Comparative susceptibility of rainforest timbers to attack by bostrychid beetles.
The 10 timbers investigated were: Anisoptera thurifera var. polyandra, Pometia pinnata, Pterocarpus indicus, Amoora cucullata, Endospermum medullosum, Buchanania heterophylla, Dracontomelon dao, Homalium foetidum, Hopea iriana and Vitex cofassus. The last 3 species are not Lyctus-susceptible according to the natural durability test results for Papua New Guinea timbers. The bostrychid species used in the test was Mesoxylin cylindrieus. The sapwood timber samples were dried in the air, and a predetermined number of beetles left in cages made of flywire. The 4-replicated trial used a randomized complete block design. The timber samples were obtained from logs stacked in the log yard awaiting milling. After drying, the moisture content of the samples was determined and noted. The beetle population was bred at the University. The number of holes/tunnels made by the beetles was used to assess the comparative susceptibility of the timbers. The period of the trial was 13 wk. Two associated aspects of the investigation were the determination of (1) the starch content of the timber species and (2) the length of the life cycle of the beetles. The starch content was determined using the iodine/iodide starch test and was expressed as: (a) the 'starch factor', obtained from the product of starch density (varying from 0 to 4) and sapwood width; and (b) the 'mean starch density', obtained by dividing the starch factor by the sapwood width, and expressed as starch factor/mm. Anisoptera thurifera, which had the highest starch factor and mean starch density, was the most heavily attacked species. Statistical analysis on the 3 species that were attacked (A. thurifera, Pterocarpus indicus and D. dao) showed a positive correlation of 0.998 at the 5% level between starch factor and number of holes/tunnel made by the beetles, so it is concluded that starch content is a strong determinant of the susceptibility of timber species to attack by the beetles. The life cycle of the beetle was 8-9 wk.