Induced terpene accumulation in Norway spruce inhibits bark beetle colonization in a dose-dependent manner.
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 m(-2)) and 2.6% as much gallery length (0.029 m m(-2) vs. 1.11 m m(-2)) 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 g(-1) dry phloem trees suffered only moderate beetle colonization, and above a high threshold of ∼200 mg terpene g(-1) dry phloem trees were virtually unattacked.


CONCLUSIONS
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.
Ecological Chemistry Group, Department of Chemistry, Royal Institute of Technology, Stockholm, Sweden.
Tao
T
Zhao
Paal
P
Krokene
Jiang
J
Hu
Erik
E
Christiansen
Niklas
N
Björklund
Bo
B
Långström
Halvor
H
Solheim
Anna-Karin
AK
Borg-Karlson
D059827:Agricultural Inoculants / Q000502:physiology; D000818:Animals; D001203:Ascomycota / Q000502:physiology; D002929:Cinchona / Q000378:metabolism* / Q000382:microbiology; D004305:Dose-Response Relationship, Drug; D028222:Picea / Q000378:metabolism* / Q000382:microbiology*; D013729:Terpenes / Q000378:metabolism* / Q000494:pharmacology*; D013997:Time Factors; D014197:Trees / Q000378:metabolism / Q000382:microbiology; D048889:Weevils / Q000187:drug effects* / Q000254:growth & development
D013729:Terpenes