Termite resistance of pine wood treated with nano metal fluorides

Termites cause significant structural damage to timber resulting in high economic losses (Pimentel et al. 2005). The invasive Formosan subterranean termite species Coptotermes formosanus, originating from China and Taiwan, has been successful in spreading to southern parts of the United States such as Florida (Chouvenc et al. 2016). Globally, the estimated economic costs from damage to goods and services due to C. formosanus was found to be > US$30.2 billion per year (Bradshaw et al. 2016). These costs are predicted to rise as the population of C. formosanus is expected to increase by 15−20%, with the warming of the environment due to climate change (Buczkowski and Bertelsmeier 2017). Thus, there are ongoing research efforts to find effective wood preservatives and wood modification technologies to control the consequent damage to timber products from the spread of C. formosanus.

Wood modification techniques such as acetylation and furfurylation have been evaluated for their resistance to termites. Acetylated radiata pine (both leached and unleached) in EN 117 tests was found to be durable against Reticulitermes banyulensis resulting in mass loss lower than 3% (Gascón-Garrido et al. 2013). However, acetylated wood was not durable against C. fomosanus in a forced feeding test as even 20% acetylated larch resulted in a mass loss of 4.2% (Imamura and Nishimoto 1986). Besides wood modification treatments, current termiticides available on the market contain active ingredients such as bifenthrin, cypermethrin, fipronil, and permethrin (Xie et al. 2013). Although effective, some of these ingredients may be toxic to human health and environment. Fipronil poisoning was reported to cause seizures, while permethrin was found to be toxic to aquatic organisms (Weston et al. 2005; Dhang 2011). An alternative to commercial termiticides are nanoparticles as their smaller size allows for improved penetration into wood (Evans et al. 2008; Taghiyari 2015). Recently, several studies have reported on the efficacy provided by the reduction in size of commercial biocidal actives like copper, boron or zinc (Clausen et al. 2011; Civardi et al. 2015; Ibáñez et al. 2019). In the form of nanoparticles, they were tested for their efficacy against termites (Kartal et al. 2009; Mantanis et al. 2014; Terzi et al. 2016; Lykidis et al. 2018). In a four week no-choice test, unleached southern yellow pine wood treated with nanoboron had lower mass loss than those treated with nanocopper and nanozinc (Kartal et al. 2009). In addition, unleached nanoboron-treated specimens resulted in 100% termite mortality (Kartal et al. 2009). Another study tested black pine wood treated with nano formulations of zinc oxide, zinc borate and copper oxide against C. formosanus (Mantanis et al. 2014). In this three week no-choice test, nanozinc borate treated wood had mass loss lower than specimens treated with nanozinc and nanocopper (Mantanis et al. 2014). Another wood preservative that could be studied for their biocidal efficacy in the form of nanoparticles are fluorides. There are several studies on the efficacy of NaF, but no study has reported on termite resistance using nano metal fluorides.

One of the advantages of fluoride in wood preservatives is that they are effective in low concentrations (Freitag and Morrell 2005). A study on dry wood and subterranean termites found that NaF was more effective compared to disodium octaborate tetrahydrate (DOT) as it led to lower weight losses (Kartal et al. 2019). Concentration of 1% NaF in particleboard (unleached) resulted in a mass loss of 2.3% after three weeks of exposure against C. formosanus (Tascioglu et al. 2017). However, the termite resistance was lost when the NaF-treated particleboard was leached. Another study reported on field efficacy of NaF treatment against the subterranean termites Reticulitermes flaviceps (Pan et al. 2015). A choice test done with 0.057% (w/w) NaF-treated wood inhibited attack from R. flaviceps (Pan et al. 2015). The research also reported that a concentration of 0.1% (w/w) would protect timber from wood-degrading organisms. However, a follow-up study by the same research group concluded that more than 90% of fluoride from the NaF-treated blocks was leached into the surrounding soil (Pan and Wang 2015). Thus, the studies show that there is evidence of the efficacy of fluoride against termites, however, there is a need to improve its resistance against leaching.

The high leaching susceptibility of NaF is expected because of its high water solubility of 40 g/L (Roark 1926). Therefore, in the past, fluoride was combined with co-biocides to reduce its leaching and increase the biocide efficacy. Mixture of fluorine-chromium-arsenic was found to be toxic to three termite species, Nasutitermes exitosus, Coptotermes acinaciformis, and Coptotermes lacteus (Gay and Schulz 1965). However, awareness of environmental and health risks associated with chromium (Cr) and arsenic (As) has led to their restricted application in wood preservation. Without these co-biocides it becomes challenging to use approved fluoride compounds in wood protection products for outdoor application. This is one of the reasons for the decline in use of fluoride compounds in wood preservation. A potential way to overcome this challenge of high leachability is to test metal fluorides (MF₂) with low water solubility such as calcium fluoride (CaF₂) and magnesium fluoride (MgF₂), but it is difficult to impregnate wood with crystalline low water-soluble fluorides. A way to overcome this limitation is to synthesize low water-soluble fluorides in the form of nanoparticles.

Nano metal fluorides are increasingly accessible since the fluorolytic sol gel synthesis was reported in 2003 (Kemnitz et al. 2003). Based on this synthetic accessibility, nano metal fluorides have been applied to anti-reflective coatings, bio-medicine and bio-plastics (Sun and Chow 2008; Krahl et al. 2016; Mahn and Kemnitz 2019). The homodispersed nanoparticles have been characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and dynamic light scattering (DLS) (Usmani et al. 2018). The mean particle size distribution of these nanoparticles was below 25 nm. It was shown that wood can be successfully impregnated with these nano metal fluoride particles. The distribution of these nano metal fluorides in cross-cut sections of treated wood was confirmed with scanning electron microscopy (Usmani et al. 2018). The average retention of MgF₂ and CaF₂ in wood specimens was 5 and 6%, respectively (Usmani et al. 2018). More importantly, it was found that wood treated with nano metal fluorides had average mass loss of only 2% when attacked by brown-rot fungi, Coniophora puteana. Thus, it was shown that nanoscopic homodispersed MgF₂ and CaF₂ synthesized via the fluorolytic sol-gel synthesis can be successfully used for timber treatment against C. puteana (Usmani et al. 2018). Consequently, the following study reports on biocide efficacy of nano metal fluorides against termites, Coptotermes formosanus. Here, two different formulations of CaF₂ were evaluated, one in ethylene glycol and another one in lactic acid. In addition to ethylene glycol, lactic acid as a solvent was tested as it is known to be biodegradable and compostable (Garlotta 2001). Thus, in the present study, low water-soluble fluorides MgF₂ and CaF₂ synthesized as nanoparticles were investigated for their efficacy against the subterranean termites, C. formosanus.

The following results are separated into two sections based on the solvents used for synthesis of the respective sols. MgF₂ sol was synthesized with ethylene glycol as solvent. Two sols of CaF₂ were synthesized, one in ethylene glycol and another one in lactic acid. Combinations of MgF₂ and CaF₂ were also tested for efficacy in different formulations. The first formulation was the combination of MgF₂ and CaF₂ synthesized as a single sol in ethylene glycol. The second was the combination of separately synthesized sols of MgF₂ in ethylene glycol and CaF₂ in lactic acid. For comparison, NaF in H₂O as solvent was also tested.

Figure 1(a) and (b) present average termite mortality caused by respective treatment of test specimens. Termite mortality in control specimens in both series (Fig. 1a, b) was less than 50%, which is important for the overall validity of the EN 117 test. Additionally, termite mortality was significantly higher for almost all treated specimens compared to control specimens (p value < 0.01), except for specimens treated with 0.1 M CaF₂ which had a p value of 0.02 compared to control specimens.

In Fig. 1a, the termite mortality due to MgF₂-treated specimens was similar for concentrations below 0.5 M but did not reach 100%. Higher concentrations of 0.5 and 0.8 M MgF₂ resulted in 100% termite mortality. Termite mortalities caused by CaF₂ treatment in concentrations ranging from 0.1 to 0.4 M were similar. Again, termite mortality was lowest at 0.1 M CaF₂, while with higher concentrations of 0.2 and 0.4 M, the termite mortalities increased but did not reach 100%. A higher concentration of 0.8 M of CaF₂ was assumed to be more effective than a lower concentration of 0.5 M CaF₂, because the solubility of CaF₂ is lower than that of MgF₂. Hence, CaF₂ at 0.5 M was not tested for efficacy against termites. Nonetheless, wood specimens treated with the single sol combination of MgF₂ and CaF₂ at 0.5 M caused 100% termite mortality. Treatment with NaF at 0.1 M also resulted in 100% mortality (Fig. 1b). Both, CaF₂ in lactic acid at 0.8 M and the combination of CaF₂ and MgF₂ at 0.5 M led to 100% termite mortality as well. The 100% termite mortality caused by nano metal fluorides at higher concentrations indicates that the toxicity of nano metal fluorides is the same as that of NaF and can be compared to nanoboron and nanozinc borate which also led to 100% termite mortality (Kartal et al. 2009; Mantanis et al. 2014; Tascioglu et al. 2017).

Surprisingly, the specimens treated only with lactic acid also resulted in 100% termite mortality (Fig. 1b). This suggests that lactic acid might provide resistance similar to that observed for acetylated radiata pine wood exposed to R. banyulensis, which also showed 100% termite mortality (Gascón-Garrido et al. 2013).

From Fig. 2a, it can be seen that the control specimens had significantly higher average mass loss due to termite attack than treated specimens (p value < 0.01). For MgF₂ and CaF₂ treated specimens respectively, mass loss was not affected by treatment concentration. However, MgF₂ treated specimens had lower average mass loss compared to CaF₂ treated specimens. The combination treatment of MgF₂ and CaF₂ (0.5 M) had a lower mass loss than CaF₂ treatment alone, but a similar average mass loss when compared to the MgF₂ treatments at higher concentration of 0.5 and 0.8 M. Therefore, the lower concentration of 0.5 M seems to be sufficient to promote antifeedant effect against C. formosanus similar to that observed for nanozinc borate (Mantanis et al. 2014).

From Fig. 2b, it is observed that the control specimens had significantly higher average mass loss compared to fluoride treated specimens (p value < 0.01). Average mass loss in lactic acid treated specimens was not significantly different compared to control specimens (p value – 0.70), suggesting that termites consumed lactic acid treated specimens as they consumed control specimens. NaF-treated wood had mass loss less than 2%. Mass loss was even lower for CaF₂ (in lactic acid) treatment alone and for the combination treatment of CaF₂ (in lactic) and MgF₂ (in ethylene glycol), respectively. This is somehow surprising since the freely available fluoride concentration in both, MgF₂ and CaF₂, due to their lower solubility is significantly lower than in the case of NaF, although the nominal concentration of the latter is lower.

Figure 3 a shows the control specimens had higher visual ratings for damage than wood specimens treated with nano metal fluorides, which corresponds well with losses of mass (Fig. 2a). For both MgF₂ and CaF₂ treated specimens in the concentration from 0.1 to 0.4 M, the average visual assessment was above rating “3”. Based on this and according to EN 350 (2016), treated samples would be considered as “not durable”. For specimens treated with higher concentrations of MgF₂ (0.5 and 0.8 M), the visual ratings were below “2”. Similarly, test specimens treated with the single sol combination of MgF₂ and CaF₂ showed only minor damage with the average visual rating below “2”.

From Fig. 3b, it is observed that all the control specimens had an average visual rating of “4”, which was higher than those observed for treated specimens. Specimens treated with only lactic acid had an average visual rating of “2.5”, while NaF and CaF₂ (in lactic acid) treated specimens had average visual ratings below “2”. Therefore, fluoride treated specimens were less damaged than lactic acid treated specimens. Even though lactic acid treated specimens caused 100% termite mortality, they were more damaged compared to wood specimens treated with nano metal fluorides at concentrations of 0.5 and 0.8 M, which suggests that the effects of lactic acid had to accumulate over time with delayed mortality after the termites had consumed larger amounts of lactic acid treated wood. The delayed toxicity effect might be triggered by the low pH of lactic acid, which might be too acidic for the protists in the termite gut that are responsible for digesting the cellulose. However, these are unleached specimens and it is likely that lactic acid would be leached out under leaching conditions and thus reduce its toxic effect on the gut microorganisms. Finally, the two-step combination treatment of CaF₂ and MgF₂ led to even lower visual ratings of “1”, which would be considered “durable” according to durability classification in EN 350 (2016).

Nano metal fluoride treated specimens (MgF₂ and CaF₂) performed similar to NaF treated specimens. Wood specimens treated with NaF at 0.1 M had a lower rating for visual damage, lower mass loss, and higher termite mortality compared to control specimens. These results show that nano metal fluorides can be suitable alternatives to water soluble NaF. Evaluating the durability of nano-MgF₂ and -CaF₂ in comparison to NaF, at the first glance, it seems that higher concentrations of the nano-MF₂ systems are necessary. However, the opposite is true. The solubility of NaF is 40 g/L, while that of MgF₂ (130 mg/L) and of CaF₂ (16 mg/L) is significantly lower (Lide 2003). That means although nominally higher concentrations of MgF₂ and CaF₂ are needed for resistance against C. formosanus, just a very small part of the nanoparticles becomes dissolved, consequently a very low concentration of free fluoride ions is available. Thus, the latter fluorides are evidently more sustainable since the fluoride anions are released over a significantly larger time scale as they are mainly present as insoluble MF₂ and only a small part is dissolved in wood specimens depending on moisture conditions.

Even the fact that MgF₂-treated specimens performed better than CaF₂ with lower average mass loss and lower visual ratings after termite attack by C. formosanus is due to the differences in their solubility. With 130 mg/L solubility of MgF₂ in water compared to just 16 mg/L solubility of CaF₂ in water, the same nominal concentration of both sols provides a free fluoride concentration in water being nearly higher by the factor of 10 for MgF₂. Surprisingly, the combination of CaF₂ and MgF₂ has similar effects at a lower concentration of 0.5 M than using either CaF₂ or MgF₂ alone at a higher concentration of 0.8 M. A reasonable explanation might be that not both metal fluorides exist as such in these combined systems but instead CaMgF₄ is formed, which has a solubility between that of CaF₂ and MgF₂ alone. Therefore, the combination provides an increased fluoride concentration compared with the single MgF₂ system. Although the formation of CaMgF₄ complexes has been reported previously, solubility data is not available for this complex compound (Krahl et al. 2016).

Previous research has reported that fluoride can be effective at low concentrations against fungi and termites (Gay and Schulz 1965; Freitag and Morrell 2005; Pan et al. 2015). This study shows that specimens treated with nano metal fluorides are less damaged than the severely attacked controls when exposed to termites. In this work, the low water-soluble nano metal fluorides CaF₂ and MgF₂ alone performed very well, but their combination performed even better than NaF in visual ratings of treated wood specimens. This test was done over a duration of 8 weeks, while the termite resistance test with other nano treatments such as nanoboron and nano zinc borate reported in the literature were done for four weeks or less. Thus, it is expected that nano metal fluoride treated samples provide a longer-term protection compared to other nano formulations. Test specimens in this study were not leached prior to biological testing. In future experiments, leached wood specimens treated with nano metal fluorides have to be tested in comparison to leached NaF treated specimens. It can be assumed that susceptibility to leaching is significantly reduced with nano metal fluorides compared to NaF. The lower water solubility of MgF₂ and CaF₂ means that they can be used without fixatives making them more environmentally friendly and sustainable alternatives for wood protection against termites. The results from this study evidently show the potential of low water-soluble nano metal fluorides as effective and sustainable wood preservative formulations against termites.