## 2 Materials and Methods
### Electrophysiological Assays  
The electroantennogram (EAG) responses of virgin females to Tenax(r) bacterial extracts were measured in Australia. In New Zealand, using GC coupled with an electroantennogram detector (GC/EAD), we separated and identified the bioactive MVOCs using SPME collection samples from the mixed bacteria grown under different pH concentrations. For both techniques (i.e., EAG and GC/EAD), each fly was anaesthetized at \(-20\)\({}^{\circ}\)C for 2 min or by using carbon dioxide gas. The head was then removed and mounted between two recording electrodes in a salt-free hypoallergenic gel (Spectra 360, Parker Laboratories Inc., Fairfield, NJ, USA) and placed under a constant charcoal-filtered flow of humidified air (400 mL/min). For the EAG, the presentation of odorant samples was made manually in front of the antennae by inserting in the airflow a Pasteur pipette containing 10 \(\upmu\)L of the bacterial Tenax(r) extract applied on a filter paper and puffed for 0.5 s. Prior recording, the stimulus pipettes were kept for 10 min in a fume hood to facilitate solvent evaporation. For each sample, air was puffed (for 0.5 s at 1.5 L/min) through the Pasteur pipettes, which were inserted into a glass tube, located at 10 cm from the antennal preparation. A constant airflow was maintained to compensate for the air-puff. The samples were presented at 30 s intervals between stimuli to allow the olfactory receptor neurons to return to a basic activity rate as previously described [27]. The four bacterial Tenax(r) extracts were tested in concentrated form, as well as in a series of dilutions (10-1 and 10-2) prepared and tested for a dose-response. All odorant stimuli were delivered from the lowest to the highest concentrations. Ten replicates were run for each bacterium and dilution. The EAG responses of female QFF to bacterial Tenax(r) extracts were recorded with a high-input impedance preamplifier and analyzed with the recording software AutoSpike (version 3.9, Syntech, Hilversum, The Netherlands).  
For GC/EAD, the SPME fiber passively collected MVOCs for 1 h at 29 \({}^{\circ}\)C before desorption in the GC injection port. The SPME samples were presented to the insect antennae via an Agilent GC equipped with a 30 m by 0.32 mm i.d. by 0.25 \(\upmu\)m HP5 capillary column (Agilent Technologies, Santa Clara, CA, USA) using an FID coupled to an electro-antennographic detector (EAD) acquisition controller (IDAC 4, Syntech, Hilversum, The Netherlands). The fiber was desorbed for 5 min in the injection port, which was held at 250 \({}^{\circ}\)C. The samples were subsequently passed through the column at 1 mL/min with helium carrier gas, which was split between the insect antennae and the FID. The injector of the GC was set at 250 \({}^{\circ}\)C and run in split-less mode; the detector was set at 300 \({}^{\circ}\)C, and the GC oven temperature programmed from 60 \({}^{\circ}\)C, held for 1 min, then increased to 250 \({}^{\circ}\)C at 20 \({}^{\circ}\)C/min and held for 10 min. The EAD exit port temperature was maintained at 250 \({}^{\circ}\)C, and the antennal preparation was placed in a charcoal-filtered and humidified air system (400 mL/min). The recording software AutoSpike (Version 3.9, Syntech, Hilversum, The Netherlands) was used to record the antennal response and FID response. Kovats retention indexes (KI) were calculated for the antennally active compounds, and the samples were run by GC/MS for identification.