Comparison of termite resistance of wood modified with lignin cleavage products-phenol-formaldehyde and phenol-formaldehyde resins against subterranean and drywood termites
-
Johannes Karthäuser
,
Kévin Candelier
Abstract
Wood modification with phenol-formaldehyde (PF) impregnation resins increases the resistance against fungal decay and subterranean termite attacks. However, phenol is obtained from non-renewable resources. Prior research explored partially replacing phenol with lignin cleavage products (LCP), and the good fungal decay resistance remains. However, the influence on termite resistance has not been studied. In this study, 30 % (w/w) of phenol in PF resins was replaced with LCP (LPF resin). Pinus sylvestris sapwood was treated with PF and LPF resins, and its resistance against European subterranean termites (Reticulitermes flavipes) and tropical drywood termites (Cryptotermes dudleyi) was evaluated by non-choice screening tests. Additionally, choice tests were carried out to confirm that the modifications were non-biocidal. The PF and LPF resin treatments improved termite resistance (reducing the mass loss) against both termite species. Low termite survival rates were observed after the non-choice test against subterranean termites. The choice tests confirmed that both treatments were non-biocidal. Visual evaluation (EN 117 (2023)) led to worse resistance ratings of treated specimens against tropical drywood termites, compared to European subterranean termites, due to different mode of actions of the termites’ attacks. Thus, new standards (or appendixes) could be useful to evaluate treated woods resistance against different termite species.
1 Introduction
The mitigation of climate change is one of the major challenges in modern times and necessitates the development of renewable and environmentally friendly technologies. Hence, the application of wood as a building material, which acts as a carbon sink during its use, is increasing (van der Lugt and Harsta 2020). However, compared to energy-intensive and non-renewable inorganic building materials, the organic nature of wood posts new challenges, and suitable treatments are required to ensure a long lifespan.
Biological degradation is one of the major challenges during the application of wood as a building material. Wood can be decayed by various organisms, for example bacteria, fungi, insects, or marine borers, depending on the environment in which it is applied. In tropical or subtropical Mediterranean regions, termites appear to be the most important wood consumer, due to favorable temperature and humidity conditions for their proliferation (Law et al. 2024; Zanne et al. 2022). While termites play important ecological roles, some species have strong invasive capabilities and termite attacks cause major structural damage and lead to high economic damage (Chouvenc 2025). As of 2010, the damage to structures resulting from termite infestation and the associated treatment cost and repair was estimated to have an annual economic impact of over $US 40 B worldwide (Rust and Su 2012).
Termites are generally separated into three categories: subterranean termites, drywood termites, and dampwood termites (Oi et al. 2008). Dampwood termites are usually not important in structural wood degradation, because they consume cellulose in dead or rotting wood (or even rotting leaves), even if they occasionally can attack decayed wood in damp areas of homes, like leaky bathrooms or roofs (Pervez 2018).
Subterranean termites are the most widely distributed termites, and they may also be found in temperate regions. Subterranean termite colonies require a certain level of moisture; they live in soil and typically feed on wood that is in contact with the soil or moisture, causing the most economic damage (Morrell 2012). If the wood is not in contact with water, subterranean termites create mud shelter tubes extending from a water source towards the wood, and can by this also attack dry wood (Mallis 1990).
Drywood termites fully live in the wood they are feeding on, not requiring any contact with the ground. They get the moisture they need from the atmosphere, thus, preferring humid environments. Drywood termites can inhabit wood with a much lower moisture content than subterranean termites. While they prefer moisture contents above 10 %, it was reported that they can attack and remain active in wood with a moisture content below 3 % (Goodell 2001). Due to their tunneling inside of wood, drywood termite colonization is more difficult to identify (despite the presence of fecal pellets) than subterranean termite infestation, so they may remain unnoticed for a long period of time, while causing significant damage to wood structures (Goodell and Nielsen 2023).
The use of naturally durable tropical wood species, wood impregnation, and wood modification processes represent the most effective methods to reduce or prevent damage due to subterranean and drywood termite attacks.
One of the major methods for wood protection is the use of biocidal preservatives. Traditional heavy-duty wood preservatives (lindane, dieldrin chromated arsenicals, creosote, and pentachlorophenol) are now forbidden or highly restricted worldwide (Boucard and Denize 2022). More recent treatments based on water-borne or oil-borne preservative systems, associated with organic fungicides and/or quaternary amines are introduced into the wood, oftentimes by applying reduced and/or high pressure (Khademibami and Bobadilha 2022). Although these treatments remain affordable in terms of price, such preservatives are biocidal and oftentimes toxic towards humans, and their water-leaching from the treated wood posts for environmental and health issues, leading to restrictions and limitations in use by government regulations (Boucard and Denize 2022). An additional challenge regarding the application of preservatives is the waste disposal, which oftentimes increases costs (Townsend et al. 2004).
An alternative to preservatives is wood modification. Wood modification is the treatment of wood with chemical, biological, physical or thermal methods, with the requirement that the modified wood should be nontoxic during service life and disposal (Gérardin 2016). The mode of action by wood modification against biological attacks must be nonbiocidal (Hill 2006). One of the main wood modification methods is the impregnation of wood with resins, which are thereafter cured inside of the woods cell walls. This treatment increases, amongst others, the dimensional stability and biological resistance (Augustina et al. 2023).
A well-known impregnation resin is phenol-formaldehyde (PF) resin. PF resins are known to effectively increase the dimensional stability, weathering resistance and biological resistance, while being easy to synthesize (Fleckenstein et al. 2018).
The termite resistance of wood modified with PF resins is described in several studies. Takahashi and Imamura (1990) modified Japanese cedar (Cryptomeria japonica), western hemlock (Tyromyces palustris), and Japanese beech (Fagus crenata), and tested the resistance against Coptotermes formosanus. Deka et al. (2002) treated Anthocephalos cadamba, a fast-growing hardwood species, with PF resins and tested the resistance against unspecified termites, and in another study against Ontotermes spp (Deka and Saikia 2000). Loh et al. (2011) treated oil palm stem and tested the resistance against Coprotermes curvignathus. Another study about the treatment of oil palm stem and termite resistance against the same species was carried out by Bakar et al. (2013). Abdullah et al. (2013) modified oil palm trunks and analyzed the resistance against Coptotermes gestroi. Gascon-Garrido et al. (2015) describe the modification of Pinus sylvestris and the modified woods resistance against Reticulitermes flavipes. Finally, Nunes et al. (2022) treated Pinus radiata and tested the resistance to attacks by Reticulitermes grassei. In all the above-mentioned studies, the modification significantly increased the termite resistance. This highlights the high potential of the modification technique as a non-biocidal wood protection against termite attacks. However, it must be mentioned that all termite species described above are subterranean species; studies on the resistance of PF-treated wood against drywood termites or a comparison of the resistance against drywood or subterranean species in laboratory or field tests have, to the authors knowledge, not been published. Only Sulistyono et al. (2019) highlighted that PF-impregnation of coconut wood improved the drywood termite (Cryptotermes cyanocephalus) resistance from durability class IV (non-resistant) to durability class II (durable), based on visual rating according to SNI 01–7207 (2006). However, there was no statistical difference in specimen mass loss due to termite degradation between untreated and treated coconut wood. Hence, further efforts to study the termite resistance of PF-resin treated wood against drywood termites could provide new perspectives on non-biocidal wood protection methods. Additionally, comparisons between the termite resistance against subterranean and drywood termite species could help to understand in what regions the application is a suitable wood protection – and where it would be prone to decomposition.
While the treatment of wood with PF resins leads to significant improvements in the wood properties, there are drawbacks. The main challenges connected to phenol application are its high toxicity and its non-renewable nature. Because of this, renewable (and environmentally friendly) alternatives to phenol have been a research topic for decades (Klašnja and Kopitović 1992; Sarika et al. 2020). One of the most promising candidates for replacing phenol is lignin (Huang et al. 2022; Sarika et al. 2020). Lignin is one of the main constituents of wood and the most abundant aromatic biomass on earth (Qu et al. 2015; Wang et al. 2020). It is a side-product in the pulping industry, which is currently mainly used for regaining energy and pulping chemicals, lacking higher-value applications (Bajwa et al. 2019). Lignin, a natural polymer with phenolic structures, appears to be a sustainable substitute for phenol to synthesize lignin-phenol-formaldehyde (LPF) in the production of economical and eco-friendly bio-based resins (Zhao et al. 2024). Several studies have suggested that the substitution of commercial monomers with lignin would have environmental benefits (Hildebrandt et al. 2019; Lettner et al. 2018; Yuan and Guo 2017). In addition, lignin and some cleavage monomers exhibit antioxidant and antimicrobial properties (Vasile and Baican 2023), which may contribute to fungal and termite resistance. However, the substitution of phenol with lignin is still mainly used and studied today for the formulation of adhesive resins rather than in the field of wood impregnation (Karthäuser et al. 2021).
To substitute phenol for wood impregnation, the lignin must be cleaved, because the penetration of the cell wall is limited for the large lignin macromolecules (Biziks et al. 2019; Grinins et al. 2021).
First studies on the substitution of phenol by lignin cleavage products (LCP) for the application in PF resins for wood modifications indicated that 40 % of phenol could be substituted with similar performance with regard to the dimensional stability, leaching performance, mechanical properties, and weathering resistance (tested on beech wood laminated veneer lumber) (Fleckenstein 2018; Fleckenstein et al. 2018). Beech wood laminated veneer lumber treated with resins with a 40 % treatment load of the wood had good fungal decay resistance against Coniophora puteana and Trametes versicolor (Fleckenstein et al. 2018). Similarly, Scots pine sapwood treated with a PF resin with 30 % substitution of phenol by LCP with a treatment load of 10–30 % had good performance against fungal decay by Gloeophyllum trabeum, Rhodonia placenta, and T. versicolor (Karthäuser et al. 2024), while maintaining good dimensional stability, low formaldehyde emission, good weathering resistance, and good leaching resistance (Karthäuser et al. 2023, 2024, 2025). While these studies indicate a good resistance against fungal decay, the resistance to termite attacks of wood treated with LCP-containing PF resins has not been studied. This regionally limits the applicability of the modification method.
The purpose of this study was to determine if the partial substitution of phenol in PF resins by LCP influences the termite resistance of modified wood. A special focus was to study differences between the resistance against attacks by subterranean and drywood termites. Although their living and feeding patterns are different, both termite species were tested under the same tests conditions (wood sample size, number of termites and exposure duration) so that the results on raw and modified wood resistances could be compared on the same basis (the temperature and humidity were adjusted to the termites' natural environment). For this purpose, Scots pine (P. sylvestris) sapwood was modified with PF resin, and PF resin in which 30 % of the phenol was substituted by LCP. Both treatments were applied in three different concentrations to determine if there is a threshold in concentration at which the termite resistance is affected. The modified (and reference) specimens were subjected to termite attacks by subterranean termites (R. flavipes, ex. santonensis) and tropical drywood termites (Cryptotermes dudleyi), in both non-choice and choice tests, and evaluated according to a modified EN 117 (2023)and EN 350 (2016).
2 Materials and methods
2.1 Resin synthesis
To test the influence of the substitution of phenol by LCP on the termite resistance of the modified wood specimens, two resins were synthesized. The first resin is a pure PF resin with molar ratios of phenol:formaldehyde:NaOH = 1.0:1.5:0.1. The second resin is a PF resin with substitution of 30 % of the phenol by LCP and reduced formaldehyde content (LPF) with molar ratios of phenol + LCP:formaldehyde:NaOH = 1.0:1.4:0.1. The chemicals used were phenol (99.5 %) and formaldehyde (37.5 % solution) obtained from Th. Geyer GmbH & Co. KG, Renningen, Germany, as well as NaOH (50 % solution) provided by Honeywell International Inc, Seelze, Germany. The LCP were obtained by microwave-assisted pyrolysis and the resin synthesis was carried out both as described in Karthäuser et al. (2023) In short: the phenol (and LCP) was weighed in and molten at 58 °C. Nitrogen atmosphere was applied. The formaldehyde was added to start the reaction. The reaction was carried out at 65 °C for 4 h under constant stirring.
To determine the solid content of the resins, about 3 g of resin were weighed into an aluminum cup. Butanol was added to obtain a flat surface. The resin was heated to 140 °C for 2 h. The final mass was measured, and the solid content was calculated. This procedure was repeated once.
Prior to impregnation, the resins were diluted to a solid content of 6.25, 12.50, and 18.75 %, respectively, using demineralized water, to study the influence of the resin concentration on the woods termite resistance.
2.2 Termite resistance of modified wood
2.2.1 Wood specimen’s preparation and modification
Scots pine (P. sylvestris) sapwood specimens from southern Germany with dimensions of 5 × 10 × 25 mm (tan × rad × long) were modified. Prior to impregnation, the specimens were dried at 103 °C to collect the dry mass and dimensions. Their dry density was 498 ± 26 kg/m3.
For modification, sixty oven-dried specimens were submerged into the respective resin. Vacuum (1 h, 80–100 mbar) followed by pressure (2 h, 11 bar) were applied to achieve a thorough impregnation. After impregnation, the specimens’ wet weights were determined, and the solution uptake was calculated. The specimens were carefully dried, first at room temperature, then at increasing temperatures up to 80 °C. Finally, the resin was cured by heating the specimens to 140 °C for two days. The dry weight and dimensions of the cured specimens were determined, and the weight percent gain (WPG) and volume increase (bulking) were calculated. To study the influence of the thermal treatment on the termite resistance of the wood, half of the reference specimens were heated to 140 °C. Both the reference specimens and the specimens heated to 140 °C were impregnated with water to be able to compare the solution uptake and changed materials properties after treatment.
2.2.2 Cold water leaching tests
To determine if leaching may affect the woods termite resistance, half of the specimens were leached according to EN 84 (2020) with the difference, that the dry weight (103 °C) was collected prior to and after leaching (in the standard, the equilibrium weight at 20 °C, 65 % relative humidity (RH) is measured). In short, the specimens were submerged in demineralized water and impregnated at increased pressure (20 min, 4 bar). The water was replaced after 2 h and nine more times in the following 14 days. The dry weight before and after this procedure was measured, and the weight loss was calculated. While for the modified wood half of the specimens were subjected to the leaching procedure, all the reference specimens were leached.
Below, the treated specimens are abbreviated with the treatment (PF or LPF) followed by the solid content of the resins (6.25, 12.50 or 18.75 %) and an “L” for the leached specimens. Reference specimens treated at 140 °C are referred to as TT (thermally treated).
2.3 Termite tests
2.3.1 Reticulitermes flavipes tests
All wood specimen modalities were exposed to subterranean termites (R. flavipes, ex. santonensis) in non-choice tests and choice tests. Non-choice tests were carried out to assess the conferred wood durability against termites, whereas choice tests were used to confirm the non-biocidal mode of action of the modification. Termites were collected from Oléron Island, France (Lat. 45° 49′ 5.9″ N; Long. −1° 13′ 47.8″ W). The colony was reared in a climatic chamber regulated at 27 ± 2 °C and RH > 75 %. All the specimens were oven-dried at 103 ± 2 °C to obtain their initial anhydrous mass (m1).
The non-choice tests were carried out according to the main criteria of EN 117 (2023), with some adjustments concerning the specimen size, the termite number, and the exposure period. Five replicates per modality were tested separately. Five Scots pine sapwood specimens with similar dimensions were also tested against termites as virulence controls.
Each specimen was placed on a plastic grid support at the center of a 9-cm diameter Petri dish, containing 35 g of Fontainebleau wet sand (4 vol. of sand/1 vol. of deionized water). Considering the dimension of the specimens, and according to the method used in previous studies (Afzal et al. 2017; Elaieb et al. 2020; Leroy et al. 2023; Martha et al. 2024; Mohareb et al. 2017; Salman et al. 2017), 50 termite workers, one nymph, and one soldier were then introduced into each test device. The test devices remained in a dark climatic chamber conditioned at 27 ± 2 °C and RH > 75 % for 4 weeks. Once a week, water was added and termite behavior was checked.
After the exposure, the specimens were removed and cleaned of sand. The termite survival rate was calculated. Specimen’s degradations were visually rated according to EN 117 (2023) and EN 350 (2016) (adjusted to the specimen size). Then, the specimens were oven-dried at 103 ± 2 °C for at least 24 h to obtain their anhydrous mass after termite exposure (m2), and their mass losses due to termite attack (MLT) were calculated according to Equation (1).
The choice tests were carried out according to the main criteria in EN 117 (2023), with some adjustment concerning the specimen size, the number of termites, the exposure period and including one treated and one untreated wood specimen (5 × 10 × 25 mm (tan × rad × long)) in the same test device.
Five replicates per treatment modality, associated with their untreated Scots pine sapwood specimen, were tested. In addition, five tests, including two untreated Scots pine sapwood specimens, were performed as control devices. Moreover, eight Scots pine sapwood specimens were tested separately as termite virulence control. Specimens (one treated and one untreated) were placed on a plastic grid support, side by side, at the center of a 9-cm diameter Petri dish, containing 35 g of Fontainebleau wet sand. 50 termite workers, one nymph, and one soldier were then introduced into each test device. These test devices remained for 4 weeks in a dark climatic chamber conditioned at 27 ± 2 °C and RH > 75 %. Once a week, water was added and termite behavior was checked.
The evaluation of the specimens (treated and untreated assorted) was carried out as described above regarding their visual rating and MLT.
2.3.2 Cryptotermes dudleyi tests
All wood specimen modalities were exposed to tropical drywood termites (C. dudleyi) in non-choice tests and choice tests to confirm the non-biocidal mode of action of the wood modification. Termites were collected from infested plywood in Kourou, French Guiana (Lat. 5.153245; Long. −52.671036). The species of drywood termites used in the framework of this study was identified as C. dudleyi by microscopic imagery analyses (Olympus BX60, Olympus Europa SE & Co. KG, Hamburg, Germany) of several soldiers from the colonies used for the non-choice and choice tests. All wood specimens were oven-dried at 103 ± 2 °C to obtain their initial anhydrous mass.
The non-choice tests were carried out according to the main criteria of EN 117 (2023), with some adjustment concerning the specimen size, the termite species, the number of termites, and the exposure period. Three replicates per modality were tested separately. Three Pine sapwood (P. sylvestris) and three Virola (Virola michelii) specimens with dimensions of 5 × 10 × 25 mm (tan × rad × long) were also tested against the termites as virulence controls.
Each specimen was placed on a plastic grid support at the center of a 9-cm diameter Petri dish, containing 0.35 g of Cryptotermes termite feces from the termite sampling area (infested plywood) to maintain the mood of the colony during testing. Although the living and feeding patterns of drywood termites differ from those of subterranean termites, specially by the fact that Cryptotermes species may require more time or wood mass to undergo full damages, tests using C. dudleyi were carried out under the same tests conditions (wood sample size, number of termites and exposure duration) so that the modified wood resistances could be compared on the same basis. Compared to subterranean termite species, there is still little literature that uses drywood termite species to determine the durability of solid wood blocks based on standard EN 117 (2023). Considering the dimension of the specimens to be tested, and according to the method used in previous studies (Batista et al. 2016; Gonçalves and da Silva Oliveira 2006; Santos 1982, Supriana 1985), 50 nymph-workers and one soldier were introduced into each test device. These test devices were placed for 4 weeks in a dark chamber at tropical ambient conditions (Tmin = 24.0 °C; Tmax = 28.7 °C; Tmoy = 26.5 °C; Hmin = 50.9 %; Hmax = 99.8 %; Hmoy = 94.5 %). Once a week, termite behavior was checked.
At the end of the exposure, the specimens were removed, cleaned of termite feces, and the termite survival rate was calculated. Although no standardized methodology allowing to assess specifically the durability against drywood termites exists, specimen degradations were given a visual rating according to EN 117 (2023) and EN 350 (2016) (adjusted to the specimen size). Then, the specimens were oven-dried at 103 ± 2 °C for at least 24 h to obtain their anhydrous mass after termite exposure, and their MLT were calculated as described in Equation (1).
In addition, all wood specimen modalities were exposed to tropical dry-wood termites (C. dudleyi) in choice screening tests. The choice tests were carried out according to the main criteria of EN 117 (2023)(2023), with some adjustment concerning the specimen size, the termite species, the number of termites and the exposure period, as well as including one treated (P. sylvestris) and one untreated wood specimen (V. michelii sapwood) in the same test device. Three replicates were carried out per treatment modality. In addition, three Petri dishes, including two native V. michelii sapwood specimens, were prepared as choice test control devices, and three Petri dishes containing only one V. michelii sapwood specimen were tested separately as termite virulence control, using the same test modalities.
Specimens (one treated and one untreated) were placed on a plastic grid support, side by side, at the center of a 9-cm diameter Petri dish, containing 0.35 g of Cryptotermes termite feces from the termite sampling area. 50 nymph-workers and one soldier were then introduced into each test device. The test devices were placed in a dark chamber at tropical ambient conditions (Tmin = 25.0 °C, Tmax = 29.5 °C, Tmoy = 27.1 °C; Hmin = 78.1 %, Hmax = 99.7 %, Hmoy = 89.4 %) for four weeks. Once a week, termite behavior was checked.
The evaluation was carried out as described above regarding their visual rating and MLT.
2.4 Statistical analysis
For comparison of means, Tukey HSD was performed using OriginPro 2020 (OriginLab Corporation, Northamptom, MA, USA). A probability of 0.05 was applied as significance level.
3 Results and discussion
3.1 Resin synthesis
The resins were obtained as a light brownish liquid (PF) and a dark brown liquid (LPF). The LPF had a slightly higher solid content compared to the PF (54.4 ± 0.5 % and 52.3 ± 0.0 %, respectively).
The slight increase in solid content of the LPF resin was expected due to the decrease in formaldehyde (in aqueous solution) during synthesis. The solid contents are in line with earlier results with similar resins (Karthäuser et al. 2024).
3.2 Wood modification
To confirm a successful wood modification, the solution uptake (wet after impregnation), WPG, bulking (both dry after treatment), and mass loss after leaching according to EN 84 (2020) were calculated (Table 1). The solution uptake for the resin-treated specimens was higher than for the water-impregnated reference and reference heated to 140 °C and increased with increasing resin solid content. The solution uptake of the PF and LPF treated specimens was similar. The mass loss was below 1.5 % for all specimens and did not vary largely between the different treatments.
Solid content, weight percent gain (WPG), bulking, and mass loss after leaching tests (EN 84 (2020)) of the wood specimens treated with different resins and resin solid contents.
| Treatment | Solution uptake (%) | WPG (%) | Bulking (%) | Mass loss (%) |
|---|---|---|---|---|
| Ref | 146 ± 13 | −1.8 ± 0.9 | −0.9 ± 1.6 | −0.1 ± 0.5 |
| TT | 147 ± 12 | −2.4 ± 0.8 | −0.2 ± 1.4 | 0.7 ± 1.2 |
| PF 6.25 | 162 ± 14 | 8.2 ± 1.7 | 4.9 ± 1.2 | 1.2 ± 0.8 |
| PF 12.50 | 162 ± 14 | 17.8 ± 1.9 | 6.6 ± 1.7 | 0.8 ± 0.6 |
| PF 18.75 | 170 ± 15 | 28.3 ± 2.8 | 7.9 ± 1.7 | 0.7 ± 0.6 |
| LPF 6.25 | 159 ± 14 | 8.8 ± 1.3 | 4.4 ± 1.1 | 0.7 ± 0.8 |
| LPF 12.50 | 167 ± 14 | 17.2 ± 1.7 | 6.1 ± 1.5 | 0.5 ± 0.7 |
| LPF 18.75 | 170 ± 18 | 27.6 ± 3.0 | 7.6 ± 1.3 | 0.3 ± 0.4 |
The increased solution uptake in relation to resin load was expected due to the higher viscosity and density of resins compared to water. The solution uptake and WPG were slightly lower than described in literature, but in the expected range (Karthäuser et al. 2024). The bulking was slightly lower than described in Karthäuser et al. (2023) with a similar treatment, most likely due to the different geometry of the wood specimens. The low mass loss of all specimens indicates a good fixation of the resins inside of the modified wood, which is in line with literature (Karthäuser et al. 2024).
3.3 Reticulitermes flavipes non-choice tests
The MLT, termite survival rate, and durability class of specimens according to EN 350 (2016) after non-choice tests against R. flavipes are depicted in Figure 1, pictures of representative specimens of non-choice and choice tests are presented in the supplementary information (Figure S2). The MLT of- and termite survival rate for all the treated specimens (PF or LPF) was significantly reduced compared to the reference (Ref L) and virulence specimens. Among all the treatment modalities, the highest MLT was observed for LPF 6.25 L (2.7 ± 0.7 %), whereas the lowest one was obtained with PF 18.75 L (0.6 ± 0.5 %), however, statistical analysis indicates no significant difference. The termite survival rates were also reduced by each treatment (maximum of 6.4 ± 3.9 %, PF 6.25 L) compared to those obtained with the virulence control specimens (85.9 ± 2.8 %). The TT L specimens had a slightly reduced MLT (8.0 ± 1.7 %) and termite survival rate (48.8 ± 6.1 %) compared to the reference specimens (MLT = 10.5 ± 2.4 %; Termite survival rate = 60.4 ± 10.3 %) and the virulence (MLT = 10.3 ± 1.4 %; Termite survival rate = 85.9 ± 2.8 %). The leached reference specimens had a reduced termite survival rate compared to the virulence control specimens.
Mass loss (black) and termite survival rate (red) of the differently treated wood specimens after exposure to Reticulitermes flavipes in non-choice tests for four weeks.
According to the visual rating of the specimens, the attributed durability classes of all of the modifications were “D-Durable”, with exception for PF 6.25 L, PF 12.5 L, and LPF 6.25 L, which were “M-Moderately durable”. Thermally treated, reference, and virulence specimens were classified as “S-Susceptible”, according to EN 350 (2016).
The comparison of the pure PF and the LPF resin treated wood indicated that the substitution of phenol did not have a significant influence on the resistance against R. flavipes. While the MLT was slightly higher for the wood treated with LPF resins, the termite survival rate was slightly decreased, no statistical significance was determined in either case, and the durability classes according to EN 350 (2016) were improved. This indicates that the substitution of phenol by LCP is a viable option to reduce the use of non-renewable resources.
Weekly monitoring of non-choice test devices, using R. flavipes, highlighted that most termites died after 3 weeks of exposure to wood treated with PF resins, and after 2 weeks of exposure to wood treated with LPF resins, regardless of concentration level and water leaching process. These observations may indicate that PF and LPF resins possess more repellent than toxic activity against R. flavipes, and that termite mortality is mainly due to starvation (lack of food). Indeed, past studies have shown that Reticulitemes flavipes are able to survive for approximately three weeks without any food source (Candelier et al. 2020; Kieny et al. 2025). Another explanation given by Kajita and Imamura (1991), could concern the inability of subterranean termites (C. formosanus) to digest PF resulting in increased mortality rates over time. Finally, it could be that PF acts as a slow poison to subterranean termites (Coptotermes curvignathus), which induces mortality after an extended period (Bakar et al. 2013).
The results indicate that water-leaching influenced the termite resistance, as all modified specimens classified as “M-Moderately durable” were leached after treatment. However, due to the low mass reduction after leaching, a major part of the resins is expected to be fixed well inside of the wood cell wall, and it is likely that further leaching would not significantly affect the termite resistance in terms of MLT. Additionally, for MLT and termite survival rate, no significant differences were observed. An explanation for the improved classification could be a slight biocidal effect of the leachates. To confirm that the mode of action of the modification was not biocidal, choice tests were carried out and the results are described below.
The thermal treatment, which all the treated specimens were subjected to during the curing of the resin, positively influenced the termite resistance. An explanation for this could be the initial degradation of the wood, which changes the chemical composition and could pose a challenge for the termites (Candelier et al. 2017; Salman et al. 2017). Another explanation could be the reduction of edible components due to the evaporation of volatile chemicals and the migration of extractives to the outer wood which could offer a slight protection (Ripoll de Medeiros et al. 2023).
3.4 Reticulitermes flavipes choice tests
One of the main motivations behind the development of wood modification methods is the non-biocidal mode of action, which assures that upon weathering no environmental damage is done due to the leaching of biocidal compounds (Hill 2006; Martha et al. 2024). To confirm that the termite resistance was not improved by PF or LPF treatments, due to the biocidal effects of chemicals, laboratory screening choice-tests using R. flavipes were carried out (Figure 2).
Mass loss (black), virulence mass loss (grey), and termite survival rate (red) of the differently treated wood specimens after exposure to Reticulitermes flavipes in choice tests for four weeks.
The virulence specimens presented a low termite resistance according to MLT (>10 %) and had a strong termite attack represented by a visual rating of “S-Susceptible”. The high termite attack on virulence specimens confirms the validity of termite resistance tests according to EN 117 (2023). As observed in the non-choice test, the MLT to R. flavipes attacks on treated woods (PF and LPF) were notably lower (maximum of 1.7 ± 0.2 % for LPF 6.25 L) than those obtained for TT L (2.9 ± 0.5 %), reference (4.5 ± 1.6 %), and control (13.1 ± 3.9 %) specimens (however, the MLT of the TT L is statistically similar to some of the treated specimens: PF 6.25, PF 12.5, PF 12.5L, and all LPF specimens other than the LPF 18.75 L). These results indicate that termites preferred to attack pine wood as a control compared to PF, LPF, and TT L wood. The virulence, reference, and TT L specimens exhibited high termite survival rates of 92.6 ± 1.2 %, 84.0 ± 5.7 %, and 83.6 ± 7.4 %, respectively. Average termite survival rates observed for PF and LPF treated wood including setups ranged from 64.0 ± 33.5 % (PF 6.25) to 90.4 ± 4.1 % (PF 6.25 L). These values were notably higher than those from the non-choice test.
The visual rating of the all the modified specimens exposed to R. flavipes in choice tests, was “D-Durable”, with only one exception for LPF 6.25 L and TT L, which were “M-Moderately durable”, whereas control and virulence specimens were classified as “S-Susceptible”, according to EN 350 (2016).
A biocidal wood treatment leads to a reduced termite survival rate, even in choice tests, due to the poisoning of the termites (Kartal et al. 2004; Martha et al. 2024; Salman et al. 2015; Usta et al. 2009). The results herein indicate that the mode of action of the treatment is not biocidal, as a high termite survival rate and MLT for virulence specimens due to termite activity were observed. The low MLT and comparable durability class after evaluation according to EN 350 (2016) are in line with the results described above.
3.5 Cryptotermes dudleyi non-choice tests
The MLT, termite survival rate, and durability class of the non-choice tests against drywood termite C. dudleyi are depicted in Figure 3, pictures of representative specimens of non-choice and choice tests are presented in the supplementary information (Figure S3). Test conditions using C. dudleyi species appeared to be efficient to assess the wood resistance and compared it to the result obtained with R. flavipes, due to the fact that the Virola and Pine control samples are completely infested from the inside (Figure S3), with a visual rating of 4, a mass loss around 10 % and a termite survival rate higher than 90 %. Unlike for the non-choice tests against subterranean R. flavipes, the termite survival rate was as high for treated specimens (ranged from 78.0 ± 7.2 % for PF 12.5 to 90.7 ± 6.1 % for PF 6.25 L) as for TT L (92.0 ± 2.0 %), reference (86.7 ± 6.1 %), and control specimens (93.3 ± 1.2 % for Pine and 94.0 ± 2.0 % for Virola) with no significant difference. Past studies underlined a higher survival rate of C. dudleyi compared to R. flavipes, when termites were exposed to native tropical wood species (Leroy 2024), treated wood specimens (Hadi et al. 2010) or chemically impregnated cellulosic paper (Kieny et al. 2025). Despite the high termite survival rate, the MLT of the specimens was notably reduced for all treated specimens (ranged from 0.6 ± 1.0 % for PF 12.5 L to 4.2 ± 0.9 % for LPF 6.25 L) compared to those obtained with TT L (9.5 ± 1.4 %), reference (7.1 ± 3.7 %), and control (9.2 ± 2.7 % for Pine and 11.2 ± 3.6 % for Virola) specimens. This was especially obvious for specimens with higher resin load, where significant differences to the control specimens were measured. Nevertheless, according to the visual rating of the specimens exposed to C. dudleyi in non-choice tests, the attributed durability class of all of the modifications was “S-Susceptible”, with exception for PF 18.75 and PF 18.75 L which are “M-Moderately durable”, whereas TT L, control, and virulence specimens were also classified as “S-Susceptible”, according to EN 350 (2016).
Mass loss (black) and termite survival rate (red) of the differently treated wood specimens after exposure to Cryptotermes dudleyi in non-choice tests for four weeks.
The reduced MLT indicated that the treatment had a positive influence on the wood’s termite resistance against C. dudleyi, which is in line with the MLT against R. flavipes. Nevertheless, the durability class has not improved for almost all of the treatments. The reason for this may be found in the standard used for specimen evaluation. According to EN 117 (2023), the durability class is determined by visual evaluation. The specimens are rated from 0-4, with 0 being no attack and 4 being strong attack. Then, the durability class is determined according to EN 350 (2016), depending on the visual evaluation. However, it must be mentioned that the standard EN 117 (2023) was developed for evaluation of attacks by Reticulitermes ex. santonensis, a subterranean species (as R. flavipes). Drywood and subterranean termite species have entirely different lifestyles and wood degradation ways (Kalleshwaraswamy et al. 2022). Subterranean termites eat along the wood grain, focusing mainly on the softer rings of the wood (Goodell 2001), while drywood termites will eat along and across it (Himmi et al. 2016). Drywood termites make little holes in the wood, known as “kickout” holes, and they push their debris and feces out of their nests through these holes, leaving little piles of debris outside infested wood (Ebling 1996). In this sense, drywood termites (Cryptotermes sp.) mainly cause perforation and cavities within the wood, whereas subterranean termites (R. flavipes) mainly cause erosions on the wood surface, even if they sometimes make cavities (Su and Scheffrahn 2000).
The visual rating depends on the type of damage caused by termites. Although results show that C. dudleyi and R. flavipes caused a similar MLT to chemically treated wood with average values of 1.7 ± 1.1 % and 1.5 ± 0.7 %, respectively, the damages typologies caused by both termite species are different. R. flavipes caused mainly erosions on the wood surface, whereas C. dudleyi degrade the wood by digging cavities, which have a major impact on the visual rating and therefore on the decrease in the termite durability classification. For better comparison, a graph depicting the visual grading of all treated wood specimens after the termite tests is presented in the supplementary information (Figure S1). These results highlight that the development of a standard (or appendix) for a suitable evaluation of wood materials implemented in different climate conditions and exposed to various termite species could be useful. Indeed, only the non-choice termite test using subterranean termite (R. flavipes) are concerned by the European standards. Although these tests demonstrated the effectiveness of the treatments used against R. flavipes termites the other non-choice tests using drywood termites (C. dudleyi) highlighted lower efficiency of treatments, showing the importance of considering the biological risks and climatic environment to which the material will be exposed during its lifetime, as well as the determination process of the wood material durability class. Current European standards could implement appendices suggesting that tests be carried out under specific conditions/parameters related to the target environment in which the wood products concerned will be used. Such normative improvements will be useful for European overseas departments and territories, to use wood materials in the best way and extend the lifespan of wood products.
3.6 Cryptotermes sp. choice tests
To confirm that the resistance of treated wood specimens (PF and LPF) against drywood termites was not improved due to biocidal effects of chemicals, laboratory screening choice-tests using C. dudleyi were carried out (Figure 4). Hadi et al. (2010) confirmed that biocidal treatments of wood using Borax-5% were toxic to Cryptotermes sp.
Mass loss (black), virulence mass loss (grey), and termite survival rate (red) of the differently treated wood specimens after exposure to Cryptotermes dudleyi in choice tests for four weeks.
The virulence specimens presented a low termite resistance according to MLT (>22.00 %) and visual rating (4 “S-sensible”). As in the non-choice test, the MLT due to C. dudleyi attack on chemically treated woods (PF and LPF) were notably lower (maximum of 2.1 ± 3.0 % for LPC 6.25) than those obtained for control specimens (23.4 ± 1.0 %). Additionally, the TT L and Ref L specimens exhibited significantly lower MLT than the virulence specimens. The virulence, reference, and TT L specimens exhibited high termite survival rates of 94.0 ± 2.0 %, 95.3 ± 3.1 % and 93.3 ± 3.1 %, respectively. Termite survival rates observed for PF and LPF treated wood containing setups ranged from 87.5 ± 3.1 % (LPC 6.25 L) to 99.3 ± 1.2 % (PF 18.75), with no significant difference to the virulence specimens. These values are notably higher than the termite survival rates during non-choice tests.
Moreover, the durability class of the modifications according to the visual rating of the specimens exposed to C. dudleyi in choice tests were “D-Durable” or “M-Moderately durable”, whereas they were all classified as “S-Susceptible”, with exception for PF 18.75 and PF 18.75 L which were “M-Moderately durable”, with non-choice tests.
The significantly different MLT for virulence and (pine) reference indicated, that C. dudleyi preferred to attack Virola wood compared to PF, LPF, and also TT L Pine wood (ML= 0.94 ± 0.31 %). The results confirm the observation made during choice-tests with R. flavipes, that the wood treatment described is not biocidal.
4 Conclusions
The goal of this study was to analyze whether the substitution of phenol in PF resins used for wood modification by impregnation with LCP influences the modified woods’ termite resistance against European subterranean termites (R. flavipes) and tropical drywood termites (C. dudleyi). The termite resistance was studied in choice- and non-choice tests.
The MLT was notably reduced by the treatments, indicating an improved resistance to termite attacks against both European subterranean termites and tropical drywood termites. The visual evaluation of the treatments according to EN 350 (2016) indicated that the treatment is effective against European subterranean termite species, while being more susceptible to attacks from tropical drywood termites. The reason for this is that the visual evaluation in the standard does not consider the differences in mode of action of termite attacks between tropical drywood termites and European subterranean termites.
These findings suggest that the development of standards (or appendixes) for a suitable evaluation of treated wood materials designed for use in areas with variable climate conditions and several types of material-degrading termites could be useful. Such normative improvements would be suitable for European overseas departments and territories, to use wood materials in the best way and extend the lifespan of wood products.
The choice tests proved that chemical modification based on PF and LPF are non-biocidal for both termite species. The comparison of the pure PF and the LPF resin treated wood indicates that the substitution does not have a strong influence on the termite resistance.
Finally, the results indicate that the substitution of phenol by LCP is a promising approach to reduce the use of non-renewable resources while maintaining the same wood performance and enhancing the woods termite resistance without the application of biocides. In combination with results from past research, this indicates that a wood material with good biological resistance (against both fungi and termites), high dimensional stability, and improved weathering resistance can be created, while at the same time reducing the consumption of non-renewable phenol in the application. This could be particularly promising for countries with a strong biomass conversion economy.
Future studies could include termite tests according to the standard, as herein a smaller specimen size was used. Additionally, further studies on differences in termite resistance of modified wood against tropical drywood- and European subterranean termites could be helpful for assessing the potential of modified wood for application in areas, where both termites are present. In this sense, the test conditions could also be adapted according to the termite species used to assess the wood durability. For example, drywood termite species may require longer test duration and a bigger wood sample mass than those used in this present study, to undergo higher damages (than those recorded here) due their mode of life and feeding behavior, which differs from that of subterranean termites.
Funding source: Forschungszentrum Jülich GmbH
Award Identifier / Grant number: 031B0952
Funding source: Agence Nationale de la Recherche
Award Identifier / Grant number: ANR-24-PEFO-0011
Funding source: Norges Forskningsraad
Award Identifier / Grant number: RCN Project No. 305120
Acknowledgments
The authors thank Prof. Ivan Paulmier from FCBA Institute (France) for his help in the drywood termite species identification.
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Research ethics: Not applicable.
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Informed consent: Not applicable.
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Author contributions: Conceptualization: all authors; investigation: JK, KC, JD, LP; data curation: JK, KC, JD; visualization: JK, KC, JD; writing – original draft: JK, KC; writing – review and editing: all authors; supervision: HM, KC. All authors have accepted responsibility for the entire content of this manuscript and approved its submission.
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Use of Large Language Models, AI and Machine Learning Tools: Not applicable.
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Conflict of interest: The authors state no conflict of interest.
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Research funding: The authors acknowledge support from the Federal Ministry of Education and Research (Germany) and its project promoter Forschungszentrum Jülich GmbH (funding number: 031B0952), as well as Norges Forskningsraad (RCN Project No. 305120) as part of the funding program “Bioeconomy in the North 2018”. This work is part of the project Gouvernance of the research program FORESTT and received government funding managed by the Agence Nationale de la Recherche under the France 2030 program, reference ANR-24-PEFO-0011, allowing to carry out the screening termites test in Montpellier and Kourou.
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Data availability: The raw data can be obtained on request from the corresponding author.
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