The natural occurrence, distribution (within a plant) and roles of four phenylbutanoid compounds (anisyl acetone, cue-lure, raspberry ketone and zingerone) are elucidated for the Asia-Pacific and Oceania regions. These phenylbutanoids may act individually or in combination to attract true fruit fly males belonging to a tribe Dacini of subfamily Dacinae (Diptera: Tepritidae). Of special interest are the mutualistic interactions between the Dacini fruit fly males and the tropical daciniphilous (attracting exclusively Dacini fruit flies) orchids - leading to cross pollination for the orchids and enchanced mating success for the flies. When offered to male flies, anisyl acetone and cue-lure are generally converted to raspberry ketone. Upon consumption, raspberry ketone and zingerone are individually sequestered in the male rectal (pheromonal) gland unchanged. Attracted male flies readily imbibe the phenylbutanoid(s) in the floral synomone to compliment the endogenously synthesized male sex pheromonal components - to enhance attraction of conspecific females during courtship as well as attract conspecific males to form 'leks'. The phenylbutanoid(s) may also act as an allomone to deter vertebrate predators, especially geckos, besides possessing antimicrobial and antioxidant activities. Cue-lure, raspberry ketone and zingerone are important attractants/lures used in pest surveillance and mass trapping under the integrated pest management (IPM) program against quarantine Dacini fruit fly pest species, particularly Bactrocera tryoni and Zeugodacus cucurbitae.
Bulbophyllum apertum flower (Orchidaceae) releases raspberry ketone (RK) in its fragrance, which attracts males of several fruit fly species belonging to the genus Bactrocera. Besides RK as a major component, the flower contains smaller amounts of 4-(4-hydroxylphenyl)-2-butanol, plus two minor volatile components, veratryl alcohol and vanillyl alcohol. Within the flower, the lip (labellum) had the highest concentration of RK with much smaller quantities present in petals; other flower parts had no detectable RK. Male fruit flies attracted to the flower belong to RK-sensitive species--such as Bactrocera albistragata, B. caudatus, B. cucurbitae (melon fly), and B. tau. Removal and attachment of the pollinarium to a fly's thoracic dorsum occurred when a male of B. albistragata was toppled into the floral column cavity, due to an imbalance caused by it shifting its body weight while feeding on the see-saw lip, and then freeing itself after being momentarily trapped between the lip and column. During this process, the stiff hamulus (the pollinia stalk protruding prominently towards the lip) acted as a crowbar when it was brushed downwards by the toppled fly and lifted the pollinia out of the anther. If the fly was big or long for the small triangular lip, it would not be toppled into the column cavity and would just walk across the column, during which time the pollinarium could be accidentally removed by the fly's leg, resulting in a failed transport of the pollinarium. This suggests an unstable situation, where the orchid relies only on a particular pollinator species in the complex ecosystem where many RK-sensitive species inhabit. Wild males of B. caudatus (most common visitors) captured on Bulbophyllum apertum flowers were found to sequester RK in their bodies as a potential pheromonal and allomonal ingredient. Thus, RK can act either as a floral synomone (pollinarium transported) or kairomone (accidental removal of pollinarium leading to total pollen wastage), depending on the body size of the male fruit flies visiting the flowers.
The major fruit fly attractant component in the floral fragrance of Bulbophyllum cheiri (fruit fly orchid) is methyl eugenol (ME). In the lowland rain forest of Malaysia, the solitary and nonresupinate flowers of the fruit fly orchid attract only males of the ME-sensitive fruit fly species (Bactrocera carambolae, B. papayae. and B. umbrosa. During the morning, the fruit fly orchid flower is visited by many fruit flies, which can sometimes cover the whole flower. The number of visitors dwindles in the afternoon. Headspace analysis of the flower shows a high ME peak in the morning, a small one between 12:00 and 14:00 hr, and no detectable ME peak after 14:00 hr. The process of pollination in the wild is initiated by attraction of fruit flies to floral ME. The flower, with the aid of its specialized hinged see-saw lip (labellum), temporarily traps (< 1 min) a fruit fly pollinator between its lip and column. Just prior to this, the fly is rewarded by the opportunity to feed on the floral attractant found on surfaces of petals, sepals, and lip. The pollinaria borne by two wild B. papayae males (caught on and near the fruit fly orchid flower) are identical in morphology and structure with those obtained from the flower. Many of the B. papayae males (17 of 22 analyzed) attracted to the fruit fly orchid already possessed both ME metabolites, trans-coniferyl alcohol and 2-allyl-4,5-dimethoxyphenol, in their rectal glands. indicating that they had previously consumed ME. In this orchid-fruit fly association, both organisms gain direct reproductive benefits: the orchid flower gets pollinated without having to offer nectar, while the fruit fly boosts its pheromone and defense system, as well as its sexual competitiveness by feeding on the ME produced by the flower.
After pharmacophagy of methyl eugenol (ME), males of Bactrocera carambolae (Diptera: Tephritidae) produced (E)-coniferyl alcohol (CF) along with its endogenously synthesized pheromonal compounds. CF was shown to be released into the air by the ME-fed males only during the courtship period at dusk and attracted significantly more males and females than the ME-deprived males in wind tunnel assays. However, earlier onset of sexual attraction and a higher mating success were observed only in the wind tunnel and field cage assays on the third day posttreatment of ME. Field cage observations on the male-to-male interaction indicated that the ME-deprived males did not exhibit aggregation behavior, but that ME feeding promoted aggregation behavior in B. carambolae. Field cage observations revealed that the ME-deprived males were not only attracted to the ME-fed males, but also appeared to feed on their anal secretions. The secretions were subsequently confirmed to contain CF along with endogenously produced pheromonal compounds. Results obtained for B. carambolae were compared to those previously obtained from its sibling species, Bactrocera dorsalis, and are discussed in light of species advancement in fruit fly-plant relationships.
It is widely believed that most orchid flowers attract insects by using deception or chemical rewards in the form of nectar. Flowers of Bulbophyllum vinaceum produce a large array of phenylpropanoids that lure tephritid fruit fly males and also act as floral reward, which the flies subsequently convert to pheromone components. The major floral volatile components identified are methyl eugenol (ME), trans-coniferyl alcohol (CF), 2-allyl-4,5-dimethoxphenol (DMP), and trans-3,4-dimethoxycinnamyl acetate, whereas the minor components are eugenol, euasarone, trans-3,4-dimethoxy cinnamyl alcohol, and cis-coniferyl alcohol. Among the various floral parts, the lip (which is held in a closed position up against the sexual organs) has the highest concentration of the major compounds. An attracted male fly normally lands on one of the petals before climbing up onto and forcing the "spring loaded" floral lip into the open position, hence exposing the floral sexual organs. The architecture and location of chemical attractants of the lip compel the fly to align itself along the lip's longitudinal axis in a precise manner. As the fly laps up the compounds and moves towards the base of the lip, it passes the point of imbalance causing the lip to spring back to its normal closed position. The fly is catapulted headfirst into the column cavity, and its dorsum strikes the protruding sticky base of the hamulus and adheres to it. The momentum of the fly and the structural morphology of the long stiff hamulus act to pry out the pollinia from its anther cover. Hence, the pollinarium (pollinia + hamulus) is detached from the flower and adhered to the fly's dorsum. In this unique mutualistic association, both species receive direct reproductive benefits--the flower's pollinarium is transported for cross pollination, and the fly is offered a bouquet of phenylpropanoids (synomone) that it consumes, converts, and/or sequesters as sex pheromonal components, thus enhancing sexual attraction and mating success.
An FAO/IAEA-sponsored coordinated research project on integrative taxonomy, involving close to 50 researchers from at least 20 countries, culminated in a significant breakthrough in the recognition that four major pest species, Bactrocera dorsalis, Bactrocera philippinensis, Bactrocera papayae and Bactrocera invadens, belong to the same biological species, Bactrocera dorsalis. The successful conclusion of this initiative is expected to significantly facilitate global agricultural trade, primarily through the lifting of quarantine restrictions that have long affected many countries, especially those in regions such as Asia and Africa that have large potential for fresh fruit and vegetable commodity exports. This work stems from two taxonomic studies: a revision in 1994 that significantly increased the number of described species in the Bactrocera dorsalis species complex; and the description in 2005 of Bactrocera invadens, then newly incursive in Africa. While taxonomically valid species, many biologists considered that these were different names for one biological species. Many disagreements confounded attempts to develop a solution for resolving this taxonomic issue, before the FAO/IAEA project commenced. Crucial to understanding the success of that initiative is an accounting of the historical events and perspectives leading up to the international, multidisciplinary collaborative efforts that successfully achieved the final synonymization. This review highlights the 21 year journey taken to achieve this outcome.
The Oriental fruit fly, Bactrocera dorsalis, is a highly destructive pest of various fruits. The reproductive and host-finding behaviors of this species are affected by several plant semiochemicals that are perceived through chemosensory receptors. However, the chemosensory mechanisms by which this perception occurs have not been fully elucidated. We conducted RNA sequencing analysis of the chemosensory organs of B. dorsalis to identify the genes coding for chemosensory receptors. We identified 60 olfactory receptors (ORs), 17 gustatory receptors and 23 ionotropic receptors-including their homologs and variants-from the transcriptome of male antennae and proboscises. We functionally analyzed ten ORs co-expressed with the obligatory co-receptor ORCO in Xenopus oocytes to identify their ligands. We tested 24 compounds including attractants for several Bactrocera species and volatiles from the host fruits of B. dorsalis. We found that BdorOR13a co-expressed with ORCO responded robustly to 1-octen-3-ol. BdorOR82a co-expressed with ORCO responded significantly to geranyl acetate, but responded weakly to farnesenes (a mixture of isomers) and linalyl acetate. These four compounds were subsequently subjected to behavioral bioassays. When each of the aforementioned compound was presented in combination with a sphere model as a visual cue to adult flies, 1-octen-3-ol, geranyl acetate, and farnesenes significantly enhanced landing behavior in mated females, but not in unmated females or males. These results suggest that the ORs characterized in the present study are involved in the perception of plant volatiles that affect host-finding behavior in B. dorsalis.