SCOTS PINE SUSCEPTIBILITY TO ATTACK BY TOMICUS-PINIPERDA (L) AS RELATED TO PRUNING DATE AND ATTACK DENSITY

The susceptibility of young Scots pine to bark beetle attack was increased by pruning trees to a similar crown size almost-equal-to 10, 7 and 1 month(s) prior to beetle flight. Beetle population in the study area was high, and spontaneous attacks were expected to occur on the pruned trees. Half of the trees were baited with split pine bolts in order to attract more beetles to attack these trees. Thus, experimental trees carrying almost-equal-to one-third of their original foliage and with different vigour indices due to the pruning history were exposed to 2 levels of beetle attack. The pine shoot beetles preferentially attacked baited trees, whereas attack rates did not differ between pruning dates. Six wk after attack, beetle performance was better in trees pruned shortly before attack than in trees pruned earlier. Vigour indices differed between the 2 treatments, but phloem starch, secondary resinosis (expressed as lesion size and resin acid content) and tree survival did not. Trees that eventually survived were significantly less attacked than those that died. but the 2 groups did not differ in tree characteristics (except in cambial electrical resistance).