Dendroctonus species pheromones

Evidence for de novo synthesis of pheromone components was obtained by showing that labeled acetate and mevalonate were incorporated into ipsdienol by male Ips pini [103,104]. Similarly, labeled acetate and other labeled intermediates were shown to be incorporated into frontalin in a number of Dendroctonus species [105]. Possible precursors to frontalin include 6-methyl-6-hep-ten-2-one, which was incorporated into frontalin by D. ruffipennis [106]. The precursor 6-methyl-6-hepten-2-one also was shown to be converted to bre-vicomin in the bark beetle, Dendroctonus ponderosae [107]. In addition, the expression patterns of HMG-CoA reductase and HMG-CoA synthase are tightly correlated with frontalin production in Dendroctonus jeffreyi [108, 109]. A geranyl diphosphate synthase cDNA from I. pini was also isolated, functionally expressed, and modeled [110]. These data indicate that the de novo isoprenoid biosynthetic pathway is present in bark beetles. A variety of other monoterpene alcohols such as myrcenol, pityol, and sulcitol are probably synthesized through similar pathways [111]... 

In Dendroctonus species, stridulation may trigger the release of pheromones that deter further arrivals. Thus arriving male D. pseudotsugae (Douglas fir beetle) stridulate outside the females entrance hole, stimulating her to release... 

A second example is the spruce beetle, Dendroctonus rufipennis (Kirby) (Coleoptera Scolytidae), a major cause of mortality in mature spruce stands. Gries et al. (1992) found that the terpene verbenene (31 in Figure 19.5), was emitted from the beetles, predominantly from females, and concluded that it is a pheromone component. The oxygenated compounds seudenol, frontalin, and l-methyl-cyclohex-2-en-l-ol, were previously identified as pheromone components in this species. Attraction to verbenene alone was demonstrated in field traps, and it enhanced captures to the other pheromone components. The absolute configuration of verbenene has not been investigated. 

The aggregation pheromone of Dendroctonus brevlcomls Leconte consists of a synergistic mixture of exo-brevicomin (IV) produced by females, myrcene (V) from the host tree, and frontalin (VI) produced by males which respond to compounds IV and V. The mixture of all three compounds then attracts males and females (10). Subsequently, verbenone (VII) and trans-verbenol (VIII) are produced and deter further attack (11.). Additionally verbenone deters the response of I. paraconfusus which utilizes the same host tree species in the same area (16,17). Thus the two species may be found colonizing different parts of the same tree. Again, as in the Ips pheromones, chirality plays a role. The naturally occurring enantiomers of exo-brevicomin and frontalin in combination with myrcene elicit a greater response from both male and female D. brevlcomls than do the other enantiomers. 

Thus, both enantiomers of frontalin, a pheromone of several species of beetles belonging to the genus Dendroctonus, were separately synthesized by applying this asymmetric reaction sequence. A reversal of order of the addition of the requisite Grignard reagents (Scheme 5) affords either enantiomer at will... 

Evidence accumulated for and against the paradigm that bark beetle pheromone biosynthesis involved direct modification of host precursor monoterpenes. For 1. pini, the issue was laid to rest with the demonstration that male tissues incorporate radio-labeled acetate into ipsdienol in a manner consistent with pheromone production. Similar experiments proved the de novo biosynthesis of frontalin, an important isoprenoid-derived semiochemical produced by male Dendroctonus jeffreyi It is probable that other Coleoptera can also synthesize monoterpenes, either as pheromone components " or defensive compounds. Despite the capacity for de novo biosynthesis, plant precursor modification is likely an important source of pheromone components for some species. In these cases, plant chemicals could enter the pheromone biosynthetic pathway at later steps. 

In none of his experiments could Moeck demonstrate that primary attraction is used in host selection by the three principal species in the area Dendroctonus brevicomis (the western pine beetle), D. ponderosae (the mountain pine beetle) and Ips paraconfusus (the California five-spined engraver beetle). What Moeck s results did show, however, was that beetles landed indiscriminantly on healthy and stressed trees at about one beetle a day on each tree. Theoretically only one beetle is needed to initiate mass attack, since as soon as it releases pheromone, landing rates would increase markedly on that tree. In Moeck s study, landing rates on trees that became attacked by D. brevicomis increased to up to 800 beetles a day. 

Many compounds have been isolated and identified from various bark beetle species and claims made as to their effects on behavior. A few of the claims are conflicting and some others are not well proved. It is therefore difficult to state categorically exactly which compounds do what and to whom. Also, the behavioral chemicals to which most bark beetles respond appear to be a complex blend of components and the actual behavioral contribution and meaning of each component part remain essentially unknown. However, it is known that the pheromonal blends, so far identified, to which beetles in the genera Ips, Dendroctonus and Scolytus respond, have a number of similarities. 

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