BIOLOGICALLY SYNTHESISED COPPER OXIDE AND ZINC OXIDE NANOPARTICLE FORMULATION AS AN ENVIRONMENTALLY FRIENDLY WOOD PROTECTANT FOR THE MANAGEMENT OF WOOD BORER, Lyctus africanus

The management of Lyctus africanus, one of the major dry wood pests in the tropical region is difficult due to its secluded habits and long lifecycle and therefore, its control measures are limited to the usage of insecticides. The insecticides particularly the metal salts are effective, but in some cases their leaching leads to concerns about environmental pollution. Nanometal particles are found to be more effective than metal salts. Presently available metal nanoparticles are synthesized using physical or chemical methods and their production results in toxic by-products and are costly. The current investigation deals with synthesis and use of metal nanoparticle for wood protection in an environmentally friendly and cost-effective way. The plant extracts that are reported to have wood preservative properties were used for the synthesis of metal nanoparticles. Copper oxide and zinc oxide nanoparticles were synthesized using leaf extracts of Lantana camara. The efficacy of the synthesized Lantana camara leaf extract and copper oxide or zinc oxide nanoparticle formulation as a wood protectant was tested against Lyctus africanus according to standard BIS. The formulation of copper oxide nanoparticle and Lantana camara leaf extract effectively protected the treated rubberwood blocks from Lyctus africanus attack, when compared to zinc oxide nanoparticle Lantana camara leaf extract formulation and can be developed into a stable, ecofriendly wood preservative.


INTRODUCTION
The powder post beetle Lyctus africanus, a destructive wood borer of timber and its products is one of the most serious pests of economic importance (Helal 1983, Creffield 1996. Timber characteristics such as moisture, starch content and vessel size suitable for the insertion of the ovipositor of the lyctid beetles are the factors that determines the susceptibility of the timber to the pests (Peters et al. 2002, Ito 1983. Kartika and Yoshimura (2013) confirmed the importance of starch and sugar to attract adult females of L. africanus to lay their eggs on starchy sites. In general, it is difficult to detect the initial stages of infestation of this beetle and is unnoticed until attack is much advanced. The main sign of an active infestation of L. africanus is small accumulations of fine powder below exit holes in infested wood (Peters et al. 2002). The efforts to study and control them is complicated due to their reclusive habits and relatively long generation time (Tan et al. 2003).
Wood preservative treatment is an effective control measure and it protects wood and wood products from deteriorating organisms like decay fungi and wood boring insects (Lebow 2010). It involves use of toxic or repellent wood preservatives and chemical pesticides for increasing the durability and decay resistance of wood (Lepage et al. 2017). Many chemicals and pesticides have been recommended as preservatives for the protection of non-durable timbers against borers (Gnanaharan andMathew 1982, Gnanaharan et al. 1983, Ali and Hashim 2019) which includes commercial chlorpyriphos formulations (Remadevi and Muthukrishnan 1997), diffusion formulation of Boracol-40 (Creffield et al. 1983), lindane or cypermethrin (Van Acker et al.1990), chlorpyriphos in solution (Fears andLeca 1995, Tolley et al. 1998) and copper ethanolamine (Kalawate 2013). The conventional wood preservatives are very effective to protect wood but their usage often lead to environmental pollution. The leaching of these toxic constituents from the treated wood in use is a matter of concern as these are hazardous to animals and human beings even after disposal (Venmalar 2017, Lepage et al. 2017. Groenier and Lebow (2006) reported that there are efforts to restrict, replace or eliminate the use of wood preservatives currently in use and, researchers are in search of developing new wood preservatives which are environmentally friendly, efficient, less toxic and cost effective.
Metals especially salts of copper, zinc and boron are reported to be effective as wood preservatives alone or in combination with other compounds (Richardson 1997, Zabel and Morrel 1992, Ash and Ash 2004. The disadvantage of using metal salts is their leachability (Lebow 1996, Temiz et al. 2014). Humar et al. (2001 has discussed the leaching of copper from treated wood and suggested amines as a fixative, a substitute for toxic chromium. Freeman and McIntyre (2008) and Kartal et al. (2009) reported that metal nanoparticles are less leachable compared to metal salts and can provide much higher retention resulting in higher biological durability of wood. Many researchers have reported the use of nanometals for protecting wood (Clausen 2007, Kartal et al. 2009, Clausen et al. 2010, Akhtari and Arefkhani 2010, Bak et al. 2012, Terzi et al. 2016, Lykidis et al. 2016, Nair et al. 2017. Borges et al. (2018) has reviewed the advances and application of nanotechnology in wood protection.
But there are concerns about the impacts of nanoparticles on various systems and environment which requires proper understanding and investigation. Platten et al. (2014) reported the potential of the release of copper nanoparticles from pressure treated lumber. Civardi et al. (2015) reported the possible environmental fate of Cu-based nanoparticles from wood preservatives. Reports are available in literature showing the negative impacts of nanoparticles on plant system, soil microbial community, and rhizosphere (Raffi and Husen 2019, Wagay et al. 2019, Husen and Iqbal 2019. Accumulation of nanoparticles cause human health problems (Siddiqui et al. 2013, Dwivedi et al. 2018. There are reports of nanoparticles used for plant growth improvement, increased seed germination, nutrient quality, pest management, food additives, packaging materials and as health supplements also (Khot et al. 2012, Sekhon 2014, Oliveira et al. 2015, Kumar et al. 2017, Duhan et al. 2017, Cao et al. 2018, Husen and Iqbal 2019. Therefore, issues related to optimal use of nanoparticle and its ethical and social implications in different systems needs appropriate consideration. Nanometals which are reported to protect wood are synthesized by employing physical or chemical methods and these procedures are energy intensive with the use of toxic chemicals and are expensive (Vijayaraghavan and Ashokkumar 2017). Singh et al. (2018) has summarized the current state of research on the green synthesis of metal and metal oxide nanoparticles with their advantages over chemical synthesis methods. Biological synthesis of nanoparticle is possible by using biomaterials such as fungi (Dhillon et al. 2012), virus (Dujardin et al. 2003, bacteria (Hulkoti and Taranathm 2014), microalgae (Schrofel et al. 2011), yeast (Moghaddam et al. 2015), plant extracts (Mittal et al. 2013) and macroalgae (Singaravelu et al. 2007) as an alternate, ecofriendly, non-toxic method of synthesis. Among the biomaterials, plants are preferred over other biomaterials, because of their availability, renewability, simplicity of process, efficiency, cost effectiveness and most importantly, stability of the synthesized nanoparticles (Vijayaraghavan and Ashokkumar 2017, Iravani 2011). Singh et al. (2018) has reviewed research on green synthesis of metal and metal oxide nanoparticles, their use in environmental remediation and their environmental impact or toxicity. Shiny et al. (2019) reported a preliminary study on copper oxide nanoparticle biosynthesis using selected leaf extracts and its effectiveness as wood preservative against decay fungi and termites. Current research work is the first study to examine the efficacy of biologically synthesized copper oxide (CuO) and zinc oxide (ZnO) nanoparticle formulation using Lantana camara leaf extract against the wood borer Lyctus africanus.

Maintenance of laboratory cultures of Lyctus africanus
Culture of Lyctus africanus (Lesne) was maintained on dried tapioca chips in the laboratory as suggested by Nair and Mathew (1984). Thin slices of raw tapioca were dried in an oven at 60 °C in the laboratory or in sunlight. Adults of L. africanus collected from the field were confirmed using light microscope and released into glass bottles containing dry tapioca chips. The glass bottles were covered either by lids fitted with fine mesh or by muslin cloths and kept in a dry place in the laboratory. After two months, the establishment of culture was verified by taking out the tapioca slices from the culture and examining the stages under light microscope. Adults emerged from the culture were used for the study.

Preparation of test specimens
Rubberwood, Hevea brasiliensis (Willd. ExAdr. Juss.) Muell. Arg., which is classified as perishable wood (Findlay 1985) was procured from M/s Starline Packers Pvt. Ltd, Bengaluru, India. Wood specimens of rubberwood with the specification of 50×25×15 mm 3 , free from knots, mould, stain, and any visual defects, with long axis parallel to the grain of the wood were prepared according to IS 4873 Part 2 (2008) with some modification. These wood specimens were tested for the presence of adequate starch content as described by Kartika and Yoshimura (2013). Rubberwood specimens containing abundant starch grains were selected and six matched replicates were used for the treatment and six matched untreated rubberwood specimens served as control. The required rubberwood specimens were numbered, dried and stored in desiccators prior to treatment.

Synthesis of copper oxide and zinc oxide nanoparticles by using Lantana camara leaf extract
Fresh leaves of Lantana camara (Lantana) were collected from Nallal, Karnataka and were used for the synthesis of CuO and ZnO nanoparticles. Leaf extract 20 % by weight was prepared as per the procedure adopted by Shiny et al. (2019). Precursor solution of CuSO 4 , 5H 2 O and Zn (NO 3 ) 2 , 6H 2 O were prepared (0,025M) and preheated separately in a water bath at 60 °C for 30 minutes. The leaf extract was also preheated in a water bath at 60 °C for 30 minutes and was mixed with respective precursor solutions at 1:4 proportions (Majumder 2012). The resultant formulations were kept in a water bath at 80 °C for 10 minutes, stirred at 104,71 rad/sec for 10 minutes and kept at room temperature for 1h. The formulations containing copper oxide or zinc oxide nanoparticles and Lantana leaf extract were used for the testing. The formulation was composed of Lantana leaf extract at 20 % w/v having an estimated concentration of 0,499 % CuSO 4 with an active ingredient of copper at 0,13 %. Similarly, ZnO nanoparticle Lantana leaf extract formulation was composed of 20 % w/v of Lantana leaf extract with an estimated concentration of 0,594 % of Zn (NO 3 ) 2 with an active ingredient of zinc at 0,2 %.

Characterization of biologically synthesized CuO and ZnO nanoparticles
Scanning electron microscopy (SEM) coupled with Energy dispersive analysis X-ray (EDAX) was used for the characterization of the synthesised CuO and ZnO nanoparticles as per the method described by Shiny et al. (2019).

Pressure impregnation method and retention
Rubberwood specimens were pressure impregnated with CuO and ZnO nanoparticle Lantana formulations, Lantana leaf extract and reference solutions respectively. Rubberwood specimens were completely immersed in the treatment solution kept in the pressure impregnation chamber, a vacuum of 3,99 kPa was applied for 15 min followed by a pressure 344,74 kPa for 60 min. After the treatment rubberwood specimens were taken out of the treatment solution, drained completely, weighed and then air dried to constant weight. Average preservative retention for rubberwood specimens treated with the solution was calculated.
Retention was calculated as per ASTM D1413-05 (2005) using the following Equation 1.

Laboratory testing of the preservative formulation against borer Lyctus africanus
The performance of a newly developed preservative formulation is bioassayed as per IS 4873 Part 2 (2008). In this method, the rubberwood test specimens were stored individually in glass containers provided with finely woven cotton cover to prevent mite infestation. The temperature and relative humidity during the test were maintained at 25 °C to 30 °C and 70 to 75 % respectively. The treated rubberwood specimens were exposed to four pairs of adult L. africanus. Presence of exit holes is an indication of the number of emerged beetles and is a measure of the effectiveness of the treatment. The control and treated rubber wood specimens were examined periodically for the presence of exit (emergence) holes. The number of exit holes in each test specimen was counted manually and the time taken for the appearance of first exit hole was recorded. The number of exit holes or the number of emerged beetles from treated wood and control wood exposed to L. africanus were compared. Mean and standard deviation were calculated using Microsoft Excel Package.

RESULTS AND DISCUSSION
Formulations containing copper oxide and zinc oxide nanoparticles were prepared using Lantana leaf extract. The presence of CuO nanoparticle was indicated by the change in colour from bluish green to dark green and finally to brown and ZnO nanoparticle, by the turbidity of the solution. This is in agreement with earlier reported studies of Majumder (2012)

Characterization of biologically synthesized CuO and ZnO nanoparticles
Scanning electron micrograph (SEM) of CuO and ZnO nanoparticles synthesised using Lantana leaf extract is shown in Figure 1. The synthesised nanoparticles were not agglomerated. The synthesized CuO nanoparticles were found to be spherical in shape with a particle size of 33 nm to 46 nm. Particle size of the synthesized ZnO nanoparticle was between 34 nm and 37 nm. EDAX profile confirmed the presence of copper and zinc in major fraction (Figure 1). This is in agreement with earlier reports by Lee et al.

Retention of the preservative formulation
Retention is expressed as kilograms of preservative per cubic meter of wood (Canadian wood council 2018) and is a measure of the amount of preservative that is retained in the wood after the treatment. It is one way of expressing the effectiveness of the preservative used and the treatment method employed. Table  1 shows the retentions of preservative formulations in rubberwood. It was observed that rubberwood test specimens retained similar amount for all the three preservative formulations. The uptake of reference solutions at a concentration of 0,5 % was also found to be similar.

Laboratory testing of the preservative formulation against borer L. africanus
Untreated control specimens of rubberwood developed exit holes within six months of inoculation of the L. africanus. The number of exit holes in the control specimens has increased progressively thereafter. Similarly, rubberwood specimens treated with Lantana leaf extract alone has also developed exit holes within six months (Table 2). This clearly indicates that Lantana leaf extract alone does not impart any protection to treated wood specimens against borers. After two years of inoculation, all the rubberwood specimens treated with CuO nanoparticle Lantana leaf extract formulation and tested were free of L. africanus attack evidenced by the absence of exit holes (Figure 2). Whereas, rubberwood specimens treated with ZnO nanoparticle Lantana leaf extract formulation developed exit holes after six months of exposure to L. africanus. The numbers of emergence holes in the control and Lantana leaf extract treated rubberwood blocks were 35,33 ± 2,5 and 45,66 ± 4,0 respectively. Whereas, the number of exit holes in the ZnO nanoparticle Lantana leaf extract formulation treated rubberwood blocks was 25,66 ± 9,5 (Table 3).
The results indicate that CuO nanoparticle Lantana leaf extract formulation gave better protection to the treated rubberwood specimens for a period of two years compared to ZnO nanoparticle Lantana leaf extract formulation against L. africanus. Reference solutions of Copper sulphate (CuSO 4 , 5H 2 O), Zinc nitrate (Zn (NO 3 ) 2 , 6H 2 O, Copper oxide nanopowder, Zinc oxide nanopowder, Copper Oxide (CuO) and Zinc oxide (ZnO) solutions failed to protect the treated wood specimens against L. africanus beetles. Whereas, the low resistance offered by reference solutions can be attributed to the lower concentration and to the choice of the medium used to prepare the test solution (Nair et al. 2017, Nair et al. 2018, Terzi et al. 2016. It can be concluded that, since both Lantana leaf extract and reference solutions alone does not offer any kind of protection to the treated rubberwood specimens, the protection offered by CuO nanoparticle Lantana leaf extract formulation against L. africanus is due to synergistic effect of the synthesised CuO nanoparticles and Lantana leaf extract in the formulation.

CONCLUSIONS
It has been concluded that Lantana camara leaf extract has the potential for the biological synthesis of CuO and ZnO nanoparticles. Formulations containing Lantana leaf extract and CuO, ZnO nanoparticles were tested against Lyctus africanus which is a serious timber pest. Lantana leaf extract and CuO nanoparticle formulation was found to be effective against the beetles for a period of two years where as Lantana leaf extract ZnO nanoparticle formulation was effective for a period of six months only. This can be attributed to the synergistic effect of the properties of Lantana leaf extract and the synthesized nanoparticles as both Lantana leaf extract and reference solutions alone failed to protect the treated rubberwood specimens from L. africanus attack. Since the prophylactic treatment of timber with the Lantana leaf extract and CuO nanoparticle formulation effectively protected the timber from beetle attack, it can be recommended for the management of L. africanus while lessoning potential detrimental effects on the environment. But to assess the possibility of using this formulation as a wood protectant, further investigation regarding leachability and stability of the formulation are required.