Identification of host-related volatiles attractive to pineapple beetle Carpophilus humeralis

Article abstract of DOI:10.1023/A:1020857721010

Volatiles collected from oranges fed upon by Carpophilus humeralis of either sex were consistently more attractive than volatiles from beetle-free oranges in wind-tunnel bioassays. Three compounds were identified as attractants from this system: 4-ethyl-2-methoxyphenol (1), 2,5- diisopropylpyrazine (2) (a new natural product), and 2-phenylethanol (3). Identifications were confirmed with synthetic compounds that had matching chromatographic and spectral properties. Compounds 1, 2, and 3 had only slight activity alone, but were highly synergistic with each other and with propyl acetate (PA), a fruity ester that is mildly attractive to Carpophilus beetles. Compound 2 was the most active in the wind tunnel; its threshold dose was 0.5 ng when PA was present. The structural specificity for these compounds was high. Twelve phenol analogs of 1 were tested, but only one of these, 2-methoxyphenol, was more attractive than the control. Similarly, the analogs of 2, 2-isopropylpyrazine and 2,6-diisopropylpyrazine, were completely inactive. In the field, a combination of 1, 2, and 3 was not attractive by itself, but it strongly synergized attraction to fermentation volatiles, Carpophilus pheromones, or both. Compounds 1, 2, and 3 apparently have a microbial origin because all three were detected when the host fruit was pineapples instead of oranges, because they could occur in the absence of beetles, and because autoclaved pineapple began to produce the compounds after inoculation from an attractive piece of fruit. The study demonstrated that host location for this generalist species can be far more complex than responding simply to the bouquet of low-molecular-weight volatiles normally associated with fermentation.