Susceptibility of defoliated Scots pine to spontaneous and induced attack by Tomicus piniperda and Tomicus minor

In 1990-1991, Diprion pini extensively defoliated Scots pine (Pinus sylvest ris L.) trees in Lauhanvuori National Park in southwestern Finland. Many tr ees lost all their foliage, while others had ca. 10% foliage left after the second year of defoliation. Outside the national park, many nearby stands were also heavily defoliated in 1990, but were sprayed with diflubenzuron ( Dimilin(R)) in 1991. This protected the current year needles, corresponding to ca 30% of full foliage. In spring 1992, pine trees with 0, 10, 30 and 100% foliage remaining (10 sm all and 10 large trees in each category) were baited with pine bolts to ind uce stem attacks by pine shoot beetles. All baited trees were attacked by T omicus piniperda and some by T. minor. The attacks failed in all these tree s except those that were totally defoliated and some of the small trees wit h 10% foliage left. Many unbaited trees escaped attack entirely, but only t otally defoliated trees were successfully colonized (i. e. produced brood). Attack densities and brood production figures peaked in baited, large and totally defoliated trees. None of the measures (cambial electrical resistan ce, resin flow, induced lesion length by fungal inoculation, amount of hydr ocarbons or phenolic compounds) used to describe tree vigour at the time of attack gave better information than the estimated remaining foliage. We conclude that the risk for beetle-induced mortality following defoliatio n is a function of remaining needle biomass and beetle pressure. Even at hi gh beetle densities (as was simulated by baiting of trees), trees with 10% of the foliage remaining were able to defend themselves against attacking p ine shoot beetles.