Leaf Volatiles from Nonhost Deciduous Trees: Variation by Tree Species, Season and Temperature, and Electrophysiological Activity in Ips typographus

The leaf volatiles emitted from four nonhost tree species of Ips typographus, i.e. Betula pendula, B. pubescens, Populus tremula, and Sambucus nigra, were collected outdoors by headspace sampling in situ and analyzed by GC-MS. Three major classes of compounds, aliphatics mainly green-leaf volatiles (GLVs)], monoterpenes, and sesquiterpenes, existed in all the deciduous tree species investigated. In June, when the bark beetles are searching in flight for host trees, GLVs mainly consisting of (Z)-3-hexenyl acetate and (Z)-3-hexen-1-ol were the dominant constituents in B. pendula and S. nigra. In B. pubescens and P. tremula, sesquiterpenes (and their derivatives) and monoterpenes made up the major part of whole volatile blends, respectively. Surprisingly, sesquiterpene alcohols and other oxides released from B. pubescens in considerable amounts were not found in the closely related species, B. pendula. By August, both the total volatiles and individual compounds significantly decreased, mainly due to the maturation of leaves, since the light intensity and temperatures during sampling were the same as in June. There were almost no volatiles detected from P. tremula and S. nigra leaves in August. The total emissions from these deciduous species were significantly different among the species, with B. pubescens releasing 5–10 times more than other species. Under the conditions of constant light intensity and humidity, emissions of both total volatiles and most individual components of severed B. pendula and S. nigra branches (with fresh leaves) increased according to a saturation curve from 16°C to 40°C. Ips typographus antennae responded strongly to green leaf alcohols: (Z)-3-hexen-1-ol, 1-hexanol, and (E)-2-hexen-1-ol, but not to aldehydes or acetates in GC-EAD analyses of B. pendula and B. pubescens leaf volatiles. No antennal responses to monoterpenes, sesquiterpenes, or sesquiterpene oxides were found. These three antennally active GLVs emitted from nonhost tree leaves might be indicators of a wrong habitat in the host selection of conifer bark beetles.