## 4 Materials and Methods
### Evaluation of Olfactory Responses of M. persicae and Its Parasitoids  
A glass Y-tube olfactometer was used to determine the responses of _M. persicae_ and its two species parasitoids, _A. colemani_ and _A. abdominalis_, to each of the following pairs of plant treatments. For the aphid responses, the test was (1) infested (cabbage plants infested with _M. persicae_) versus clean (filter) air; (2) non-infested versus clean air; and (3) infested versus non-infested plants (Figure 5). For the test of parasitoid wasps, _A. colemani_ and A. abdominalis_, (1) infested plant versus clean air; (2) non-infested plant versus clean air; and (3) infested versus non-infested plants. Bioassays were used to compare their olfactory responses to VOCs released from uninfested plants versus clean air or infested plants with _M. persicae_ versus uninfested plants. The infested cabbage plants that were used in this study contained aphids.  
Volatile preference experiments were made using a glass Y-tube olfactometer as previously described [45], with a 7 cm arm length and 2 cm internal diameter, ground glass fitting for the air that passed 200 mL/min through each arm, controlled by air flow meter (SCFH AIR, Dwyer Instruments, Michigan City, IN 46360, USA) (Figure 5). Each arm tube was connected to a glass chamber (2 L desiccator). Couples of blend VOCs (released from different plant treatments) were presented in a sealed glass chamber (2 L each) at the end of either arm. The compressed air was filtered by using activated charcoal passed through two glass chambers, before the treatment plant could be introduced, and then the air passed through the olfactometer. After assembly, the olfactometer was left to stabilize for 15 min prior to use [46].  
The Y-tube olfactometer work was carried out under the same conditions as the glasshouse conditions. The area surrounding the olfactometer (below and around) was covered by white paper and white light was placed directly over the olfactometer. For the bioassay, a single aphid or single parasitoid was introduced into the main arm of olfactometer and pushed 1-2 cm inside the main arm. Each aphid or was given up to 3 min in the olfactometer to respond. Once an individual moved beyond 2 cm and into one of the Y-tube arms, it was considered to have made a choice for the conforming plant treatment in that arm. Non-responders that did not make a choice in 5 min were discarded and excluded from the statistical analysis (non-responsive parasitoids counted in statistical analysis in the experiment of comparison of clean air with the uninfested plant).  
Three replicates and 30 adults of wingless aphid _M. persicae_ were assayed for each replicate, and each aphid was tested only one time. Every 10 aphids were assayed, the volatile treatment resources were removed, and all glass vessels cleaned with ethanol, then washed with water and oven dried at 100 \({}^{\circ}\)C for a minimum of 30 min. For the comparison, three replicates were carried out on different days using new aphids and fresh infested and non-infested plants. All plant resources were the same age and same size.  
Figure 5: The diagram of the olfactometer, including the glass Y-tube where the aphid _Myzus persicae_ and the parasitoid _Aphidus colemani_ and _Aphelinus abdominalis_ were released individually and exposed to two plant VOCs, blends from uninfested and plants infested with _M. persicae_ as shown by the blue and red small circles.  
The same procedure above was carried out for the parasitoid _A. colemani_ and _A. abdominalis_. Additionally, three replicates were used for the parasitoids with 15 wasps for each replicate and wasps were used only once. Throughout the experiments, after all 15 wasps were assayed for each replicate, the apparatus was cleaned with water and ethanol, then dried and heated in the oven at 100 \({}^{\circ}\)C for more than 30 min. Statistical significance between wasp responses to pairwise combinations of plant treatments was determined using Chi-square tests at the 5% level.