Males of many tephritid fruit fly species of the genus Bactrocera show a very strong affinity to methyl eugenol (ME). An attracted male compulsively ingests ME, which is then biotransformed before its metabolites are accumulated into the rectal gland. The glandular organ is known to serve as a reservoir for sex pheromone in some species. Upon ME-feeding, males of the oriental fruit fly, Bactrocera dorsalis, selectively accumulated two metabolites, 2-allyl-4,5-dimethoxyphenol (DMP) and (E)-coniferyl alcohol (E-CF), in the rectal pheromone gland. We compared the profiles of phenylpropanoid metabolites accumulated by three other species of very high economic and quarantine importance-Bactrocera invadens, Bactrocera zonata and Bactrocera correcta, with that of B. dorsalis. Males of each species were fed artificially on ME and the metabolites stored in the rectal glands were examined by means of chromatography and spectroscopy. Similar to B. dorsalis, males of laboratory-raised B. invadens accumulated DMP and E-CF, in almost equal quantities, in the rectal sac. The sum of DMP and E-CF increased gradually with time after ME consumption and reached as high as 150 lg/male 2 days post ME-feeding. Wild males of B. invadens captured in Kenya also possessed both the compounds in varying quantities. In contrast, males of B. zonata accumulated DMP and (Z)-coniferyl alcohol (Z-CF) in an approximate ratio of 1:1; whereas B. correcta is known to convert ME to (Z)-3,4-dimethoxycinnamyl alcohol (Z-DMC) and Z-CF also in an approximately 1:1 ratio. Thus, there are three types of binary combinations of rectal phenylpropanoid volatiles (i.e. DMP ? E-CF; DMP ? Z-CF; Z-CF ? Z-DMC) utilized among the four Bactrocera species. Such differences in phenylpropanoid ingredients may play a critical role in differentiating these species if encountered in the natural habitat. In this context, the two putative sibling species-B. invadens and B. dorsalis, possess the identical subset of rectal volatiles (DMP and E-CF) in a similar proportion. Furthermore, the phylogenetic analyses of the four Bactrocera species by comparing nucleotide sequences in the mitochondrial genes showed that B. invadens clearly belonged to the same clade as B. dorsalis species. Therefore, we consider the two as the same biological species, and certainly not distinct.

The guava fruit fly, Bactrocera correcta, is widely distributed in Thailand and other surrounding Southeast Asian countries, and, like the closely related sympatric species, the oriental fruit fly, B. dorsalis, infests various fruits, including guava, peach, and mango. Males of both B. correcta and B. dorsalis are strongly attracted to, and compulsively feed on, methyl eugenol (ME). Bactrocera dorsalis males fed on ME sequester its metabolite phenylpropanoids, (E)-coniferyl alcohol and 2-allyl-4,5-dimethoxyphenol, in the rectal pheromone gland. In contrast, B. correcta males fed on ME sequester two different metabolites, (Z)-coniferyl alcohol (ZCF) and (Z)-3,4-dimethoxycinnamyl alcohol (DMC), in the rectal gland. Examination of the temporal changes of ME metabolites in B. correcta male rectal glands revealed that the total of ZCF and DMC was as high as 100 μg/male at 24 hr after ME feeding. ZCF and DMC were detected in a large proportion of wild B. correcta males captured at various sites in Thailand. Since B. correcta and B. dorsalis are sympatric species in Thailand, these two different subsets of rectal phenylpropanoids could play a role to avoid interbreeding between the species. Further survey of wild flies in Thailand revealed that a large proportion of males of B. correcta store large quantities (over 250 μg/gland) of sesquiterpene hydrocarbons, including β-caryophyllene, α-humulene, and alloaromadendrene in the rectal gland in addition to, or instead of, ZCF and DMC. Laboratory-reared males also sequestered β-caryophyllene and α-humulene, along with ZCF and DMC, when the sesquiterpenes were artificially supplied together with ME. A field test demonstrated that a mixture (1:1) of β-caryophyllene and α-humulene attracted male B. correcta, albeit in smaller numbers than in traps baited with ME. The sequestration of sesquiterpenes, in addition to the different ME metabolites in the pheromone gland in B. correcta males, contrasts with the situation in B. dorsalis males, suggesting a potential role in intra and/or inter-specific interactions between these sympatric species.

The Japanese orange fly, Bactrocera tsuneonis, infests various citrus crops. While male pheromone components accumulated in the rectal glands are well-characterized for Bactrocera, but information regarding the chemical factors involved in the life cycles of B. tsuneonis remains scarce. Herein, several volatile chemicals including a γ-decalactone, (3R,4R)-3-hydroxy-4-decanolide [(3R,4R)-HD], were identified as major components, along with acetamide and spiroketals as minor components in the rectal gland complexes of male B. tsuneonis flies. The lactone (3R,4R)-HD was also identified in female rectal gland complexes. The amount of this compound in mature males was significantly higher than those observed in females and immature males. The lactone (3R,4R)-HD was detected in flies fed with sucrose only, indicating that this lactone is not derived from dietary sources during adulthood, but biosynthesized in vivo. The predominant accumulation of (3R,4R)-HD in mature males also suggests a possible role in reproductive behavior.