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RESISTANCE OF BANGKIRAI (SHOREA LAEVIS)
TO ATTACK BY
THE FORMOSAN SUBTERRANEAN TERMITE
Construction materials used in Hawaii and other tropical and subtropical regions are exposed to severe risk of attack by subterranean termites. The Formosan subterranean termite, Coptotermes formosamis Shiraki, in particular is a severe pest in many parts of the world, and other Coptotermes species are serious pests in the South Pacific, Australia, and Southeast Asia. In Hawaii, this termite is responsible for over $ 100 million in costs for control and damage repairs each year. In these regions, the ability of building materials to resist termite attack is a critical factor in architectural design and construction decisions.
Naturally durable woods represent an alternative to the use of preservative-treated wood in building construction. For example. Western red cedar (Thuja plicala), Alaska cedar (Cliamaecyparis nooikaiensts), redwood (Sequoia sempervitae) and teak (Tecfoiia grandis) ail exhibit some degree of resistance to termite attack (Grace & Yamamolo 1991, Grace et al. 1999). Recent surveys of tree species grown in Hawaii (Grace et al. 1996) and in Malaysia (Grace et al. 1999) also identified several different trees with potential for greater use in the tropics.
Sliorea laevis Ricil. (family Diptocarpaceae) is a dense tropical hardwood species with favorable strength properties that reported to have some degree of resistance to both insects and decay fungi (Soerianegara & Lemmens 1994; Indonesia Dept. of Forestry, undated; Lopez 1984). This rather dark and attractive wood is known as Bangkirai in Indonesia, and Balau kumus in Malaysia. As with almost all durable tree species, only the heartwood of bangkirat is reputed to be durable, and the sapwood does not carry any such resistance. This need to differentiate between heart wood and sapwood in specifying naturally durable woods for construction purposes is important to bear in mind. In recent laboratory evaluations, we found that heartwood of both Hinoki (Japanese cypress) and Alaska cedar (also known as yellow cypress) was very resistant to termite attack, but the sapwood was extremely susceptible (unpublished). Fortunately, most tropical durable trees are largely heartwood.
In the present study, wafers, cut from bangkirai heartwood lumber (of Indonesian origin) were evaluated for resistance to attack by Formosan subterranean termites in a rigorous laboratory test (American Wood-Preservers' Association Standard Method El-97). This lest method consisted of both a no-choice (or single choice) assay, in which termites were provided with only a single sample of either bangkirai or susceptible Douglas-fir upon winch to feed; and a two-choice assay in which termites had the option of feeding upon either bangkirai or a Douglas-fir wafer. In both cases, each replicate with each individual wood sample was exposed to 400 termites for a 4-week (28-day) period. This test represents severe termite exposure, since the termites are freshly collected from field locations immediately before the test, and then kept under warm and humid conditions ideal for survival and feeding. Typically, Douglas-fir wafers are virtually destroyed in the 4-week test period.
Materials and Methods
Wafers cut from bangkirai (Shoiea laevfs) heartwood lumber were provided by Mr. Michael Dalke, 11AX-E (Habitat and Living Environments, LTD), 4224 Waialae Avenue, Suite 5/248, Honolulu III 96&21. These were precut lo the standard test wafer dimensions of 1 x 1 x l/4-inch specified in AWPA Standard Method El-97 (AWl'A 1993). Wafers were ovendried (90" C. 24 hours) to obtain dry weights prior to termite exposure. Wafers of the same dimensions cut from Douglas-fir heartwood, which is quite Susceptible to termite attack, were treated similarly. Two tests were conducted: (1) a no-choice (or single-choice) lest in which termites were presented either with a wafer of bangkirai or a wafer of susceptible Douglas-fir, und (2) a two-choice test in which a wafer of bangkirai 'Vas paired with a wafer of Douglas-fir within a single test container, thereby offering a choice of food to the termites.
For the no-choice lest, a single dry wafer of either Bangkirai or Douglas-fir was placed on top of a on rhr surface of 150 g of damp silica sand (moistened with 30 ml distilled water) inside a screw-top jar (8 cm diameter. 10 cm high). For the two-choice test, a wafer of bangkirai was paired will) a Douglas-fir wafer within a similar test container, under the same conditions as the no-choice test.
Formosan subterranean termites, Coptotermes formosanus Shiraki, were collected from an active field colony at the Poamoho Experiment Station (Oahu, Hawaii) immediately before the laboratory test using a trapping technique (Tamashiro et al. 1973). 400 termites (360 workers and 40 soldiers, to approximate natural caste proportions in field colonies) were added to each test jar. The no-choice test was replicated 10 times, and the two-choice test was replicated 5 times. We also included 3 additional wafers of each wood as "environmental controls" - exposed lo the same test conditions as the other wafers, but without addition of any termites to the jar - in order to recognize any weight change in the wafers due to absorbing moisture or any oilier factor; unrelated to termite attack
After adding termites, the jars were placed in an unlighted controlled-temperature cabinet at 28° C for 4 weeks (28 days), as specified in AWPA El-97. Each jar was inspected weekly for evidence of termite activity in the soil and on the test materials. At the conclusion of the 4-week test period, percentage termite mortality was recorded, the wafers were rated visually according to a 0*10 scale (where 10 is sound, 9 is light attack, 7 is moderate attack and penetration, 4 is heavy attack, and 0 is total failure of the wood sample), and the ovendry weight change was recorded for each wafer./p>
Results and Conclusions
As can be seen in the attached table summarizing the results of both tests, the Douglas-fir wafers were virtually destroyed by termite feeding over the 4-week test period, (visual ratings of 0-4). The bangkirai wafers, however, were largely untouched (average ratings of 9.8 in both the no-choice and two-choice tests), even the termites had no other food source available to them in the no-choice test and literally had to eat or starve. Out of the IS bangkirai wafers exposed to termites, 12 received visual ratings of "JO" (completely sound) and only 3 received ratings of "9" (small surface abrasions by the termites on one of the wafer surfaces.
The Douglas-fir wafers lost more than half of their original weight due to termite attack, while the bangkirai wafers actually gained weight very slightly (due to moisture uptake that was not completely removed by the short-term ovendrying at the end of the test) in the two-choice test, and lost almost no weight (overall adjusted average loss of less than 4%) In the no-choice test. Obviously, the termites avoided the bangkirai in favor of feeding on the Douglas-fir when it was available, and still avoided it to the point of starvation when no other option was available. The increased mortality among the termites exposed to bangkirai only indicates that it is extremely deterrent to them.
Clearly, these very high visual ratings and negligible weight lasses demonstrate that bangkirai heartwood is not only comparable in terms of termite resistance to Alaska cedar and leak, but also to wood pressure treated with the commonly-used preservatives CCA (rated 9-lOby Grace 1998) and disodium octaborate tetrahydrate (rated 7-10 by Grace & Yamamoto t°94). It is important to note, though, that it is only the heartwood of bangkirai that has been demonstrated to be termite and decay resistant, and that the sapwood is reported to be susceptible to attack (Lopez 1984). Fortunately, heartwood is more predominant than sapwood in this tree, but for construction purposes it is important to specify bangkirai heartwood lumber and to minimize any sapwood content.
These evaluations were performed using bangkirai of Indonesian origin. Shorea laevis is also harvested in other countries, such as Malaysia where it is known as balau kumus. Timber harvested in these other countries may also be extremely termite resistant, and S. laevis from Malaysia has indeed been reported locally to be quite durable (Lopez 1984). However, site specific differences in the environment, soil conditions, and age of trees at harvest have been shown to influence the amount of heartwood in other durable trees and the extractive content of the heartwood. Both of these factors can impact termite resistance. For example, teak from Laos, Burma, and Indonesia has been reported to be very durable, while teak from younger trees harvested in Malaysia was less durable (Grace et al. 1999, Martawijaya 1065). Thus, although S. laevis (bangkirai) is generally reputed throughout Southeast Asia to be a very durable heartwood species (Soerianegara & Lemmens 1994), it would be a wise precaution to first evaluate lumber samples from the specific countries of harvest against the Formosan subterranean termite, if bangkirai lumber from countries other than Indonesia is to be imported for construction purposes.
In conclusion, this study demonstrates that bangkirai (Shorea laevis) heartwood lumber from Indonesia is extremely resistant to termite attack, and is a viable replacement for preservative-treated lumber or other naturally durable limbers for construction use in Hawaii and other regions where the Formosan subterranean termite occurs. Care should be taken to specify heartwood lumber and minimize any sapwood content. However, if lumber is to be imported from sources other than Indonesia, we would recommend as a precaution that it also be evaluated before use, since natural durability may vary among trees grown in different regions of Southeast Asia.
References
American Wood-Preservers' Association. 1998. Standard method for laboratory evaluation to determine resistance to subterranean termites. El-97. AWP A Book of Standards.
Grace, J.K. 1998. Resistance of pine treated with chromated copper arsenate to the Formosan subterranean termite. Forest Products Journal 48(3): 79-82
Grace, J.K., and R.T. Yamamolo. 1994. Natural resistance of Alaska-cedar, redwood, and teak to Formosan subterranean termites. Forest Products Journal 44(3): 41-45.
Grace, J.K., 1XM. Ilwart, & C.H.M. Tome. 1996. Termite resistance of wood species grown in Hawaii. Forest Products Journal 46(10): 57-60.
Grace, J.K., A.H.H. Wong, and C.H.M Tome. 1999. Termite resistance of Malaysian and exotic woods with potential for plantation growth. Forest Products Journal. In Press. Indonesian Dept. of Forestry. Undated. Bangkirai. 4 pages.
Lopez, D.T. 1983. Malaysian timbers - balau. Malaysian Forest Service Trade Leaflet No. 78. Malaysian Timber Industry Board. 9 pages.
Martawijaya, A. 1965. The influence of Iree age on the durability of teak. Laporan No.98. Purest, Products Research Institute, Bogor, Indonesia. 12 pp.
Soerianegara, I., and R.H.M.I. Lemmens (Editors). 1994. Timber trees: Major commercial lumbers. Plant Resources of Soulh-Easl Asia 5(1). Proses Foundation, Bogor, Indonesia.
Tamashiro, M., J.K- Fujti & P.Y. Lai. 1973. A simple method to observe, trap and prepare large numbers of subterranean termites for laboratory and field experiments. Environmental Entomology 2:721-122.
Source: J. Kenneth Grace and Carrie H.M. Tome, Department of Entomology, University of Hawaii at Manoa, 3050 Maile Way, Room 310, Honolulu, HI 96822-2271 - A Report for Mr. Michael Dalke, April 1999
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