5.5.2 Precursors of pheromones and allomones for avoiding intra-and interspecific competition

5.5.2 Precursors of pheromones and allomones for avoiding intra-and interspecific competition

As mentioned in part 5.2.4, many prevalent bark beetle species in Dendroctonus and Tomicus produce verbenone, which affects behavior in their own species as well as competing species in these genera and others (Ips, Pityogenes) by reducing their response to pheromone. Exposure of male and female D. brevicomis to (+)- and (- )- enantiomers of alpha-pinene for several hours caused them to produce large amounts of (+)- and (-)-trans-verbenol in their hindguts (Fig. 9, Byers, 1983c). However, the biosynthesis of verbenone in these beetles was not affected by exposure to alpha-pinene enantiomers, even though verbenone is structurally similar to alpha-pinene (Figs. 4 and 5) and is found in males landing on trees (Renwick and Vité, 1968, 1970; Byers et al., 1984). The (-)-enantiomer of trans- verbenol inhibits female D. brevicomis from entering holes and may serve as a signal to arriving females that they should avoid areas colonized by conspecifics (Byers, 1983c).

Both verbenone and trans-verbenol are produced by D. brevicomis beetles in the greatest amounts early in colonization so it was suggested that they play a role in reducing intraspecific competition (Byers et al., 1984) as well as interspecific competition with I. paraconfusus (Byers and Wood, 1980). However, verbenone (and possibly trans-verbenol) are also produced increasingly in ageing logs infested by bark beetles (Birgersson and Bergström, 1989; Byers et al., 1989c). A common bacterium, Bacillus cereus, also isolated from I. paraconfusus can make cis- and trans-verbenol from alpha-pinene (Brand et al., 1975). Several yeasts from I. typographus can interconvert the verbenols, and when grown in a phloem medium they produced the oxygenated monoterpenes à-terpineol, borneol, myrtenol, terpenene-4-ol and trans- pinocarveol, compounds also shown to be released increasingly from bark beetle holes with age of attack (Leufvén et al., 1984, 1988; Birgersson and Bergström, 1989). A mycangial fungus grown in culture media converted alcohol products of alpha-pinene to verbenone, the end product (Brand et al., 1976). These microorganisms are introduced by bark beetles during colonization and after buildup may release verbenone, thus signalling to flying beetles that remaining in these bark substrates would entail competition with established bark beetle colonies.

Myrcene and alpha-pinene may be used as precursors of pheromones and allomones in some species of conifer-feeding bark beetles. D. brevicomis, I. paraconfusus and I. pini occur sympatrically in California and Oregon and compete for ponderosa pine bark. Myrcene is converted sex-specifically by male D. brevicomis to (+)- ipsdienol, also a pheromone component of its competitor I. paraconfusus (Hughes 1973, Byers, 1982). (+)-Ipsdienol inhibits response of both D. brevicomis and I. pini to their synthetic aggregation pheromones (Birch et al., 1980a; Lanier et al., 1980; Byers, 1982). In Europe, ipsdienol, a pheromone component of I. duplicatus could act as an allomone to inhibit response of I. typographus (Byers et al., 1990; Schlyter et al., 1992). Mated males of I. typographus produce small amounts of ipsdienol and ipsenol during colonization that might function in avoiding competition (Birgersson et al., 1984; Birgersson and Leufvén, 1988). Although ipsdienol was earlier thought to be an aggregation pheromone component of I. typographus (Bakke et al., 1977), other studies have shown the compound to inhibit attraction of these beetles to cis-verbenol and methyl butenol, the most potent components (Schlyter et al., 1987b, c).

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.