Scolytid pheromones

Our comprehension of how JH III interacts with the biosynthesis of fatty acid-or amino acid-derived scolytid pheromones is limited to what has been learned from several preliminary studies, and in contrast to the isoprenoid pathway there is no information on any endocrine effects on the expression or activity of specific pathway enzymes. Hughes and Renwick (1977a) found that topical application of JH III to newly emerged female D. brevicomis resulted in the production of more than 1 pig of ejco-brevicomin per beetle. Feeding on fresh P. ponderosa logs also stimulated exo-brevicomin production in females as did... 

In most insects, pheromones are synthesized in specialized cells or tissues associated with the epidermis (Tillman et al., 1999). Biochemical analyses traced the localization of Scolytid pheromone accumulation to portions of the alimentary canal, particularly the hindgut (e.g. Borden et al., 1969 Byers, 1983), but the actual tissue source of pheromone components was unknown. Fortunately, the tight correlation of HMG-R gene expression with pheromone component biosynthesis meant that hybridization techniques could be used to map the location of pheromone biosynthesis. Northern blots provided the first maps, while in situ hybridizations definitively showed which tissues were elevating HMG-R mRNA in response to feeding or JH III treatment. As with endocrine regulation studies, the molecular and biochemical data complemented each other. 

There are many other scolytid species that utilize these and similar compounds to effect intraspecific communication. Francke et al. (18) have discovered several spiroketals as active components of scolytid pheromones. Additionally, several parasites and predators utilize the pheromones produced by their scolytid prey as kairomones. For example, the predatory beetle, Temnochlla chlorodla, responds specifically to exo-brevicomin produced by female D. brevlcomls (19). This same phenomenon has been demonstrated recently in Lepidoptera the egg parasite Trlchogramma sp. uses the Hellothls zea (Boddie) sex pheromone to locate the H. zea eggs (20). 

It is clearly advantageous for parasites and predators to respond to scolytid pheromones and to synchronize their arrival at a tree with large numbers of their host species. No parasites or predators appear to have exploited the sex pheromones of Lepidoptera by responding to them, and it seems that only where the responding insect is likely to encounter aggregations of prey individuals in the vicinity of a pheromone source, has this behavior been a selective advantage. 

Nardi J. B., Gilg Young A., Ujhelyi E., Tittiger C., Lehane M. J. and Blomquist G. J. (2002) Specialization of midgut cells for synthesis of male isoprenoid pheromone in two scolytid beetles, Dendroctonus jeffreyi and Ips pini. Tissue and Cell. 226, 221— 231. 

Specialization of midgut cells for synthesis of male isoprenoid pheromone components in two scolytid beetles, Dendroctonus jeffreyi and Ips pini. Tissue Cell. (in press). 

In contrast to the rutelines, the melolonthine scarabs generally use terpenoid-and amino acid-derived pheromones (reviewed in Leal, 1999). For example, the female large black chafer, Holotrichia parallela Motschulsky, produces methyl (2.S, 3. Sj - 2 - am ino-3-methy lpcn tanoatc (L-isoleucine methyl ester) as an amino acid-derived sex pheromone (Leal et al., 1992 Leal, 1997). There is no direct evidence that the chafer beetles or any other Coleoptera use the shikimic acid pathway for de novo pheromone biosynthesis, but some scarabs and scolytids (see section 6.6.4.2) may convert amino acids such as tyrosine, phenylalanine, or tryptophan to aromatic pheromone components (Leal, 1997,1999). In another melolonthine species, the female grass grab beetle, Costelytra zealandica (White), the phenol sex pheromone is produced by symbiotic bacteria (Henzell and Lowe, 1970 Hoyt et al. 1971). 

Byrne K. J., Swigar A. A., Silverstein R. M., Borden J. H. and Stokkink E. (1974) Sulcatol population aggregation pheromone in the scolytid beetle, Gnathotrichus sulcatus. J. Insect Physiol. 20, 1895-1900. 

Gries G. (1992) Ratios of geometrical and optical isomers of pheromones irrelevant or important in scolytids J. Appl. Ent. 114, 240-243. 

Gries G., Borden J. H., Gries R., LaFontaine J. P., Dixon E. A., Wieser H. and Whitehead A. T. (1992a) 4-Methylene-6,6-dimethylbicyclo[3.1.1]hept-2-ene (verbenene) new aggregation pheromone of the scolytid beetle, Dendroctonus rufipennis. Naturwissenschaften 79, 367-368. 

A population aggregation pheromone has been identified from males of the scolytid, Gnathotrichus sulcatus (69). A 65/35 mixture of the (S)-(+) and the (R)-(-) enantiomers of 6-methyl-5-hepten-2-ol (sulcatol) was isolated from the boring dust and shown to attract both females and males in a ratio of 2.65 1, respectively. 

The terpenoid-exocrine theme emphasized by scolytid beetles was again evident when the chemical constitution of the secondary attractant for the smaller European elm bark beetle, Scolytus multistriatus, was elucidated. The aggregation pheromone was identified as a mixture of (-)-4-methyl-3-heptanol, 2,4-dimethyl-5-ethyl-6,8-dioxabicyclo [3.2.1]octane (multistriatin) (XXII), and (-)-a-cubebene (XXIII), a host-derived synergist (70). All three compounds are required for the maximum attraction of beetles. The inactive diastereomers of 4-methyl-3-heptanol and multistriatin did not inhibit the responses of airborne beetles. 

Some of the aggregation pheromones of scolytid beetles also appear to be synthesized with great chiral specificity. The flight response of both sexes of the western pine beetle Dendroctonus brevicomis to (lR,5S,7R)-(+)-exo-brevicomin (XX), host terpenes, and racemic frontal in (XXI) was much greater than the response when the antipode of brevicomin was substituted (160). Similarly, (lS,5R)-(-)-frontalin was a much more powerful attractant than its antipode when tested in admixture with... 

Mori, K. Synthesis of optically active forms of sulcatol, the aggregation pheromone in the Scolytid beetle Gnathotrichus sulcatus. Tetrahedron 31, 3011—3012 (1975). 

Libbey, L. M., M. E. Morgan, T. B. Putnam, and J. A. Rudinsky Pheromones released during inter- and intra-sex response of the scolytid beetle, Dendroctonus brevicomis. J. Insect Physiol. 20, 1667—1671 (1974). 

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