Epsilon Open Archive

Abstract

Background: Tree-killing bark beetles (Coleoptera, Scolytinae) are among the most economically and ecologically important
forest pests in the northern hemisphere. Induction of terpenoid-based oleoresin has long been considered important in
conifer defense against bark beetles, but it has been difficult to demonstrate a direct correlation between terpene levels and
resistance to bark beetle colonization.
Methods: To test for inhibitory effects of induced terpenes on colonization by the spruce bark beetle Ips typographus (L.) we
inoculated 20 mature Norway spruce Picea abies (L.) Karsten trees with a virulent fungus associated with the beetle,
Ceratocystis polonica (Siem.) C. Moreau, and investigated induced terpene levels and beetle colonization in the bark.
Results: Fungal inoculation induced very strong and highly variable terpene accumulation 35 days after inoculation. Trees
with high induced terpene levels (n = 7) had only 4.9% as many beetle attacks (5.1 vs. 103.5 attacks m22) and 2.6% as much
gallery length (0.029 m m22 vs. 1.11 m m22) as trees with low terpene levels (n = 6). There was a highly significant rank
correlation between terpene levels at day 35 and beetle colonization in individual trees. The relationship between induced
terpene levels and beetle colonization was not linear but thresholded: above a low threshold concentration of ,100 mg
terpene g21 dry phloem trees suffered only moderate beetle colonization, and above a high threshold of ,200 mg terpene
g21 dry phloem trees were virtually unattacked.
Conclusion/Significance: This is the first study demonstrating a dose-dependent relationship between induced terpenes
and tree resistance to bark beetle colonization under field conditions, indicating that terpene induction may be
instrumental in tree resistance. This knowledge could be useful for developing management strategies that decrease the
impact of tree-killing bark beetles.