# Detection of Volatile Constituents from Food Lures by Tephritid Fruit Flies
## 4 Discussion  
In the current study, our aim was to identify key compounds that attract fruit flies to feeding sites, with a view to develop synthetic lures that are more specific than the currently available broad-spectrum protein baits. We have identified a total of 13 physiologically active compounds from food-based fermentation products that are detected by the four species in our study. Most compounds identified in this study are signature compounds for microbial activity and protein breakdown products. As microorganisms constitute important resources, supplying the much-needed proteins, their detection through associated volatiles induce strong behavioral responses in Tephritidae. The majority of the compounds identified here are also ligands of \(D\). _melanogaster_ olfactory receptors (see heatmap Figure 3, [33]), a species that shares the strong association with microbes, which underlines their importance in signaling fermentation and protein sources [36; 37; 38; 39; 40]. Pyrazines are heterocyclic aromatic volatile compounds associated with gut bacteria of fruit flies and other insects. The importance of pyrazines in insect-microbe-plant interactions is noted in other contexts. _Drakaea glyptodon_ (Orchidaceae) flowers emit pyrazines that are also found as pheromone of the female wasp _Zaspilothynnus trilobatus_ (Hymenoptera: Thynninae) pheromone, to attract males for pollination [41]. Different forms of pyrazines have been reported as components of tephritid male sex pheromones [42; 43; 44]. Three of the compounds identified from the headspace extracts of the protein-based baits in the current study are pyrazines, including 2-methylpyrazine, 2,5-dimethylpyrazine and 2,6-dimethylpyrazine. 2-methylpyrazine has been isolated from several bacterial filtrates including _Klebsiella pneumoniae_ and _Citrobacter freundii_ and its attractiveness to the fruit fly _A. ludens_ was evaluated singly, but without success [38]. In the current study, 2-methylpyrazine was detected by the antennae of the three polyphagous pests, _B. dorsalis_, _B. zonata_ and _C. capitata_. It was also part of the specific blends which elicited a behavioral response in the olfactometer for each of these species. Although, 2-methylpyrazine did not induce a response in the antennae of female _Z. cucurbiae_, it is reported as a male rectal gland component of _Z. cucurbiae_, and suggested to have a role in sexual communication [42]. The lack of response in the current study may be either due to the below threshold amounts in the sample, or that this compound is detected by another olfactory organ, such as the maxillary palp. Figure 6: Female and male _B. dorsalis_ fruit flies caught per trap in commercial lures (GF-120 & torula yeast) compared to the synthetic dorsolateral. Different letters indicate significant differences. Boxplot shows the spread and the violin plot shows density of flies per trap. Several organic volatile compounds from food-based fermentation products identified in this study are products of microbial modifications of fatty acids, aromatic amino acids and carbohydrates [45]. [PERSON] et al. [46], identified 3-methyl-1-butanol from _Enterobacter agglomerans_ as a main component attracting _Anastrepha suspensa_. This substance was detected as a minor component in corn protein hydrolyzate bait [47], and also from fermented host fruit of _A. ludens_[38; 48]. However, the compound is not unique to fermentation products, as it is also a main component of several fruits and elicits responses in other fruit flies such as the spotted wing _Drosophila suzukii_[49]. In our study, 3-methyl-1-butanol was part of the dorsolateral, zonablend and capiblend, but not part of the cucublend, which may be a reason why this blend was not attractive to _Z. cucurbiae_. A main limitation of utilizing the general proteinaceous or fermentation baits for control of tephritid flies is that they are not target-specific [50].