Abstract
The effects of seasoning on resistance of sapwood and heartwood of Dalbergia sissoo Roxb., Acacia nilotica Wild., and Pinus wallichiana A. B. Jacks against consumption by Odontotermes obesus (Ramb.) was evaluated in no-choice and choice laboratory and field experiments. Seasoning was done in an oven at 60, 80, and 100 °C for 10 and 15 days. The amount of wood consumed generally decreased as the drying temperature increased, indicating that the drying process contributed to termites’ resistance of the woods and made them unpalatable for the termites. The woods that were dried at 100 °C for 15 days showed a significant reduction in weight after the consumption by termites compared to the woods dried at 60 and 80 °C and control both in laboratory and field trials. Similarly, the wood that was dried at 100 °C for 15 days showed highest termite mortality rate in laboratory no choice and choice tests. Consequently, the termites showed maximum feeding propensity on unseasoned P. wallichiana and the minimum on seasoned D. sissoo measured with significant differences in weight loss and mortality. Based on the feeding indicated by wood weight loss, the descending order of preference was Dalbergia sissoo > Acacia nilotica > Pinus wallichiana. The importance of wood seasoning for termites’ resistance is also discussed.
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Full Article
Effect of Seasoning of Different Woods on Resistance against Odontotermes obesus (Ramb.) under Laboratory and Field Choice and No-Choice Tests
Rushda Fatima,a,* Sohail Ahmed,a Muhammad Arshad,a and Shahbaz Talib Sahi b
The effects of seasoning on resistance of sapwood and heartwood of Dalbergia sissoo Roxb., Acacia nilotica Wild., and Pinus wallichiana A. B. Jacks against consumption by Odontotermes obesus(Ramb.) was evaluated in no-choice and choice laboratory and field experiments. Seasoning was done in an oven at 60, 80, and 100 °C for 10 and 15 days. The amount of wood consumed generally decreased as the drying temperature increased, indicating that the drying process contributed to termites’ resistance of the woods and made them unpalatable for the termites. The woods that were dried at 100 °C for 15 days showed a significant reduction in weight after the consumption by termites compared to the woods dried at 60 and 80 °C and control both in laboratory and field trials. Similarly, the wood that was dried at 100 °C for 15 days showed highest termite mortality rate in laboratory no choice and choice tests. Consequently, the termites showed maximum feeding propensity on unseasoned P. wallichiana and the minimum on seasoned D. sissoo measured with significant differences in weight loss and mortality. Based on the feeding indicated by wood weight loss, the descending order of preference was Dalbergia sissoo > Acacia nilotica > Pinus wallichiana. The importance of wood seasoning for termites’ resistance is also discussed.
Keywords: Wood; Seasoning; Termites; Choice and no choice; O. obesus
Contact information: a: Department of Entomology, University of Agriculture, Faisalabad, Pakistan; b: Department of Plant Pathology, University of Agriculture, Faisalabad, Pakistan;
* Corresponding author: r_482_1980@yahoo.com
INTRODUCTION
Dalbergia sissoo (Roxb.), Acacia nilotica (Wild), Eucalyptus globulus (Labill), and Pinus wallichiana (A. B. Jacks) are major furniture woods; the latter is also important for paper and crate making in parts of Pakistan (Nouman et al. 2006). D. sissoo is internationally known as the most durable timber species. These woods, like several others, are subjected to various types of biodeterioration; termites and fungi are the leading causes of destruction at different stages of wood processing (Istek et al. 2005; Goncalves and Oliveira 2006). Termites, active in various ecological zones, destroy wood quantitatively and qualitatively, and thus a large amount of money is required for preservation of wood. Odontotermes obesus Ramb. (Termitidae: Isoptera) is one of the most damaging termite species in Pakistan (Sheikh et al. 2010). It has a broad range of preference toward many types of wood and for living trees as well (Roonwal 1978; Sheikh et al. 2010). D. sissoo is reported to be the most resistant to this species of termite in many studies (Akhtar and Ali 1979; Manzoor et al. 2010; Malik et al. 2012; Qureshi et al. 2012). Nevertheless, extent of resistance varies with the methods used to assess it.
In spite of the natural resistance of wood to termite infestation, protection is still needed to avoid monetary losses. Two important strategies to protect wood from termite infestation are heat treatment and application of toxic chemical preservatives. Because of disadvantages associated with the use of chemical preservatives, heat treatment can be regarded as the more suitable option. It has yet to be determined whether heat treatment is a long- or short-term strategy. The defining feature of heat treatment (i.e., seasoning) is the drying of wood to reduce the moisture content to a level beyond which there is no further reduction in the weight of wood. The drying of wood depends on the temperature and the duration to which wood is subjected. In European thermal processes, the treatment of wood at high temperatures varies between 160 and 260 °C, and the main difference is due to the processing conditions, i.e., steaming, use of oils, and exposure to oxygen or nitrogen. The most common treatment temperatures range from 160 to 190 °C for 4 to 5 h (PLATO-process) (Tjeerdsma et al. 1998), 180 to 220 °C for 18 h (oil-heat treatment) (Rapp and Sailer 2000), and 185 to 215 °C for 2 to 3 h (ThermoWood process) (Militz 2002; FTA 2003).
Air-dried Japanese cedar was exposed to saturated water vapor at 105 to 150 °C for 6 to 72 h, and the termite resistance capabilities were lost (Momohara et al. 2003). Kiln drying over 100 °C is known to shorten the period of wood seasoning. In this reported case, the seasoning time was reduced to 6 to 72 h (Hisada 2001; Momohara et al. 2003). Still, the thermal treatment of woods has revealed a significant correlation between termite resistance and weight loss after the heat treatment process (Mburu et al. 2007). In another study, thermally compressed pine wood panels were tested against Reticulitermes flavipes, at 120 to 150 °C for 1 h. The mass loss in the specimens progressively decreased at 120 °C as compared to control specimens (Unsal et al. 2009). Irrespective of the method of seasoning, sun- or oven-drying reduced the subterranean termites’ attack on sapwood and heartwood of Ficus religiosa (L.) when they were sun-dried for 5 to 60 days, or oven-dried at 60, 80 and 100 °C for 10 or 15 days (Ahmed et al. 2013, 2014). Although thermal treatment has proven to be an effective method for deterring termites, recent studies have found contradicting results where drying has attracted termites and heightened weight loss was observed in heat-treated wood species (Rasib 2008; Sheikh et al. 2010; Aihetasham and Iqbal 2012). The differences in the methods of exposing the wood and termite species may account for the variation in results. However, wood seasoning may not only prevent termite infestation, it may also improve the wood’s dimensional stability. Thus, the present study was conducted to investigate the effects of the reduction in moisture content of D. sissoo, A.nilotica, and P. wallichiana woods on termite feeding and mortality in laboratory and field choice and no-choice tests.
EXPERIMENTAL
Wood blocks of fresh sapwood and heartwood after splicing logs of D. sissoo, A. nilotica, and P. wallichiana were purchased from a local timber market, Jhang Road, Faisalabad, Pakistan, under the guidance of a wood expert in the Department of Forestry, Range Management and Wildlife at the University of Agriculture, Faisalabad.
Collection of Termites
The termite species, O. obesus, is naturally abundant in the Entomological Research Laboratories Building at the Postgraduate Agricultural Research Station (PARS), Jhang Road, Faisalabad, Pakistan. The termites were collected using some corrugated cardboard traps (Ahmed et al. 2006). The termite-laden cardboards were brought to the Termite Management Laboratory, Department of Entomology, University of Agriculture, Faisalabad, Pakistan, for colony maintenance and subsequent use in the experiments.
Seasoning of Wood
The woods were seasoned by oven-drying to reduce the moisture content to below 20% of the initial moisture content. The sapwood and heartwood blocks, in three replicates, were oven-dried at 60, 80, and 100 °C for 10 and 15 d. The combination of 60 °C/10 d, 60 °C/10 d, 80 °C /10 d, 80 °C/10 d, 100 °C/10 d, and 100 °C/10 d were denoted as T1, T2, T3, T4, T5, and T6, respectively, while T0 was fresh/unseasoned wood.
Laboratory Tests
Completely Randomized Design (CRD) was followed in laboratory (no choice and choice) experiments to determine the effect of seasoning on weight loss of the different wood species and the termite mortality rate (%).
No choice laboratory test (wood weight loss) ASTM D 3345-08 (2008) was followed for no choice laboratory test. Wood blocks (25 × 25 × 6 mm) were weighed before and after oven-drying. A control treatment of fresh sapwood and heartwood, without oven-drying, was also included. The wooden blocks were labelled and exposed to the termites for four weeks in a specially designed apparatus. The apparatus consisted of two plastic boxes: one box housed a main/central chamber for introducing termite workers and the second box had four food chambers for placing wood specimens. The main/central chamber was connected to the other four chambers with plastic piping. The four chambers were equally spaced among each other and from the central chamber. The boxes contained moistened soil at the bottom, and a lid was placed at the top of each box. For the no choice test, blocks of only one type of wood dried at the same temperature were tied in a bundle and placed in the food chambers as the only food source for the termites along with a control wood in a separate food chamber. One thousand termite workers and 50 termite soldiers were introduced into the central chamber. After four weeks, the wood blocks were removed from the food chambers and dried at a specified seasoning temperature for 24 h. Infested blocks were cleaned with a brush and electric blower to remove debris, dirt and fecal material and weighed on a digital balance to determine the percent weight loss using the following formula:
No choice laboratory test (termites’ mortality) The mortality of the termite workers that were exposed to the seasoned wood pieces was calculated by placing seasoned blocks in 1-L glass jars filled with 500 g of sterilized soil. The test blocks were weighed before and after seasoning, and each treatment was replicated three times, along with a control treatment. Wooden blocks of the same type, dried at similar temperatures, were placed on the soil surface in a glass jar. Three replicates of unseasoned wood were exposed to termites in the separate jars. The jars were kept in the laboratory at 27 ± 2 °C and 85 ± 5% relative humidity and were exposed to the termites for four weeks. Two hundred workers and 20 soldiers were released into each jar. The termites’ activity was checked after every 10 days, and the dead termites were counted and removed to calculate the percent mortality after four weeks by applying the formula:
Choice laboratory test (wood weight loss and termites’ mortality) Wood blocks (25 × 25 × 6 mm) were weighed before and after seasoning. The dimensions of the specimens were made according to ASTM D 3345-08 (2008). A series of paired choice tests was conducted to determine the termites’ preference for various wood species. Wood blocks of A. nilotica (sapwood and heartwood), P. wallichiana(sapwood and heartwood), P. wallichiana (sapwood and heartwood), and D. sissoo (sapwood and heartwood), were seasoned and individually tied in a bundle and kept in a food chamber. Using this method, three replicates of each wood species were prepared and placed in food chamber for four weeks, along with unseasoned wood in a separate food chamber. A similar arrangement of wood blocks was also used to determine the mortality rate of the termite workers. The methods used to determine weight loss and mortality rate were the same as those used for the no choice tests.
Field Tests
The arrangement, packing, and bundling of the woods for the choice and no choice tests under field conditions were identical to that of laboratory tests. Wood blocks were exposed to the termite workers in underground/subsoil concrete chambers of 60 × 60 × 75 cm, in which a narrow border was left for the termites to enter but prevented rats and other rodents from gaining access to the wood blocks (Ahmed et al. 2014). Each wood type was cut into 13 × 5 × 2 cm and weighed before and after oven-drying. The weight loss was calculated in the same way as the laboratory tests. When the consumed woods were brought back to the laboratory, the wood blocks were dried at a specified seasoning temperature for 24 h and the hygroscopicity (equilibrium moisture content) was evaluated using the AWPA E6-05 (2008) method.
Statistical Analysis Field experiments were conducted using a randomized complete block design (RCBD) in three replications. The data were analyzed using a multivariate generalized linear model technique, following a factorial ANOVA test, using Statistix version 8.1 (Analytical Software, Tallahassee, FL). The means of significant parameters or interaction between parameters were compared using Tukey’s Honestly Significant Difference (HSD) test for paired comparisons, and significance was accepted at P < 0.05.
RESULTS
The weight loss of seasoned and unseasoned woods after termite exposure as an effect of wood, wood type, and wood treatment showed significant differences for the no-choice laboratory test (Table 1). The drying of the wood resulted in less weight loss compared to fresh wood. The heartwoods showed less weight loss than sapwoods.
The weight loss in woods treated with high temperatures over a longer duration was significantly less than that at a low temperature and a shorter duration (F= 6.18; df= 4.75, 0.77; P < 0.001). Maximum weight loss was recorded in the control treatment (fresh wood) of P. wallichiana sapwood (53.85%) after four weeks of termite exposure, while the lowest weight loss was observed in D. sissooheartwood (14.41%) dried at 100 °C for 15 d, which resulted in less weight loss (29.14%) in comparison to woods seasoned at 60 °C for 10 d. The weight loss in A. nilotica seasoned at 100 °C for 15 d (17.34%) was in between the above two extremes. The weight loss in the seasoned wood of P. wallichiana also showed a reduction (19.37%) in weight loss in comparison to the controls. The weight loss of A. nilotica sapwood (38.67%) and P. wallichiana sapwood (38.45%) was not significantly different between treatments T1 (60 °C for 10 d) and T2 (60 °C for 15 d), respectively. Similarly, the weight loss of A. nilotica heartwood and P. wallichiana sapwood (33.00% and 31.93%, respectively) were not significantly different from the D. sissoo sapwood (32.36%) for treatments T1, T2, and T4, respectively (Table 2).
The weight loss of the seasoned woods in the choice laboratory bioassay showed interactions among the treatments, woods, and wood types that were significantly different (F= 20.41; df= 5.42, 0.27; P < 0.01). The maximum amount of weight loss was recorded in the control treatment (T0) of P. wallichiana sapwood (51.51%) after four weeks of termite exposure, while the lowest amount of weight loss was recorded in the D. sissoo heartwood (16.27%) treated at 100 °C for 15 d (Table 3). The weight loss (30.67%) of A. nilotica sapwood in T3 (80 °C for 10 d) was not significantly different from the weight loss (30.41%) in the heartwood of P. wallichiana in T3. The weight loss (34.43%) in sapwood of A. nilotica in T2 (60 °C for 15 d) was not significantly different from the weight loss (34.29%) in D. sissoo sapwood in T1 (60 °C for 10 d). The seasoned A. nilotica wood showed a significant reduction in weight (20.33%) at 100 °C for 15 d as compared to the weight loss (47.01%) in T0. The P. wallichiana wood also showed a reduction in weight (21.60%) after seasoning (Table 3).
Table 1. ANOVA for Weight Loss (%) of Seasoned Woods Exposed to Termites in No-Choice Laboratory Tests
Table 2. Comparisons of Weight Loss (%) of Seasoned Woods Exposed to Termites in No-Choice Laboratory Tests
Table 3. Comparisons of Weight Loss (%) of Seasoned Woods Exposed to Termites in Choice Laboratory Tests
Termites’ Mortality in Seasoned Wood
The ANOVA of termites’ mortality revealed that the interaction among seasoning, woods, and wood types was significantly different (df= 0.98, 0.25; P < 0.01; Table 4). These results exhibited a similar trend as can be observed in the percent weight loss data. The termites’ mortality increased with rise in drying temperature. The minimum mortality was recorded in the control treatment (T0) of P. wallichiana sapwood (8.36%) after four weeks; the highest mortality rate (42.59%) was recorded in D. sissoo heartwood seasoned at 100 °C for 15 d, which increased from 17.28% in T1 (60 °C for 10 d) (Table 5).
Table 4. ANOVA for Termite Mortality (%) when Exposed to Seasoned Woods in No-Choice and Choice Laboratory Test
Table 5. Comparisons of Termites’ Mortality (%) of Seasoned Woods in No-Choice Laboratory Test
The A. nilotica heartwood showed a significantly (p>0.05) higher mortality rate (34.54%) than P. wallichiana heartwood (28.87%) seasoned at 100 °C for 15 d, which was not significantly different (p<0.05) from A. nilotica sapwood. The heartwood from each wood species exhibited a higher termite mortality rate than sapwood (Table 5). Results from the choice laboratory bioassay showed that the termite mortality rate for the interactions between seasoning, woods, and wood types was also significantly different (df= 6.566, 0.021; P < 0.01). The minimum mortality rate was recorded in the T0 (control) of P. wallichiana (5.21%) wood. However, the mortality rate significantly increased (27.84%) in T6. The highest mortality rate was recorded in the seasoned D. sissoo (39.14%) wood at 100 °C for 15 d (Table 6.).
Table 6. Comparisons of Termites’ Mortality (%) of Seasoned Woods in Choice Laboratory Test
Weight Loss in Seasoned Wood after Exposure to Termites in the Field Tests The data obtained after the exposure of seasoned wood to a field no-choice test showed that interactions among seasoning, woods, and wood types were significant (df= 1.26, 0.20; P < 0.001; Table 7). The control treatment (T0) of P. wallichiana sapwood showed the highest amount of weight loss (53.50%) after four weeks of termite exposure, while the lowest amount of weight loss (20.61%) was recorded in D. sissoo heartwood in T6 (100 °C for 15 d), when compared to weight loss (35.58%) in wood treated at 60 °C for 10 d. The weights of A. nilotica and P. wallichiana woods in T6 were reduced as well (Table 8). The percent weight loss of woods in the choice field tests decreased as the drying temperature and drying time was increased. The maximum amount of weight loss was recorded in the control treatment (T0) of P. wallichiana sapwood (51.66%), after four weeks of termite exposure, while the lowest amount of weight loss was observed in D. sissoo heartwood (19.43%) at 100 °C for 15 d, which declined to 33.61% at 60 °C for 10 d. The A. nilotica wood showed an increase in resistance to termite attack after drying, and showed a reduction in weight (20.51%) after seasoning at 100 °C for 15 d. A significant reduction of weight in the P. wallichiana and A. nilotica woods was observed at the highest temperatures and over the longest durations of seasoning, when compared with the control treatment (Table 9).
Table 7. ANOVA for Weight Loss (%) of Seasoned Woods Exposed to Termites in No-Choice Field Tests
Table 8. Comparisons of Weight Loss (%) of Seasoned Woods Exposed to Termites in the No-Choice Field Tests
Table 9. Comparisons of Weight Loss (%) of Seasoned Woods Exposed to Termites in Choice Field Tests
DISCUSSION
Results of the present studies clearly showed that seasoning (heat treatment) of the woods improved the resistance in them against termite species. One effect of this practice is to reduce the moisture content of wood. O. obesus preferred the woods with higher moisture content (Ijaz and Aslam 2002; Varmaa and Swarana 2007). Agarwal (1980) linked the termite activity with moisture contents of woods and drying temperature. There was a positive relationship between the termites’ feeding and moisture contents of wood. The preference for high moisture content woods by termites may be due to the more moist wood fiber being softer, making it easier to masticate (Delaplane and La Fage 1989). Termites preferred the highest initial moisture content wood blocks and continued to feed on them (Gautam 2011). A reduction in the moisture content makes it hard for the termites to feed upon, which was evident from less weight loss in the seasoned woods. Nakayama et al. (2005) reported that medium moisture content (79 to 103%) wood was preferred over high moisture content (133 to 191%) wood by Coptotermes formosanus.
The effectiveness of seasoning to reduce termite-induced weight loss as compared to control values and from other wood types was manifested in P. wallichiana, which recorded the lowest resistance towards termite’s infestation in the present studies. Many reports agree with the present results with respect to prevention of weight loss in the seasoned woods (Matsumura et al. 1999; Boonstra and Tjeerdsma 2006; Esteves and Helena 2009; Upadhyay et al. 2010). However, more weight loss by O. obesus in the seasoned woods has also been recorded, and authors of these studies have concluded that O. obesus might have preferred dry woods (Sheikh et al. 2010). In this case the range of drying temperature and termites’ exposure time were 50 to 100 °C and 24 to 48 h, respectively, on four different woods; Fagus sp., Pinus wallichiana, Abies pindrow, and Cedrus deodara. The dissimilarity with the present results results may be attributed to the time during which the woods were left for drying. Woods treated at high temperature (135 or 150 ºC) showed larger weight loss percentages than those treated at lower temperatures, which can be due to reduction in wood durability after drying at high temperature ranges (Momohara et al. 2003, Manzoor et al. 2009; Aihetasham and Iqbal 2012). Yet, the present study results are supported by Ahmed et al. (2013, 2014) who reported that lowest weight losses of sapwood and heartwoods of Acacia nilotica and Ficus relogiosa were observed in treatments with maximum period of seasoning (oven drying) at 100 ºC for 15 days.
Mortality was also reported when termites’ workers were allowed to feed upon the seasoned woods. In no-choice laboratory tests, the lowest weight loss was observed in D. sissoo heartwood seasoned at 100 ºC for 15 days, and the termites’ mortality was highest (42.59%) in the same treatment. Similar trends were seen in laboratory choice test. Findings of different researchers are in conformity with the present results for termites’ mortality. For instance, R. flavipes workers and soldiers exposed to experimental drying at 34 to 35 °C had an average survival time of 3.0 to 6.2 h (Collins 1969); R. flavipes workers lived only 5.1 h when exposed to 30 °C (Sponsler and Appel 1990). These reports concluded that drying caused cuticular water loss to kill termites in a short period of time. Our results suggest that cuticular water loss may not be the only mechanism for water loss in subterranean termites. In the present experiments, termites under ideal temperature and RH conditions exposed to wood with low moisture content (MC) were not able to survive, which demonstrated that wood MC was a major factor in determining termite survival. It has been suggested that termites may continue to feed indefinitely on wood with 13 to 15% MC (Forschler 1999). Wood with MC ≤24% was not adequate to sustain a subterranean termite infestation with no soil contact, probably because water obtained from the wood by termite feeding did not compensate for water loss. At least 30% wood moisture was necessary for termites’ survival for more than six months. Although ambient temperature is an important factor in survival of subterranean termites, long-term aerial infestations can only be maintained at ≥30% MC of the wood (McManamy et al. 2008). The other factors that can account for these mortalities are the chemical constituents and hardness of the seasoned woods. The termites’ workers may find seasoned woods difficult to scrape, which is one of their feeding habits. The changed physical properties may provide another rationale for termites’ mortality (Boonstra and Tjeerdsma 2006; Esteves and Helena 2009; Manzoor et al. 2010).
In addition to reduction in moisture content of wood, heat treatment causes its surface to become inactivated, which results in poor bond quality (Unsal et al. 2009). Chemical processes occur in layers near the surface that result in a modified surface with new characteristics. O. obesus usually scrapes surface layers profusely and consumes whole tissue or penetrates deep inside in case of severe infestation. A renewed wood surface after seasoning at various temperatures for different time periods may affect O. obesus feeding and survival. Wood specimens, their types and drying regimes were significantly different among themselves. This interaction has shown that best treatment was found in heartwood at high temperature for the longest drying time. High temperature with extended drying time improved resistance of the most preferred wood, which indicates effectiveness of heat treatment to enhance service life of the woods. P. wallichiana is usually recorded as susceptible wood for the termite species (Manzoor et al. 2010). Weight loss (%) of seasoned (100 °C for 15 days) P. wallichianasapwood in a no-choice field test was non- significantly different from that of D. sissoo sapwood heat treated at 80 °C for 15 days.
The method of exposure of woods to termites had no significant effect on weight loss in choice and no choice tests where in both cases low weight loss was observed when seasoned woods were used. The similar results were observed where wood specimens that had been oven dried for twenty four hours were evaluated in choice and no-choice tests against Heterotermes indicola (Wasmann) and Coptotermes heimi (Wasmann) (Malik et al. 2012). Thus, temperature and duration in particular set of conditions affect preference of woods towards termites’ feeding and should be studied on the basis of standardized methods.
CONCLUSION
- The impact of drying temperature showed that the amount of wood consumed in general decreased with an increase in the drying temperature, indicating that heat contributed to the improvement of resistance to termites.
- Among all the treatment temperatures, wood dried at 100 °C for 15 days showed a minimal amount of weight loss and the maximal rate of termite mortality, when compared to the control (unseasoned) wood in both field and laboratory (no choice and choice) tests.
- When O. obesus was given a choice of woods that were offered in combination, the termite species repeatedly preferred and maximally fed on P. wallichiana, and minimally consumed the D. sissoo.
ACKNOWLEDGMENTS
The authors are grateful for the support of the Higher Education Commission of Pakistan for providing financial assistance through the Indigenous 5000 PhD Fellowship Scheme.
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Article submitted: April 24, 2015; Peer review completed: July 20, 2015; Revised version received and accepted: July 27, 2015; Published: August 6, 2015.
DOI: 10.15376/biores.10.4.6363-6377