Abstract
There are many applications for timber in the marine environment, such as piers, jetties, and groynes, as it performs well in turbulent coastal conditions. However, one of the biggest threats to the service life of wooden structures in the marine environment is the biological degradation caused by marine wood-boring invertebrates. Damage caused by these mollusc and crustacean borers amounts to billions of pounds every year to restore or replace damaged structures. The two most aggressive attackers of marine wood are the teredinid bivalves (shipworm) and the isopod crustaceans, Limnoria (gribble). Understanding of the biology of these marine wood-borers can help to improve the protection measures that are applied to wood used in the marine environment.To test these novel wood products in Europe, the standard test EN275 is followed, which involves exposure in a marine environment with high activity of marine wood-boring invertebrates. While this method provides comprehensive results about durability against marine borers in a real-world scenario, feedback time is slow, and it is difficult to identify the reason why some products have higher resistance. In order to provide rapid results to supplement this test, laboratory screening methods using Limnoria and possibly teredinids can be used. Rapid laboratory testing can allow for fast feedback time, within months rather than years, to the developers of new products, to allow them to adjust treatment procedures. In addition, the more detailed examination of samples on a smaller scale, can be used to help identify the mode of action of resistance. This can be applicable, for example, with differently modified wood or lesser utilised timber species.
This work outlines a rapid laboratory, ‘no-choice’ feeding test using Limnoria, for induction into the European Standard, EN275. Optimisation of the methodology was investigated to develop a fast, cost- effective, and user-friendly procedure, which can be streamlined into product efficacy testing, in both specialist and non-specialist labs. Analysis of faecal pellet production from individual animals, along with assessing vitality (behaviour) and mortality rates were used to evaluate resistance of different woods, by comparing them to the standard reference material, Scots pine (Pinus sylvestris) sapwood. Counting of faecal pellets is an indication of feeding rate but counting large numbers of many replicates manually, can lead to fatigue and human error. Automated particle counting methods, the free, open- source software ‘ImageJ’, and a flow imaging microscope ‘FlowCAM’, were used to reproduce pellet counts. The accuracy of the count produced by each technique was compared to the corresponding manual count, and the frequency and intensity of error were assessed in reference to manual counting. The FlowCAM rapidly processed individual samples, however loading and unloading time made this technique more time consuming as replicates increased. On the other hand, ImageJ was able to process multiple samples at once so total processing time was not influenced by sample size. ImageJ, however, does require detailed images, and trial and error of the parameter settings for optimal precision so, investing time in the preparation of samples increases accuracy. ImageJ was closer to the manual count
however, when the number of faecal pellets in the sample increased, the larger the error from the automated counting methods. ImageJ more often under-counted the sample, but the FlowCAM would consistently produce counts with errors of more than 200 pellets higher than the manual count. Where manual counting is not feasible due to large numbers of replicates, ImageJ is the best alternative, in terms of cost, time and accuracy, though can be improved to further reduce the margin of error.
This methodology was applied to evaluate the efficacy of two treatments of furfurylated pine (Pinus radiata). Two experimental furfurylation treatment processes were applied to several planks of wood. The treatments, which consisted of the same chemical mix, differed in peak temperature and total process time during the curing and drying stage. One of the benefits of observing individual Limnoria in a controlled, no-choice setting, such as this laboratory test, is that mortality due to toxicity can be detected. High early mortality of Limnoria fed on treatment 1, suggested that the leaching procedure prior to the experiment did not remove all of the extractives, and this caused some biocidal effect. Once thoroughly leached, the mortality rate was low, similar to that of treatment 2 and only began to increase after 30 days, when starvation from a prolonged reduction in feeding rate occurred. There were significant differences in feeding rate between Limnoria that were fed treatment 1 and 2 and between those fed on treated wood and the untreated control. Vitality on furfurylated wood was lower than on the untreated control, with little to no burrowing behaviour observed. The lack of burrowing can also have benefits in protection of wood, exposing Limnoria to predation, thereby further reducing attack. The reduction in feeding rate and lack of burrowing behaviour is evidence that furfurylation improves resistance against biodegradation by Limnoria.
Furfurylated wood was also harder than untreated control wood, when tested using nanoindentation. This could be a factor in the reduction of Limnoria feeding rate. The mandibles of the Limnoria, the tips of which are <20 µm so comparable to the scale of the nano-indenter, may find it more difficult to cut into harder woods. Feeding rate was lowest on the treatments with the highest hardness. This also may affect the settlement success of teredinid larvae, which was lower on furfurylated wood. There is no standard equivalent laboratory test using teredinids, however, assessing the rate of settlement attempts and failures can be beneficial in rapidly identifying potentially resistant products, and a development of this method could lead to standardisation.
The fundamental mode(s) of action of the resistance of different woods, including modified wood, against marine wood-borers is still unknown. Laboratory and field testing that assess individual animals at different stages of their life-cycle can help to identify the key features necessary for reducing borer attack, and standardising these protocols can allow novel products to be screened rapidly, providing important feedback to inform the evaluation of wood modification procedures.
| Date of Award | 18 Mar 2024 |
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| Original language | English |
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| Supervisor | Simon Cragg (Supervisor), Sarah Marley (Supervisor) & Reuben Shipway (Supervisor) |