Pheromone production

The decision to colonize or to leave their host tree, Pinus radiata, is also made by Ips grandicollis, in Australia, only after the beetles have landed and bored into the bark (Witanachchi and Morgan, 1981). In resistant trees, the exudation of resin leads to retreat of the beetles without pheromone production. 

Once the first, or first few, beetles have bored into a suitable host tree they begin to release pheromone which attracts both sexes of the same species. In polygamous species, such as Ips, males attack first and both sexes respond. Males initiate new attacks, females locate the entrance holes of established 

Isolation and identification techniques have since become more sophisticated. Extraction of frass did lead to identification of the pheromone of D. brevicomis, but other pheromones have been isolated by condensation of the air from around logs containing boring male beetles (e.g., I. pini, Browne et al., 1974 Birch et al., 1980a) (Fig. 12.2), by extraction from the hind-guts of male beetles (e.g., I. grandicollis Vit6 and Renwick, 1971), or by the absorption of pheromone-laden air on a substrate (such as Porapak ) and its later extraction by solvent (e.g., Scolytus multistriatus, Pearce et al., 1975). 

Some species incorporate host compounds into the pheromone blend, such as myrcene with D. brevicomis. Vite et al. (1972) distinguished between species like D. brevicomis and D. frontalis (southern pine beetle) which apparently contain pheromone in their hindguts when they emerge and release it on contact 

Knowledge of the biosynthesis of bark beetle pheromones has been largely conjectural, based on simple metabolic pathways from host terpenes. However, the biosynthetic routes of the pheromone components of 7. paracon-fusus are now known (Fig. 12.3). Both sexes produced m-verbenol when exposed to (-)ff-pinene (Renwick et al., 1976), and tran -verbenol from (+)a-pinene. When exposed to the vapour of myrcene, ipsdienol and ipsenol appeared in the hindguts of male beetles, but not in females (Hughes, 1974 Hughes and Renwick, 1977 Byers et al., 1979), whereas no pheromone was detected in the guts of beetles not exposed to myrcene. Deuterium-labelling techniques have now confirmed that myrcene is converted in male I. para-confusus to ipsdienol and ipsenol (Hendry et al., 1980). Thus, all three known pheromone components are obtained by simple oxidation of host plant chemicals. 

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Abstract Pheromones are utilized by many insects in a complex chemical communication system. This review will look at the biosynthesis of sex and aggregation pheromones in the model insects, moths, flies, cockroaches, and beetles. The biosynthetic pathways involve altered pathways of normal metabolism of fatty acids and isoprenoids. Endocrine regulation of the biosynthetic pathways will also be reviewed for the model insects. A neuropeptide named pheromone biosynthesis activating neuropeptide regulates sex pheromone biosynthesis in moths. Juvenile hormone regulates pheromone production in the beetles and cockroaches, while 20-hydroxyecdysone regulates pheromone production in the flies. 

The site of pheromone production is varied amongst the insects just as there are variable structures in the different orders. Several reviews are available detailing the ultrastructure of these glands [9-11]. Evidence that pheromone biosynthesis occurs in these cells and tissues requires that the isolated tissue be shown to incorporate labeled precursors into pheromone components. In the more studied model insects this criteria has been met. 

The site of pheromone production in flies and cockroaches that utilize hydrocarbons is similar to that of the moths. Oenocyte cells produce the hydrocarbon pheromone which is transported by lipophorin in the hemolymph to epidermal cells throughout the body for release from the cuticular surface in general [20,21]. 

The role of the nervous system in pheromone biosynthesis in moths is not clearly understood. Christensen and co-workers [208-211] proposed that the neurotransmitter octopamine may be involved as an intermediate messenger during the stimulation of sex pheromone production in H. virescens. These workers suggested that octopamine was involved in the regulation of pheromone production and that PBAN s role lies in the stimulation of octopamine release at nerve endings. However, contradicting results concerning octopa-mine-stimulated pheromone production were reported in the same species as well as other moth species [163,172,212-214]. 

It appears that, in beetles, pheromone production is regulated by JH III, despite the variations in biosynthetic pathways. JH apparently regulates pheromone production in beetles that utilize both fatty acid and isoprenoid biosynthetic pathways [8,98]. Environmental and physiological factors will in turn regulate production of JH. The endocrine regulation of pheromone production in the beetles has been best studied with regard to the bark beetles. 

The evidence for 20-hydroxyecdysone stimulating sex pheromone production in the housefly comes from both direct and indirect studies. A correlation was found between ovarian development and sex pheromone production in female flies [236,237]. Surgical removal of ovaries immediately after adult emergence resulted in no sex pheromone production, whereas allatectomized (which removes the source of JH production) females produced the same amount of pheromone as did normal females [238,239]. Additionally, ovariectomized females that received ovary implants produced sex pheromone [238]. These data demonstrate that 20-hydroxyecdysone and not JH regulates pheromone production in the housefly [111]. 

Additional evidence came from the finding that sex pheromone production could be stimulated in male houseflies that do not normally produce detectable sex pheromone components. Male houseflies were found to have longer chain alkenes, Z9-27 H,but did not have Z9-23 H. Implantation of ovaries into male houseflies resulted in a change in hydrocarbon biosynthesis such that the longer chain alkenes were not made but rather they produced the shorter chain length Z9-23 H [240]. Likewise, injection of 20-hydroxyecdysone into males induced sex pheromone production in a dose-dependent manner. These studies demonstrated that males possess the biosynthetic capability to produce sex pheromone, but normally do not produce the 20-hydroxyecdysone necessary to induce sex pheromone production. Males became an excellent model in which to study the hormonal regulation of pheromone biosynthesis in the housefly. 

Another aspect of the sex pheromone communication system concerns the endogenous signals that control pheromone production and release from the emitting insect. A number of hormones have been found to be involved in the control of pheromone production in various insect species (18). Juvenile hormone was found to induce vitellogenesis and sex pheromone production in some cockroach and beetle species. However, ecdysteroids were found to be involved in regulating reproductive processes, including vitellogenin synthesis, in dipteran species. 

In moths, it was discovered in Helicoverpa zea that a peptide produced in the subesophageal ganglion portion of the brain complex regulates pheromone production in female moths (19). This factor has been purified and characterized in three species, Helicoverpa zea (20), Bombyx mori (21, 22), and Lymantria dispar (23). They are all a 33- or 34-amino acid peptide (named pheromone biosynthesis activating neuropeptide, PBAN) and have in common an amidated C-terminal 5-amino acid sequence (FXPRL-amide), which is the minimum peptide fragment required for pheromon-tropic activity. In the redbanded leafroller moth, it was shown that PBAN from the brain stimulates the release of a different peptide from the bursae copulatrix that is used to stimulate pheromone production in the pheromone gland found at the posterior tip of the abdomen (24). 

One additional factor that comes into play in the overall chemistry of the communication system relates to chemical signals from host plants that can override the photoperiodic control of phermone production. With the com earworm, it was found that a volatile chemical signal from com silk, probably ethylene, was required by wild insects for stimulation of pheromone production (33). This signal probably interacts with controls on the photoperiodic release of PBAN. 

The biosynthesis and endocrine regulation of pheromone production in beetles has been reviewed [33, 34]. Nevertheless, some more general pathways will be briefly discussed here. As corresponding structures are widespread among insects [2], the examples shown here are selected mostly from taxa other than beetles. Structures representing beetle pheromones will be shown in the context of the discussion of the corresponding species. 

Attractive Compounds. Pheromones of three Bostrychid species have been identified. Males of the lesser grain borer, Rhizopertha dominica, produce (S)-l-methylbutyl (E)-2-methyl-2-pentenoate (dominicalure 1) 112 and (S)-l-methyl-butyl (E)-2,4-dimethyl-2-pentenoate (dominicalure 2) 113 (Scheme 13). Both compounds induce aggregation of males and females however, the mixture does not show synergistic effects [226]. Pheromone release and inter-male variation as well as effects of different hosts and the presence of conspecific females on pheromone production by males of Rhizopertha have been recently investigated [227,228]. 

The male-produced sex pheromone of the red flour beetle, Tribolium cas-taneum>has been identified to be (4 ,81 -4,8-dimethyldecanal 164 (tribolure) [320,321]. During bioassays, a mixture of the (4R,8R) and (4.R,8S)-stereoisomers proved to be more active than the pure (4 ,810-enantiomer [322]. The exact enantiomeric composition of the natural product remains as yet unknown. 4,8-Dimethyldecanal was found in other Tribolium species, too [323]. Factors affecting the pheromone production in T castaneum have been described by Hussain et al. [324]. 

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