Studying volatile emissions of fruit flies as chemical lures

Fruit fly species are amongst the most damaging horticultural pests globally and have a devastating impact on food production. Fruit flies typically release volatile compounds, usually interpreted as sex pheromones, as an integral element of their sexual biology. Understanding the composition and function of released volatiles is an important aspect of understanding fruit fly sexual biology and can also provide valuable knowledge for the development of attractants that can be used for fruit fly monitoring and control. 

This PhD thesis provides the chemical profiles of rectal gland contents and emissions of four pest species, Bactrocera musae, B. frauenfeldi, B. bryoniae and B. kraussi, by using gas chromatography-mass spectrometry; and evaluates the detection and function of natural blends of both sexes by gas chromatography-electroantennogram detection and/or gas chromatography-electropalpogram detection and Y-tube olfactometry. Across both sexes of all species, four chemical classes were present including carboxylic acid, esters, spiroacetals and acetamides. Analysis showed that esters and spiroacetals were the two electrophysiologically active classes of compounds in these four species. Interestingly, Y-tube olfactometer assays for B. kraussi showed that the natural blend of female rectal glands attracted males, while for B. frauenfeldi the females were attracted to male rectal glands. For B. bryoniae, both males and females were attracted to the conspecific natural rectal gland blends. Males of this species were also attracted to the conspecific male rectal glands, while this was not observed for B. kraussi and B. frauenfeldi. No females were attracted to volatiles from conspecific female rectal glands across all species. 

This PhD thesis also examined the most common techniques for sampling rectal glands and headspace volatile compounds of fruit flies, using B. tryoni as a model organism as it has been studied previously for its chemistry. Extraction of the rectal glands with the different polarity solvents n-hexane, dichloromethane and ethanol was conducted, with both crushed and non-crushed glands examined. Headspace collection of the gland volatiles was also investigated using static and dynamic methods. For the static methods, three different types of solid phase microextraction fibres (polydimethylsiloxane, polydimethylsiloxane/divinylbenzene and polyacrylate) were compared. For the dynamic methods, the performance of two common sorbent polymers, Tenax-GR and PoraPak Q, and the effect of sampling duration on performance of each sorbent, were also examined. Both male and female B. tryoni were found to emit a wide range of volatile compounds, including acetamides, spiroacetals and esters. Six new compounds that have not been reported in previous studies, ethyl propanoate, ethyl isobutyrate, ethyl 2-methylbutanoate, propyl isobutyrate, ethyl 2-methylpentanoate and diethyl succinate, were identified in the male rectal glands and headspace samples, along with six previously reported amides. Three new compounds, propyl laurate, methyl myristate and N-(2-methylbutyl)acetamide, were also identified in female rectal glands and headspace samples, as well as nineteen compounds previously reported including fatty acid esters, spiroacetals and amides. For the rectal gland extractions, dichloromethane was found to extract greater amounts of amides, while there was no significant difference between the three solvents in extraction of spiroacetals. No significant differences were observed for esters between the n-hexane and dichloromethane extracts. Ethanol was found to be an unsuitable solvent as it did not contain as many of the short chain esters. Extraction of the crushed and non-crushed rectal gland samples afforded the same range of compounds, however, the crushed glands provided higher concentrations of each compound. For static headspace samples, polydimethylsiloxane performed better in collecting spiroacetals while type of fiber did not affect the amounts of esters and amides. Polyacrylate had relatively low affinity for spiroacetals, ethyl isobutyrate and ethyl-2- methylbutanoate. The results from the dynamic methods showed that Tenax was more suitable for amides and esters except ethyl isobutyrate. Sampling duration was a critical factor for the dynamic headspace methods. These studies show that when interpreting volatile profiles of fruit flies, and when comparing species, it is important to consider the biases that can be introduced by sampling methods. 

Overall, this PhD study provides insight into the development of new lures for fruit fly control and also provides guidelines for method selection for fruit fly pheromone studies.