SUMMARYWe identified several compounds, by gas chromatographic-electroantennographic detection (GC-EAD), that were antennally active in the bark beetle Ips typographus and also abundant in beetle-attacked spruce trees. One of them, 1,8-cineole (Ci), strongly inhibited the attraction to pheromone in the field. Single-sensillum recordings (SSRs) previously showed olfactory receptor neurons (ORNs) on I. typographus antennae selectively responding to Ci. All Ci neurons were found within sensilla co-inhabited by a pheromone neuron responding to cis-verbenol (cV); however, in other sensilla, the cV neuron was paired with a neuron not responding to any test odorant. We hypothesized that the colocalization of ORNs had a functional and ecological relevance. We show by SSR that Ci inhibited spontaneous activity of the cV neuron only in sensilla in which the Ci neuron was also present. Using mixtures of cV and Ci, we further show that responses to low doses (1-10ng) of cV were significantly reduced when the colocalized Ci neuron simultaneously responded to high doses (1-10mg) of Ci. This indicated that the response of the Ci neuron, rather than ligand-receptor interactions in the cV neuron, caused the inhibition. Moreover, cV neurons paired with Ci neurons were more sensitive to cV alone than the ones paired with the non-responding ORN. Our observations question the traditional view that ORNs within a sensillum function as independent units. The colocalization of ORNs might sharpen adaptive responses to blends of semiochemicals with different ecological significance in the olfactory landscape.

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 (0.2–0.7 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 ( = 0.34, corresponding effect size d≈ 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 15–30 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 push–pull 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.

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