Alvin Kah-Wei Hee

Attraction of female and male Bactrocera papayae to conspecific males fed with methyl eugenol and attraction of females to male sex pheromone components

The attraction of female and male Bactrocera papayae to conspecific males fed with methyl eugenol (ME) and female attraction to male synthetic sex pheromone, trans-coniferyl alcohol (CF), were evaluated in a wind tunnel. Earlier and greater attraction were exhibited by both females and males to ME-fed than to non-ME-fed males as dusk approaches. Males increased their precopulatory behavior (i.e., wing fanning and mounting) during the period of higher attractancy. These data confirm that the consumption of ME enhances the mating competitiveness of males and suggest that ME also functions as a precursor to the male sex and aggregation pheromones. Three phenylpropanoid compounds biosynthesized from ME, coniferyl alcohol, 2-allyl-4,5-dimethoxyphenol, and 3,4-dimethoxycinnamyl alcohol, were detected in male rectal gland along with an endogenous rectal compound, N-(3-methylbutyl) acetamide. When offered singly to the females, coniferyl alcohol was found to be most attractive.

Comparative sensitivity to and consumption of methyl eugenol in three Bactrocera dorsalis (Diptera: Tephritidae) complex sibling species

Summary. Sensitivities to methyl eugenol of three sibling species in the Bactrocera dorsalis complex were compared. The degree of species sensitivity to methyl eugenol, i.e. B. dorsalis > B. papayae > B. carambolae (in decreasing order), was concomitant with the species age-related response to methyl eugenol as previously reported. The ability to consume methyl eugenol by the three sibling species showed similar trend - the average ME consumption per male was 0.70 ml for B. dorsalis, 0.58 ml B. papayae and 0.18 μl B. carambolae. Results obtained were discussed in relation to area-wide control of fruit fly.

Phenylpropanoids in the fragrance of the fruit fly orchid, Bulbophyllum cheiri, and their relationship to the pollinator, Bactrocera papayae

Flowers of a southeast Asian orchid, Bulbophyllum cheiri, attract males of Bactrocera papayae and other fruit fly species sensitive to methyl eugenol for pollination. In addition to the major attractive component methyl eugenol, several other related phenylpropanoids were identified as minor components from the floral extracts. 2-Allyl-4,5-dimethoxyphenol, known as an element of sex and aggregation pheromone of B. papayae males was detected as a significant volatile ingredient together with its O-methyl ether (euasarone). The attractancy of each floral component and efficiency of incorporation of these phenylpropanoids into the rectal pheromone glands were examined in B. papayae males.

Male sex pheromonal components derived from methyl eugenol in the hemolymph of the fruit fly Bactrocera papayae

Pharmacophagy of methyl eugenol (ME)--a highly potent male attractant, by Bactrocera papayae results in the hydroxylation of ME to sex pheromonal components, 2-ally-4,5-dimethoxyphenol (DMP) and (E)-coniferyl alcohol (CF). These compounds, which are also male attractants, are then sequestered and stored in the rectal gland prior to their release during courtship at dusk. Chemical analyses of the digestive tract (excluding the crop and rectal gland) showed the absence of the sex pheromonal components and their precursor, ME. However, B. papayae males were attracted to and fed on the ME-fed male hemolymph extracts but not on hemolymph extracts of ME-deprived males. After thin layer chromatography in a hexane:ethyl acetate solvent system, flies were attracted to and fed on the original point on the TLC plate where the hemolymph extract had been spotted, suggesting that the pheromone components were bound in polar complexes. Chemical analyses of the ME-fed male hemolymph and crop extracts revealed the presence of the sex pheromonal components. The presence of the ME-derived pheromonal components and the absence of ME in the hemolymph suggest that the hemolymph is involved in the transportation of sex pheromonal components from the crop to the rectal gland.

Historical perspective on the synonymization of the four major pest species belonging to the Bactrocera dorsalis species complex (Diptera, tephritidae)

Abstract 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.

Comparative sensitivity to methyl eugenol of four putative Bactrocera dorsalis complex sibling species – further evidence that they belong to one and the same species B. dorsalis

Abstract Males of certain species belonging to the Bactrocera dorsalis complex are strongly attracted to, and readily feed on methyl eugenol (ME), a plant secondary compound that is found in over 480 plant species worldwide. Amongst those species is one of the world’s most severe fruit pests the Oriental fruit fly, Bactrocera dorsalis s.s., and the former taxonomic species Bactrocera invadens, Bactrocera papayae and Bactrocera philippinensis. The latter species have been recently synonymised with Bactrocera dorsalis based on their very similar morphology, mating compatibility, molecular genetics and identical sex pheromones following consumption of ME. Previous studies have shown that male fruit fly responsiveness to lures is a unique phenomenon that is dose species-specific, besides showing a close correlation to sexual maturity attainment. This led us to use ME sensitivity as a behavioural parameter to test if Bactrocera dorsalis and the three former taxonomic species had similar sensitivity towards odours of ME. Using Probit analysis, we estimated the median dose of ME required to elicit species’ positive response in 50% of each population tested (ED50). ED50 values were compared between Bactrocera dorsalis and the former species. Our results showed no significant differences between Bactrocera dorsalis s.s., and the former Bactrocera invadens, Bactrocera papayae and Bactrocera philippinensis in their response to ME. We consider that the Bactrocera males’ sensitivity to ME may be a useful behavioural parameter for species delimitation and, in addition to other integrative taxonomic tools used, provides further supportive evidence that the four taxa belong to one and the same biological species, Bactrocera dorsalis.

Integrative taxonomy versus taxonomic authority without peer review: the case of the Oriental fruit fly, Bactrocera dorsalis (Tephritidae)

1. Major global horticultural and food security tephritid fruit fly pests, Bactrocera papayae (papaya fruit fly) and B. invadens (invasive fruit fly), were synonymised with B. dorsalis (Oriental fruit fly) by Schutze et al. (2015a) based on extensive integrative taxonomic evidence from multiple sources. This synonymy was peer reviewed by eight independent experts.

Functional characterization of olfactory receptors in the Oriental fruit fly Bactrocera dorsalis that respond to plant volatiles

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.

Involvement of the Antennal and Maxillary Palp Structures in Detection and Response to Methyl Eugenol by Male Bactrocera dorsalis (Diptera: Tephritidae)

Abstract The oriental fruit fly, Bactrocera dorsalis (Handel) is one of the most destructive pests of fruits. The discovery of methyl eugenol (ME) as a potent male attractant for this species has led to its successful use in area-wide fruit fly control programs such as male annihilation. While the antenna is recognized as primarily responsible for male flies’ detection of attractants such as ME, little is known of the involvement of the maxillary palp. Using behavioral assays involving males with intact and ablated antennae and maxillary palp structures, we seek to ascertain the relative involvement of the maxillary palp in the ability of the male fly to detect ME. In cage bioassays (distance of ≤40 cm from the source), >97% of unmodified males will normally show a response to ME. Here, we showed that 17.6% of males with their antennae ablated were still attracted to ME versus 75.0% of males with their palps ablated. However, none of the antennae-ablated males were able to detect ME over a distance of >100 cm. Furthermore, wind tunnel bioassays showed that maxillary palp-ablated males took a significantly longer time compared to unablated males to successfully detect and eventually feed on ME. These results suggest that although the antennae are necessary for detection of ME over longer distances, at shorter distances, both antennae and maxillary palps are also involved in detecting ME. Hence, those palps may play a larger role than previously recognized in maneuvering males toward lure sources over shorter ranges.

Population structure of a global agricultural invasive pest, Bactrocera dorsalis (Diptera: Tephritidae)

Bactrocera dorsalis, the Oriental fruit fly, is one of the worlds most destructive agricultural insect pests and a major impediment to international fresh commodity trade. The genetic structuring of the species across its entire geographic range has never been undertaken, because under a former taxonomy B. dorsalis was divided into four distinct taxonomic entities, each with their own, largely non‐overlapping, distributions. Based on the extensive sampling of six a priori groups from 63 locations, genetic and geometric morphometric datasets were generated to detect macrogeographic population structure, and to determine prior and current invasion pathways of this species. Weak population structure and high genetic diversity were detected among Asian populations. Invasive populations in Africa and Hawaii are inferred to be the result of separate, single invasions from South Asia, while South Asia is also the likely source of other Asian populations. The current northward invasion of B. dorsalis into Central China is the result of multiple, repeated dispersal events, most likely related to fruit trade. Results are discussed in the context of global quarantine, trade, and management of this pest. The recent expansion of the fly into temperate China, with very few associated genetic changes, clearly demonstrates the threat posed by this pest to ecologically similar areas in Europe and North America.