Semiochemical diversity diverts bark beetle attacks from Norway spruce edges

The semiochemical diversity hypothesis (SDH) states that interference with host-selection from non-host volatiles (NHV) is an important mechanism for associational resistance. Inhibition of bark beetle attraction to point sources by non-host volatiles (NHV) is well established and might be a signal serving in host-selection also at the habitat scale. In forests dominated by Norway spruce in middle and northern Europe (N Slovakia 2006 & 2007, SE Sweden 2007), we applied a blend of NHV and verbenone, released from dispensers fixed at 2 and 6 m height at forest edges with high Ips typographus populations. In Slovakia, three different doses (0.20.7 dispensers/m forest edge) were tested in 20-tree zones of spruce stand edges. The Swedish experiments used only the middle dose. In Slovakia, there was high tree mortality but dispensers with the anti-attractants reduced killed trees in a dose-dependent manner. The reduction in tree killing ranged from 35 to 76% compared to untreated zones. Regression analysis of relative tree kill on log dispenser density was highly significant ( R-Adj(2)= 0.34, corresponding effect size d approximate to 0.98). In Sweden, with lower beetle populations, most attacks (99%) were found outside the experimental areas, with high attack rates (15 trees/ha) in a range of 1530 m from treated groups, indicating an active inhibitory radius exceeding the previous estimates. The SDH as a functional aspect of biodiversity was tested by converting spruce monocultures into an artificial semiochemically mixed forests. The use of NHV provides the only non-insecticidal method of direct protection of conifer forests. The demonstrated principle of protection is still too expensive for area-wide use, but viable for high-value areas (nature reserves). Further development of pushpull strategies or area-wide applications may prove more cost efficient. In the long-term, the only sustainable approach is a forest landscape of mixed habitats.