5.5 PLANT COMPOUNDS AS PRECURSORS OF SEMIOCHEMICALS

Bark beetle pheromones used in aggregation and for avoidance of competition consist of many varied structures (Fig. 5). Many of the same pheromonal compounds are used by species in the same genus, such as ipsenol, ipsdienol, and cis-verbenol in the genus Ips, or exo-brevicomin, frontalin, trans-verbenol and verbenone in the genus Dendroctonus (see Borden, 1982; Birch, 1984; Byers, 1989a). Some compounds such as cis- and trans-verbenol may be found in Ips (Scolytinae) as well as Dendroctonus and Tomicus (Hylesininae). However, cis-verbenol has so far only been proven as an aggregation pheromone component for species in the subfamily Scolytinae, whereas trans-verbenol has semiochemical activity only in the subfamily Hylesininae (Fig. 5). Fig. 5 - Bark beetle pheromone components
Fig. 5. Pheromone components of bark beetles. Key: aggregation component (a), inhibitor of aggregation (i). Row 1: 2-methyl-3- buten-2-ol (a, I. typographus); 3-methyl-3-buten-1-ol (a, I. cembrae), 4-methyl-3-heptanol (a, S. multistriatus). Row 2: sulcatol (a, G. sulcatus, G. retusus); seudenol (D. pseudotsugae, D. rufipennis, D. simplex); MCH (i, D. pseudotsugae); lanierone (a, I. pini). Row 3: cis-verbenol (a, I. paraconfusus, I. typographus, I. calligraphus); trans-verbenol (a, D. ponderosae, T. minor; i, D. brevicomis); verbenone (i, Dendroctonus); chalcogran (a, Pityogenes). Row 4: ipsenol (a, Pityokteines curvidens, and many Ips: e.g. I. paraconfusus, I. grandicollis), ipsdienol (a, many Ips, e.g. I. paraconfusus, I. duplicatus, I. pini, I. calligraphus, I. avulsus); amitinol (a, I. amitinus); E-myrcenol (a, I. duplicatus). Row 5: (+)-exo-brevicomin (a, D. brevicomis, Dryocoetes); (-)-exo-brevicomin (a, Dryocoetes); methyl decadienoate (P. chalcographus). Row 6: frontalin (a, many Dendroctonus); endo-brevicomin (i, D. frontalis); multistriatin (a, S. multistriatus); lineatin (a, T. lineatum). References to above pheromones are in reviews by Borden (1982) and Byers (1989a), and the following (Bakke, 1975; Baker et al., 1977; Lanne et al., 1987; Borden et al., 1987; Byers et al., 1989b, 1990a, b; Teale et al., 1991; Camacho et al., 1993).
 Verbenone, although inhibiting attraction response of most bark beetle species tested so far, is produced in significant amounts only by species of Hylesininae (usually males).
The base structure of ipsenol, ipsdienol, and E-myrcenol resembles the plant monoterpene, myrcene; likewise, cis- and trans- verbenol as well as verbenone resemble alpha-pinene (Figs. 4 and 5). These structural similarities support the hypothesis, considered next, that in many cases bark beetles use plant compounds as precursors for their pheromone components. This hypothesis also seems logical in that a beetle would save energy by reducing the biosynthetic costs in the production of pheromones.
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Byers, J.A. 1995. Host tree chemistry affecting colonization in bark beetles, in R.T. Cardé and W.J. Bell (eds.). Chemical Ecology of Insects 2. Chapman and Hall, New York, pp. 154-213.