West Indian drywood termite: towards sustainable management of an invasive timber pest

William Haigh

doi:10.25907/00950

The West Indian drywood termite, Cryptotermes brevis (Walker), is a major invasive pest of timber-in-service across the globe. Its cryptic colonies reproduce within wood, causing extensive concealed damage. Their ability to remain undetected and be inadvertently transported to new locations has earned them the reputation of being the world's most destructive drywood termite species. Management of C. brevis primarily relies on the fumigation of infested structures with toxic chemicals to eliminate identified colonies. While this approach has historically slowed termite spread in Australia, it is ineffective at eliminating established populations. There is a need for effective preventative approaches against C. brevis infestations, which require a deeper understanding of the species’ biology and feeding ecology. Significant knowledge gaps remain regarding the preferences and mechanisms of their feeding, and adaptations they make to diet, the systems underpinning their damage and propagation. This is particularly true in an Australian context, where a limited understanding of termite biology, their interactions with native timbers, and recent policy changes may facilitate further spread. This thesis broadly investigated the feeding behaviour of C. brevis, focussing on several key areas. The susceptibility of six industrially important timber species to termite feeding was investigated through no-choice feeding trials. Termites fed these diets were further examined to determine how diet influences their gut microbiome using next-generation genetic sequencing. Additionally, I explored chemical differences between wood and frass to understand how termite digestion utilises different dietary components. Also, the characteristics of termite frass were studied to evaluate whether visual or chemical changes could be used to estimate frass age and, therefore, current colony activity. The feeding trial revealed that radiata pine (Pinus radiata), hybrid southern pine (P. elliottii elliotii × P. caribaea hondurensis) and shining gum (Eucalyptus nitens) were as suitable for termite survival and feeding as hoop pine (Araucaria cunninghamii), the timber species most commonly attacked in Australia. Hoop pine plywood experienced similar damage to solid hoop pine timber, suggesting that structures made from untreated Pinus or shining gum timbers or composites of susceptible species may be at significant risk of termite damage. In contrast, the other hardwood diets tested, spotted gum (Corymbia citriodora variegata) and silky oak (Grevillea robusta), exhibited natural resistance to termite attack, likely due to their higher densities compared to the more susceptible species. Though material properties are a contributing factor to susceptibility, no direct correlations were found, and other wood species-specific factors, such as secondary metabolite content, may play a role in influencing termite feeding behaviour. Anecdotal evidence has suggested that termite frass colour could indicate colony age and be used to assess the success of control measures. However, the darkening of frass over time was discredited as a helpful technique. Frass aged over a year was distinguishable from fresh frass based on its chemical composition, with compounds such as pentadecanal, 3- ethyltetracosane, hexacosane, and octacosane identified as markers for more precise age estimation. This approach could be applied post-fumigation to differentiate residual frass from previously eradicated colonies and frass of new infestations, helping to prevent unnecessary repeat fumigations. Comparisons between undigested wood and residual wood in frass confirmed that cellulose serves as the primary energy source, with only slight lignin modification. The data suggested that α-cellulose and hemicellulose were both metabolised, with the former utilised more efficiently. The termites tolerated wood secondary metabolites, with a moderate decrease in total extractives after consumption suggesting some compound degradation. A relative increase in total polyphenols was realised after digestion, attributed to concentration in the frass due to a lack of metabolism. The compounds investigated here responded differently to digestion, individually varying in abundance, suggesting that termites and/or their microbiota may either metabolise, alter or discharge the compounds encountered within different diets. Understanding how the termites utilise lignocellulose and secondary metabolites may be useful for limiting termite activity through informed material choices. The gut microbiome of C. brevis consists of obligate microbial symbiotes that facilitate lignocellulose digestion. The diversity and community abundance of microbiota were compared across termites fed different diets. Diet significantly influenced gut microbial diversity, most strongly between starved termites and those collected from natural infestations. In termites feeding on different wood species, microbial diversity varied less significantly; however, hardwood diets, which exhibited lower consumption rates, induced greater microbiome shifts than softwood diets. Core taxa, including Methanobacterium (archaea), Enterococcus and Wolbachia (bacteria), and Devescovina and Calonympha (protist), were conserved and abundant in most conditions, with no correlation between prokaryote and protist diversity. These findings revealed the ability of the gut microbiome to adapt to the external pressures of changing dietary input and suggest a potential use in innovative management strategies. This thesis expands our knowledge of C. brevis feeding and provides an interpretation of the effects of diet on termite success. The information gained here improves our understanding of processes that can be utilised to develop safer, more sustainable management of this timber pest.

West Indian drywood termite: towards sustainable management of an invasive timber pest

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