Olfaction in the spruce bark beetle, Ips typographus : receptor, neuron and habitat
Sammanfattning: The bark beetle Ips typographus regularly kills spruce trees in the Palearctic. Spruces are colonized by means of attraction to an aggregation pheromone. Attraction is modulated by anti-attractive volatiles (NHV) from non-host plants. In this thesis, olfaction in I. typographus was studied. At the molecular level, putative odorant receptors (ORs) were identified. When compared with OR sequences of Tribolium castaneum, a drastic extension of bark beetle OR function was indicated. The ORs are situated in the membrane of olfactory receptor neurons (ORNs). By recording odor responses from these neurons, 17 ORN classes that strongly responded to pheromone, host, and non-host compounds, were characterized. Surprisingly, al¬most 25 % of these responded to anti-attractive NHV. The ORN for the essential pheromone compound cis-verbenol (cV), was in some sensilla co-localized with an ORN for the host plant compound 1,8-cineole (Ci). Ci inhibited pheromone attraction in the field. In addition, while the ORN for Ci responded, the response in the co-localized cV ORN was inhibited, indicating interactions between ORNs. In electrophysiological studies of olfaction, odor stimuli are often based on a known amount of compound put on a filter paper, but the amount in the vapor phase (the actual stimulus) is unknown. Using a photoionization detector, vapor amounts of compounds released from stimulus cartridges were measured. A large variation between compounds, solvents, and successive stimulations were recorded. This indicated that stimulus doses often must be corrected for differences in volatility, and that consistent test protocols are required. In nature, insects orient in semiochemically diverse habitats with intermixing odor plumes. We studied the attraction to separated pheromone components, and pheromone and anti-attractant sources in the bark beetle and in the moth Spodoptera littoralis. While the beetle responded to spacing distances of a few decimeters, the moth responded to only a few centimeters. This may reflect different processing mechanisms in the sex pheromone system of the moth as compared to the aggregation pheromone system of the beetle. In addition, a long-distance (at least 2 m) effect of NHV was found, indicating a potential for these anti-attractants in forest protection against the beetle.
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