Potential parasitoid attractants, volatile composition throughout a bark beetle attack

Abstract

Several hymenopteran parasitoids prey on mature bark beetle larvae (Scolytidae), concealed under the bark of conifers. According to previous results, certain oxygenated monoterpenes are crucial in host location. However, synthetic baits tested in laboratory bioassays are not quite as attractive to the parasitoids as the natural volatile bouquet from conifers containing susceptible bark beetle larvae. The aim of the current study was to identify the "missing components" in previously tested synthetic baits. Volatiles were sampled throughout bark beetle attacks (Ips typographus) on Norway spruce logs (Picea abies), and analyzed (qualitatively and quantitatively) by coupled gas chromatography-mass spectrometry (GC-MS). In addition, a pilot test was done employing the relatively new technique, fast GC-surface acoustic wave detector (SAW), zNoseTM, described in the methods. Quantitative differences in volatile composition were found between infested and uninfested logs, and between different stages of the bark beetle attack. In addition, the volatile composition of logs with or without mature bark beetle larvae (susceptible to parasitism) was compared, isolating odor differences at this most suitable host stage. In general, the quantity of monoterpenes (the predominant phytochemical volatiles in conifers) was found to decrease with time after initial infestation. In contrast, certain oxygenated monoterpenes and benzenoid compounds increased as susceptible bark beetle larvae developed, which therefore could be candidates for improving parasitoid baits. To the best of our knowledge, this is the first report on long-term dynamics of the volatile composition of bark beetle infested P. abies. The results should be interpreted in the light of previous electrophysiological and behavioral investigations of larval parasitoids, and present a number of compounds that could improve synthetic parasitoid baits.