Abstract

Wood is a natural material containing cellulose and hemicelluloses, and can therefore act as food for a group of extremely active organisms, the xylophages, whose activity leads to both the decomposition of substratum and the reduction of material to its constituent elements. The xylophages of greatest economic importance are fungi and termites. Owing to the problems associated with wood biodeterioration, especially in tropical countries, treatment techniques using preservatives are often required if wood is to be used in construction (Cavalcante 1982, Costa 2000).

Concerns have been raised about the traditional preservatives used to protect wood because of the risk they may pose to both the environment and public health. Of particular concern are preservatives based on heavy metals such as chromium and arsenic, and those containing fluorine or creosote. Because the toxic compounds used often have low biodegradability, additional difficulties can arise in the post-use disposal of treated timber (Evans 2001, Hata et al. 2006).

A more environmentally friendly alternative may occur through the use of natural biocides, and some researchers have focused on the chemical components from the extracts of wood and other plants (Ohmura et al. 2000, Cookson et al. 2004, Rodrigues et al. 2012, Syofuna et al. 2012). This study considers the use of oil from the neem tree (Azadirachta indica A. Juss) as a potential eco-friendly wood preservative. Neem oil has low mammalian toxicity, while possessing biologically active compounds, such as azadirachtin and other limonoids (Kraus 1995). Azadirachtin is a highly oxidized tetranortriterpenoid that has antifeedant and growth regulating effects on insects and fungi, although its biochemical effects at the cellular level are still not understood (Mordue and Blackwell 1993, Islam et al. 2009).

Neem oil and extracts have been reported to be active against termites, including Zootermopsis nevadensis (Hagen) (Ohmura et al. 2006), Reticulitermes speratus Kolbe (Serit et al. 1992), Coptotermes formosanus Shiraki (Grace and Yates 1992, Doolittle et al. 2007), and Incisitermes marginipennis (Latreille) (Arcos-Roa et al. 2001). Methanol and hexane extracts from a similar timber species, Azadirachta excelsa (Jack) Jacobs, were also active against Coptotermes curvignathus Hohngren (Sajap et al. 2006). Neem oil impregnated into Populus deltoides Bartr. ex Marsh controlled Microcerotermes beesoni Snyder in a laboratory bioassay (Dhyani and Tripathi 2006) and was suggested as a spray treatment that could avert termite attack (Yashroy and Gupta 2005). Neem wood or mulch itself has some resistance to C. formosanus (Delate and Grace 1995) and in India is usually free from insect attack (Koyani et al. 2011).

Neem also produces fungicidal compounds (Locke 1995, Paul and Sharma 2002, Amadioha 2004). The outer heartwood of neem was found to be naturally durable against four species of basidiomycete decay fungi (Rao 1990). High concentrations (25%) of neem oil impregnated into Pinus roxburghii Sargent controlled the white-rotting fungus Trametes versicolor (L.: Fr.) Pilát and the brown-rotting fungus Postia placenta (Fr.) M. Larsen & Lombard (Dhyani and Tripathi 2006).

Neem oil is usually produced by crushing seeds and is an available commercial product in Brazil. To examine whether commercially available neem oil could be used as a wood preservative, we dissolved it in white spirit, which is a common solvent for the timber treatment industry. This formulation was assessed against termites and decay fungi using laboratory bioassays that are standard test procedures for new wood preservatives in Australia (Australasian Wood Preservation Committee [AWPC] 2007).

Materials and Methods

Results

The mean mass losses caused by C. acinaciformis and M. darwiniensis to test specimens are shown in Table 1. Both termite species caused extensive damage to the water and white spirit controls, with mean mass losses above 90 percent for C. acinaciformis and above 99 percent for M. darwiniensis. In comparison, the specimens treated with a hazard class 2 retention of CCA had negligible mean mass loss, demonstrating excellent termite control. Almost all test specimens treated with the 0.01 and 0.1 percent solutions had mean mass losses similar to the untreated controls. However, those treated with the 1.0 percent neem solution had slightly reduced attacks compared with the controls (P < 0.05), with 59.9 percent mean mass loss against C. acinaciformis and 72.5 percent mean mass loss against M. darwiniensis, demonstrating slight termite resistance. Indeed, all termites exposed to the 1.0 percent neem treatment had died by the end of the bioassay, while the majority of termites in jars containing test specimens treated with the 0.01 or 0.1 percent neem solutions were alive. This result suggests that a higher concentration of neem could give a higher level of protection against termites.

Table 1. Mean mass loss in Pinus radiata test samples subjected to bioassay against Coptotermes acinaciformis and Mastotermes darwiniensis.

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Table 2 shows the data for mean mass losses caused by the decay fungi C. olivacea, F. lilacino-gilva, S. lacrymans, G. abietinum, and P. tephropora. The treatments with water and white spirit had no clear inhibitory effect upon the growth of all brown-rotting fungi, because mean mass losses were high, in the range 35 to 65 percent. White-rotting fungi usually cause lower mass losses in softwoods than brown-rotting fungi in bioassays, and this occurred with P. tephropora, where the mean mass loss was 19 percent (Table 2). These results with the untreated controls demonstrate the viability of the bioassay methods used.

Table 2. Efficacy of treatments against wood decay fungi.

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The treatment with CCA gave the best protection against all test fungi, because mean mass losses were below the toxic threshold of 3 percent (Table 2). In contrast, test specimens treated with 0.01, 0.1, 1.0, 2.5 or 5.0 percent (m/m) neem in white spirit were heavily decayed and generally incurred similar mean mass losses to the untreated controls at all concentrations examined. Even the highest concentration of neem (5.0%) failed to protect P. radiata (Table 2). There was a small trend in unleached blocks for lower mean mass losses with higher treatment concentrations against C. olivacea and S. lacrymans (Table 2). Nevertheless, the treatments investigated had a negligible or minor ability to provide protection against fungal attack. P. radiata samples treated with 1.0, 2.5 and 5.0 percent neem concentrations were also examined after artificial leaching. The mean mass losses in the majority of leached specimens were not significantly different from, or significantly higher than (P > 0.05), the corresponding test specimens without leaching, although there were some minor exceptions against G. abietinum and P. tephropora (Table 2). Note that the distribution shows normality, and equivalence of variances was observed (P > 0.05), validating the results from ANOVA.

Discussion

In contrast to previous reports in the literature of neem oil being a promising source of biocides, the results from this research suggest that neem oil is ineffective as a wood preservative against termites and decay fungi at the treatment levels examined. Our studies were conducted by impregnating softwood specimens and followed the standard procedures used for evaluating the efficacy of new wood preservatives in Australia (AWPC 2007). These are quite rigorous tests, where test specimens are vacuum oven-dried (H2 tests) or leached and vacuum oven-dried (H3 tests) to simulate long-term ageing in service, a process that may have removed active neem components. These results corroborate the findings by Grace and Yates (1992), who found lower activity against termites (C. formosanus) compared with other insect species in filter paper trials of a commercial neem-based insecticide. Additionally, Paes et al. (2012) showed that formulations based on alcoholic solutions of neem failed to protect impregnated wooden stakes when exposed to decay fungi in soil contact.

Despite the poor results for neem oil, there may still be merit in optimizing formulations to use it as an eco-friendly wood preservative. It is worth noting that our results showed that with neem oil at 1.0 percent, attack by termites was reduced, and the termites had died by the end of the bioassay. This result suggests that a higher concentration of neem oil could give a better level of protection against termites, possibly leading to suitable levels of preservation. Other formulations of neem may be more effective, or neem may offer synergies with cobiocides (Antwi-Boasiako and Damoah 2010). For instance, Venmalar and Nagaveni (2005) formulated neem with copper and obtained control against white- and brown-rotting fungi. Certainly, further studies are warranted to obtain a better understanding of neem oil's biocidal potential.