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RESISTANCE OF BANGKIRAI (SHOREA LAEVIS)
TO ATTACK BY
THE FORMOSAN SUBTERRANEAN TERMITE
Part 3: Results and Conclusions
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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