Abstract
Bark beetles kill apparently vigorous conifers during epidemics by means of pheromone-mediated aggregation. During non-endemic conditions the beetles are limited to use trees with poor defense, like wind-thrown. To find olfactory cues that help beetles to distinguish between trees with strong or weak defense, we collected volatiles from the bark surface of healthy felled or standing Picea abies trees. Furthermore, living trees were treated with methyl jasmonate in order to induce defense responses. Volatiles were analyzed by combined gas chromatography and electroantennographic detection (GC-EAD) on Ips typographus antennae. Compounds eliciting antennal responses were characterized by single sensillum recording for identification of specific olfactory sensory neurons (OSN). Release of monoterpene hydrocarbons decreased, while oxygenated compounds increased, from spring to early summer in felled trees. In both beetle sexes particular strong EAD activity was elicited by trace amounts of terpene alcohols and ketones. 4-Thujanol gave a very strong response and the absolute configuration of the tested natural product was assigned to be (+)-trans-(1R,4S,5S)-thujanol by stereoselective synthesis and enantioselective gas chromatography. One type of OSN responded to all ketones and five other OSN were characterized by the type of compounds that elicited responses. Three new OSN classes were found. Of the eight EAD-active compounds found in methyl jasmonate-treated bark, the known anti-attractant 1,8-cineole was the one most strongly induced. Our data support the hypothesis that highly active oxygenated host volatiles could serve as positive or negative cues for host selection in I. typographus and in other bark beetles.
Highlights
Hosts with weak or absent defenses, such as damaged or wind-broken trees, are the only choice for Ips typographusL. during long periods of low, “endemic”, population levels
A total of 170 gas chromatography and electroantennographic detection (GC-EAD) runs were made for 19 different volatile samples, from seven felled and two standing spruce trees collected at ten different time points during April to June, using two GC columns of different polarity
To a bark beetle it is of utmost importance to obtain information regarding the physiological status of a potential host tree
Summary
Hosts with weak or absent defenses, such as damaged or wind-broken trees, are the only choice for Ips typographusL. during long periods of low, “endemic”, population levels. Host acceptance by the attacking bark beetles and resistance in healthy host trees are governed by the strength of induced defenses (Schiebe et al 2012; Zhao et al 2011). The ability to detect rare weakened hosts, still with sufficient nutritional quality but impaired defenses, is crucial for beetles in endemic populations, in order to avoid dispersal losses or death while attacking resistant host trees. Even during population outbreaks it is uncertain whether the first attacking pioneer beetles rely on sufficient numbers of conspecifics to overcome the defenses of strongdefending host trees, and avoid the threat to be killed. In contrast to Pinus with high constitutive defenses, P. abies has weaker constitutive, but stronger induced defenses (Franceschi et al 2005). The same relationship was stressed by Raffa and Berryman (1987) for Abies spp. and Scolytus ventralis LeConte, when compared to P. contorta and D. ponderosae
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