SOME FACTORS INFLUENCING THE SUSCEPTIBILITY OF TIMBER TO BOSTRYCHID ATTACK

In this paper common factors influencing the susceptibility of potentially susceptible timbers to bostrychid attack are outlined. In field work conducted at Grafton, spotted gum trees (Eucalyptus maculata Hook) were selected for exposure to bostrychid attack. It was evident that the initial attack on the billets was concentrated during the first month that they were exposed. No attack occurred on billets during June, July and August. The chief bostrychid species found at Grafton were Xylion cylindricus Macl., Xylion collaris Er., Xylobosca bispinosa Macl., and Xylodeleis obsipa Germar., Xylosocus gibbicollis Macl. and Bostrychopsis jesuita F. occurred in small numbers. X. cylindricus appeared consistently in large numbers and seemed to be the most common bostrychid although at times large numbers of X. bispinosa were found. From the laboratory and field work it was found that the following factors were important in relation to bostrychid attack: —

i. nature of the surface of the timber

ii. positioning of the timber

iii. relative proportion of sapwood and heartwood

iv. moisture content of the sapwood

v. climatic conditions.

Although the incidence of bostrychid attack depends on the combination of these factors (and probably others), the most important and complex criterion appears to be (iv) moisture content of the timber. The moisture content range of timber in which successful bostrychid attack can occur and the exact regions of the sapwood where the moisture content is important for the developmental stages in the bostrychid life cycle, have not yet been determined.