## 2 Materials and Methods  
### Study Design  
_Tetranychus urtice_ were reared on tomato plants (_Solanum lycopersicum_ L. cv. Admiro), in cages made of a fine nylon mesh (mesh diameter = 250 \(\upmu\)m). All cages were kept in a climate chamber at 70 \(\pm\) 10% r.h., 24 \(\pm\) 2 \({}^{\circ}\)C/22 \(\pm\) 2 \({}^{\circ}\)C, and 16 h light/8 h dark photo period. The rearing was carried out at Hepia Lullier (Switzerland). The colonies of _T. urtice_ were initiated more than 3 months before the beginning of the experiment [19]. The experiment was conducted from late August until mid-September at the field station of Agroscope Conthey (Switzerland) in a 90 m\({}^{2}\) glasshouse equipped with lateral and roof ventilations, fogging and shading for climate regulation. The greenhouse was divided into 16 experimental cages (L \(\times\) W \(\times\) H = 1.75 \(\times\) 1.75 \(\times\) 2.5 m) in a Latin square split-plot design with four cages \(\times\) four treatments: control, _T. urtice_ + treatment 1, _T. urtice_ + treatment 2, _T. urtice_ + treatment 3. To prevent potential sources of arthropod escape, each cage was enclosed in a fine nylon mesh (diameter = 250 \(\upmu\)m) on all sides. Access to the plants was possible through a small zipped opening with additional Velcro(r) closings on two edges. Cages were separated by a distance of approximately 0.40 m. In each cage, three 50-day-old tomato plants, crop variety Admiro, were placed about 0.70m from each other. Plants were grown in 10 L pots in a substrate of 1/3 black peat and 2/3 Ricoter(r) mix made of 65% blond peat, 15% sand, 20% perlite (Aarberg, Switzerland). A standard fertilizing solution for tomato plants was automatically provided by drip irrigation. A data logger (DGT-Volmatic sensor RTF-5D, Senmatic A/S, Denmark) in one cage recorded temperature and relative humidity throughout the experiments. Except for the control set, each tomato plant was inoculated with 30 mobile stages (protonymphs and deutonymphs) of _T. urticae_, on \"Day 0\". Mites were collected from the rearing units, counted under a magnifying lens and transferred with a thin paintbrush to a cotton pad as positioning support. The cotton was then attached to tomato leaves to allow the spider mites to colonize their host plant (panel a. in Figure 1). Fifteen days after infestation (\"Day 15\"), a phytosanitary treatment was applied to halt the plants' infestation. Throughout the experiment's entire duration, recordings of electrophysiological signals of 32 plants (8 control and 24 infested plants) were done at 500 Hz sampling rate using four multi-channel PhytSigns devices. This measuring device, which is designed with a very high input impedance, of the order of many M\(\Omega\), includes eight pairs of custom-built electrodes, each of them made of a 0.5 mm diameter silver-coated copper wire covered with a 2.79 mm diameter coaxial cable with impedance of 50 \(\Omega\)[15]. The importance of impedance matching in plant electrophysiology measurements has been highlighted [20]. The pair of electrodes comprises one active and one ground (i.e., reference) electrode. The active electrodes were placed above the plant's first floral bud, whereas the reference electrodes were placed below the first leaf. Figure 2 gives a schematic representation of the recordings' setup. To assure stability in the recorded signal, the electrodes need to be inserted in the conducting bundles of the plant [21]. Hence, the signal stability was monitored within the initial 72 h of the experiment during which the electrodes were repositioned when an unstable recording was observed. Figure 1: Plant infestation with spider mites. Panel (**a**) illustrates the process of infestation where each plant was inoculated with 30 mobile stage of spider mites put on a cotton pad that was attached on the leaves. Panels (**b**,**c**) show the appearance of infested tomato leaves when the plants presented strong stress related to the infestation. Figure 2: Acquisition of the plant electrophysiology signal.