Olfactory relationships of Bactrocera tryoni fruit fly and associated microbes

Bactrocera tryoni, commonly known as Queensland Fruit Fly, is an economically important pest for which monitoring and management is performed using the male-specific attractant, cuelure. Studies of plant-insect-microbe interactions suggest that attraction towards the host is not only driven by plant volatile compounds but also by the microbial volatile compounds (mVOCs) from host plant/fruit-associated microbes. Insects, including B. tryoni, acquire some of these host microbes during feeding and so the bacteria present in B. tryoni guts might trigger attraction to the host via mVOCs. Although symbiotic interactions between fruit flies and their associated gut bacteria (microbionts) have been well-studied in several tephritids, including B. tryoni, the composition of volatile chemicals in these gut bacterial emissions and their role as mediators of fruit fly behaviour are little studied. To address this gap, the present study identified the mVOCs from the headspace of the culturable microbionts of wild adult B. tryoni and evaluated their effect on the electrophysiological and behavioural responses of young and mature B. tryoni.

In this study, molecular 16S rRNA sequencing identified 24 bacterial strains from wild adult male B. tryoni and 21 bacterial strains from wild female flies. The most frequent bacterial strains obtained from both male and female guts belong to Enterobactereaceae (64.25%) followed by Bacillaceae (8.25%). In trap behavioural assays, the most attractive bacteria were Bacillus anthracis, Enterobacter cloaceae, Pseudomonas aeruginosa, Bacillus sp., Pseudomonas fluorescens, Enterococcus faecalis, Citrobacter freundii and Proteus mirabilis. Young male and female flies exhibited a significantly lower response than mature male and female flies. The attraction of mature flies to different bacterial strains varied significantly and was similar for male and female flies. Gas Chromatography-Mass Spectroscopy (GCMS) analyses of these bacterial headspace extracts identified mVOCs, with a wide range of chemical classes including pyrazines, ketones, alcohols, aldehydes, and esters. Many compounds were common to all isolates with few being species-specific. Gas Chromatography coupled with Electroantennography or Electropalpography (GC-EAD/EPD) studies identified 17 electrophysiologically active volatile compounds from the extracts across all tested bacterial strains. These EAD/EPD-active compounds were ketones (octan-3-one, undecan-2-one, 1-(2-aminophenyl)ethan-1-one, and γ-tetradecalactone (5-decyldihydrofuran-2(3H)-one)), esters (ethyl octanoate, glycerol 1,2-diacetate, geranyl formate, geranyl acetate, 6-methyl-4-heptenyl pentanoate, and dodecan-5-yl acetate), alcohols (octan-1-ol, geraniol, 4-ethyl-2-methoxyphenol, and 2-methyldodecan-1-ol), isocyanide (1-isocyano-1 2-methylbenzene), aldehyde (undecanal), and phthalate acid ester (diethyl phthalate). Mature male and female B. tryoni exhibited varying attraction to these compounds in Y-maze olfactometer bioassays. The most attractive volatile for both sexes was 1-(2-aminophenyl)-ethan-1-one, followed by geranyl acetate for females and geraniol for males.

The present study also explored B. tryoni gut associated bacterial community transfer to the infested host fruits via oviposition. The bacterial isolates that were transmitted from the ovipositing B. tryoni gut to the oviposition site of host fruit (and not from the fruit surface and environment) included eleven confirmed species, with the predominant species being Enterobacter aerogenes and Klebsiella oxytoca. Behavioural assays identified B. cereus as the most attractive isolate for both male and female B. tryoni followed by E. aerogenes for male flies and E. asburiae and B. pumilus for female flies. Similar to the studies with direct gut microbionts, young flies exhibited a significantly lower response than mature flies to the gut associated bacteria transferred to the infested fruits. The attraction of mature flies to different bacterial strains varied significantly and was similar for male and female flies. GCMS analyses identified a number of mVOCs in the headspace of these bacterial liquid cultures, among which pyrazines (2,5-dimethylpyrazine, 2,5-dimethyl-3-ethylpyrazine, 2,3,5-triethylpyrazine, 2,3,5- dimethyl-3-isoamylpyrazine, and 2-ethyl-3,6-dimethylpyrazine) and ketones (octan-3-one and acetophenone) were most abundant. 

Overall, this study advances understanding of Queensland fruit fly-bacteria chemical relationships and provides insights that can support development of bisexual synthetic lures as fruit fly pest management tools.