Investigating the threat to Sitka spruce from Ips typographus: discrimination and colonization of Britain's principal commercial conifer by a damaging forest pest

BACKGROUND: Ips typographus (L.), the eight-toothed spruce bark beetle (Coleoptera: Scolytinae), has devastated European Norway spruce (Picea abies) forests in recent years. For the first time, I. typographus has established localised breeding populations in Britain, where Sitka spruce (P. sitchensis) is a critical component of plantation forestry. The interactions between Norway spruce and I. typographus are well understood, but relatively little is known about the susceptibility of Sitka spruce to the beetle. This study aimed to determine whether I. typographus would select Sitka, compared to Norway spruce, as a host for breeding, and to study the chemical ecology underlying these host preferences.

RESULTS: Host choice assays were conducted in the laboratory using freshly cut spruce logs, and then verified in the field in an area with an endemic population of I. typographus. Overall, colonisation and breeding success were found to be similar in cut Sitka and Norway spruce material. The response of I. typographus, reared on both Norway and Sitka spruce, to headspace extracts of aged and fresh Norway and Sitka spruce billets was tested behaviourally using four-arm olfactometry. Odours of aged wood from the two species were equally attractive, and fresh Sitka was more attractive than fresh Norway spruce. Antennal responses to Norway Spruce and Sitka Spruce billet headspace extracts were located using GC-EAG and identified by coupled GC-mass spectrometry and GC co-injection with authentic standards. Norway- and Sitka spruce-reared beetles did not differentiate between synthetic Norway or Sitka spruce blends and responded similarly.

CONCLUSION: These findings suggest I. typographus will select and colonise cut Sitka as readily as cut Norway spruce, with implications for its establishment risk in Sitka-growing regions. The study advances the understanding of the role of both host-emitted volatile organic compounds (VOCs) and induced host preference in host selection by I. typographus.