Susceptibility of softwood blocks to damage by subterranean termites (Isoptera: Rhinotermitidae, Mastotermitidae)

Three field bioassays were conducted to evaluate responses of the Australian subterranean termites Coptotermes acinaciformis (Froggatt) and the giant northern termite Mastotermes darwiniensis Froggatt to untreated softwood timbers. In the first bioassay, containers with a test block of each of southern yellow pine (SYP, Pinus spp.), hoop pine Araucaria cunninghamii Ait ex D. Don and slash pine Pinus elliottii Engelm. were prepared. These and other containers with blocks of termite-susceptible timber (feeder blocks) were exposed to termites by connecting to infested trees for 10 weeks. Hoop pine sapwood was significantly more susceptible to termite damage than either SYP or slash pine sapwood. SYP is a generic name for 5 different species of pine, which are difficult to separate reliably. Therefore, caution should be exercised when using "SYP" for field bioassays against termites to ensure that samples are at least from the same species of pine and preferably from the same length of timber. The rate of mass loss per block appeared independent of the number of blocks exposed to termites within a container. Variation in mass loss of hoop pine feeder blocks was significant. In a second field bioassay, termite responses to varying masses of hoop and slash pines during several exposure periods were determined. Softwood blocks were placed in plastic containers and attached to termite mounds. Significant inter-colony variability was demonstrated. Preliminary screening to determine vigour of termites should minimise this variation. The rate of mass loss per block was independent of the number of blocks in a container and appeared linear. Numbers of termites in a container appeared to be correlated with the mass of susceptible timber available. Implications for combining block mass and exposure time to achieve a standard mass loss in untreated blocks are discussed. Responses of C. acinaciformis and M. darwiniensis to untreated sapwood and heartwood of hoop and slash pines and white cypress Callitris glaucophylla Thompson and Johnson were examined in a third experiment. Hoop pine sapwood and slash pine sapwood are susceptible to damage by both species of termites, slash pine heartwood appears resistant. Hoop pine heartwood was susceptible to damage by M. darwiniensis, but resistant to C. acinaciformis. White cypress heartwood was resistant to both species of termites, with little damage to the sapwood. To minimise the considerable intra- and inter-species variations in susceptibility of softwoods to subterranean termites, the species and the proportions of sapwood and heartwood of test material should be known. The relevance of these data to wooden bait stakes used in in-ground termite baiting stations and to synthetic baits used in above-ground termite baiting stations is discussed.