughout the year, termites have evolved to swarm when conditions provide the best chance of survival.
1 Fagundes, Tadeu & Ordonez, J. & Yaghoobian, Neda. (2021). The Role of Mound Functions and Local Environment in the Diversity of Termite Mound Structures. Journal of Theoretical Biology. 527. 110823. 10.1016/ j.jtbi.2021.110823.
2 Lucena, Emanuelly & Silva, Israel Soares & Monteiro, Sara & Moura, Flávia & Vasconcellos, Alexandre. (2021). Accumulated precipitation and air density are linked to termite (Blattodea) flight synchronism in a Seasonally Dry Tropical Forest in north-eastern Brazil. Austral Entomology. 61. 10.1111/aen.12577.
Termite Professional Australian edition, 2021
How many termite reproductives are in a nest?
For pest managers carrying out termite work, the holy grail is to find the termite queen (if they are able to find the nest). Many would assume that the termite colony comprises a ‘simple family’ – a single pair of primary reproductives, which mate monogamously. This is the most common situation, but it isn’t always the case.
Termites create two other types of colony structure. Firstly, there is the extended family where one or both of the founding pair are supported or have been replaced by secondary reproductives that have been produced from within the colony. Extended family colonies appear to be more likely in lower termites (60%) compared to higher termites (13%).1
Secondly, there are ‘mixed family’ colonies, which can arise through three different mechanisms: when two dispersing alates found a colony together; from fusion between two simple families; or through adoption of alates.
Researchers in Australia utilised DNA analysis to investigate the prevalence of the different colony types in Nasutitermes exitiosus.
Analysing 60 colonies from across NSW and ACT, the researchers determined that 61.7% were simple colonies, 16.7% were extended families and 21.7% were mixed families. Most of the mixed-family colonies contained two queens and one or two kings. In one colony, the DNA evidence suggested there were seven primary queens. How such mixed family colonies are formed is unclear, with the researchers suggesting that future investigations should focus on the potential impact of local ecological and climatic conditions on breeding patterns and colony structure.
Long live the Queen (and King)
As a general rule in animals there is a trade-off between longevity and reproductive output – you can be either long lived or have a high reproductive output, but you cannot have both. The exception to the rule are social insects, especially the termite queens and kings, which can live for 20 years or more.
In their review on the subject, Tasaki et al. (2021)2 point out that a range of external factors rather than ageing per se impact life expectancy – predation, starvation and adverse weather events will all cause death. However, in a colony situation, the termite reproductives that remain safe in the centre of the colony and are protected and fed through the activity of other colony members are largely insulated from these mortality factors. Thus the development of eusociality with the subsequent removal of these pressures on the reproductives is one of the key reasons why they live so long.
However, there must also be a physiological and molecular basis to the longevity; there must be a delay in the ageing process that is normally seen in other animals. There is evidence to suggest that the antioxidant and DNA repair systems are augmented to slow the age-associated accumulation of physiological damage. An alternative theory (not mutually exclusive) is that reproductives have a lower expression of growth signalling pathways, which prevents the reduction in gene expression that occurs later in life.
Another factor that may contribute to their longevity is the hypoxic nature of the royal chamber. Deep in the nest, the oxygen levels are low (15%) and CO2 levels are high (5%). In the atmosphere, oxygen content is around 21% and CO2 levels are less than 0.05%. Clearly, reproductives have had to adapt to this very different atmosphere inside the nest and indeed queen egg-laying increases greatly under these nest conditions, compared to normal atmospheric conditions.3 These lower levels of oxygen would reduce oxidative stress on reproductives (oxygen can be toxic) and the development of anaerobic energy producing systems in these low-oxygen environments may also contribute to their long lifespans, but this needs to be investigated further.
1 Montagu, A., Lee, T.R.C., Ujvari, B., McCarl, V., Evans, T.A., Lo, N., 2020. High numbers of unrelated reproductives in the Australian `higher’ termite Nasutitermes exitiosus (Blattodea: Termitidae). INSECTES SOCIAUX 67, 281–294. https://doi.org/10.1007/s00040-020-00764-7
2 Tasaki, E., Takata, M., Matsuura, K., 2021. Why and how do termite kings and queens live so long? PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY B-BIOLOGICAL SCIENCES 376. https://doi.org/10.1098/ rstb.2019.0740
3 Tasaki, E., Komagata, Y., Inagaki, T., Matsuura, K., 2020. Reproduction deep inside wood: a low O-2 and high CO2 environment promotes egg production by termite queens. BIOLOGY LETTERS 16. https://doi.org/10.1098/rsbl.2020.0049
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