A look at termite tunnelling behaviour, examining the evidence from some of the world’s leading entomological research bodies.
“Termites can travel up to 100 metres from their nest to a feeding site.” How many times would pest managers have written or read something similar? When you think about it, there are two big questions to answer here: firstly, how do termites find their food sources? Secondly, how do they get there? This article explores the second question.
Building a foraging tunnel allows the efficient exploitation of resources. However, the actual building of tunnels requires a significant investment of resources by the termite colony and there has been a great amount of research on termite tunnelling behaviour. Clearly studying tunnelling behaviour in the field is challenging for obvious reasons, so much of the research has focused on laboratory experiments in small arenas where sand or soil is placed between sheets of glass, creating tunnelling areas of up to one square metre.
Basic tunnel geometry varies between species. Coptotermes formosanus tend to construct a primary foraging tunnel that generates numerous branching tunnels, while Australian Coptotermes species such as C. acinaciformis and C. lacteus tend to construct a number of main tunnels radiating out from the main nest, each with their own series of branches.1 Work on C. formosanus and C. gestroi further illustrated interspecific differences in foraging patterns. In searching for food, C. gestroi created a large number of thin, highly branched tunnels, whereas C. formosanus created wider, less branched tunnels. The authors suggested this may reflect the different foraging strategies optimised for tropical (C. gestroi) or temperate (C. formosanus) environments.2
This search strategy does not appear to be influenced by the presence of wood, although once a food source was located, recruitment to that site occurred, strengthening the tunnel to that site and also initiating a new search pattern around this new foraging site.3 However, soil type, soil moisture and soil temperature can influence tunnelling behaviour.4 Termites appear to find it easier to tunnel in coarse grained substrates such as sand, and exhibit higher tunnelling efficiency and larger maximum tunnelling distances in soils with higher moisture and higher temperature. Interestingly, it is not only the physical nature of particles in the soil that impacts foraging. Irregularities in the soil surface are necessary to trigger tunnelling behaviour.5 Termites took approximately seven hours to tunnel into a smooth sand surface, but in the same sand with just the smallest of indents (1 mm), termites started tunnelling within 20 minutes.
Interestingly not all termites participate in excavation. Studies on C. formosanus indicate a division in labour, as with other tasks in the colony.6 A small percentage of foragers appear to act as chief excavators, not only in transporting excavated soil in initial tunnelling but continuing to do so on consecutive days, suggesting these termites acted as organisers in determining the orientation and branching patterns in tunnelling networks. These lead excavators are supported by other termites that excavate to a lesser degree and then there is a proportion of foraging termites that do not excavate at all. There is some evidence that older workers (main picture, above) lead the excavation efforts.7
Some recent research has focused more on the process of soil excavation and deposition by termites. After all, all the soil they excavate has to be put somewhere! For ants the process is relatively straightforward – they can excavate and carry a pellet of substrate from the excavation site to the surface. For termites, where do they put all the excavated dirt?
It had been hypothesised that tunnel space was created by compacted soil. However, research published in 2008 by Nan-Yao Su and Hou-Feng Li found that deposited sand was less dense than unexcavated sand, thus rejecting the soil compaction hypothesis.8 Instead they proposed the wood consumption hypothesis – that space generated through consumption of wood allowed space to deposit excavated soil. Further research did indeed report a significant correlation between wood consumption and the amount of soil excavated.9
In excavating individual tunnels, recent research led by Sang-Bin Lee10 has demonstrated that excavating termites deposit particles inside the tunnel against the sides, a short distance behind the excavation front. For individual termites to transport excavated dirt long distances to a deposition site would be inefficient and the authors hypothesise that if the dirt needs to be moved further, different termites would move the dirt in a series of short distances by a ‘bucket-brigade’.
Whilst interesting from a scientific point of view, understanding termite foraging and tunnelling behaviour is also important in designing termite management products, in particular termite baiting systems.
1 Su, N-Y et al. (2004). Characterization of tunnelling geometry of subterranean termites (Isoptera: Rhinotermitidae) by computer simulation. Sociobiology, 44(3):471-483.
2 Hapukotuwa, N.K and Grace, K.J (2012). Coptotermes formosanus and Coptotermes gestroi (Blattodea: Rhinotermitidae) Exhibit Quantitatively Different Tunneling Patterns. Psyche: A Journal of Entomology, 2012.
3 Campora, C.E and Grace, K.J (2001). Tunnel Orientation and Search Pattern Sequence of the Formosan Subterranean Termite (Isoptera: Rhinotermitidae). Journal of Economic Entomology, 94(5):1193-9.
4 Xu, H et al. (2019). Impacts of different environmental factors on tunnelling behaviour of the subterranean termite Odontotermes formosanus (Shiraki). Journal Ethology Ecology & Evolution, 31(3)
5 Lee, S-H et al. (2008). Lee S-H, Su N-Y, Yang L-R. Surface irregularity-induced tunnelling behavior of Formosan subterranean termites. Behavioural Processes, 78(3):473-476.
6 Cornelius, M.L and Gallatin, E.M (2015). Task allocation in the tunnelling behavior of workers of the Formosan subterranean termite, Coptotermes formosanus Shiraki. Journal of Asia- Paci c Entomology, 18(4):637-642.
7 Yang, R-L and Su, N-Y (2009). Individual Task Load in Tunnel Excavation by the Formosan Subterranean Termite (Isoptera: Rhinotermitidae). Annals of the Entomological Society of America, 102(5):906-910.
8 Li, H-F and Su, N-Y (2008). Sand Displacement During Tunnel Excavation by the Formosan Subterranean Termite (Isoptera: Rhinotermitidae). Annals of the Entomological Society of America, 101(2):456-462.
9 Brown, K.S et al. (2008). Colony characterization of Reticulitermes avipes (Isoptera: Rhinotermitidae) on a native tallgrass prairie. American Midland Naturalist, 159:21–29.
10 Lee, S-B et al. (2019). Minimizing moving distance in deposition behavior of the subterranean termite. Ecology and Evolution. 2020;00:1-8.