## 2 Materials and Methods
### Collection and Identification of Volatiles from Leaves of PaP After MeJA Treatment  
Different concentrations of MeJA (200 mL) (10\({}^{-3}\), 10\({}^{-4}\), 10\({}^{-5}\), 10\({}^{-6}\), 0 mol/L, five replicates per concentration) were sprayed per tested PaP tree (5 years old, different to the above tested trees) totally on leaves and trunks with household sprayers. Soil absorption was avoided. The spraying was carried out at 7:00 to 9:00 in the morning and on days without wind. Headspace solid-phase microextraction-gas chromatography-mass spectrometry (HS-SPME-GC-MS) was used to detect volatiles from branches and leaves. Fresh PaP (5-year-old) leaves (5 g) were collected from trees treated with different concentrations (10\({}^{-3}\), 10\({}^{-4}\), 10\({}^{-5}\), 10\({}^{-6}\), 0 mol/L, five replicates per concentration) of MeJA aqueous solutions, and each sample was placed in a clean 500 mL triangular conical flask. After 30 min of acclimation, an SPME with a DVB/CAR/PDMS (divinylbenzene/carboxoyethyl/polydimethylsiloxane) extraction head (Supelco Co., Shanghai, China) was used to extract volatiles from each sample for 30 min at 25 \({}^{\circ}\)C. The chromatographic column was a DB-WAX device (30 m \(\times\) 250 \(\upmu\)m \(\times\) 0.25 \(\upmu\)m). The heating program was as follows: 40 \({}^{\circ}\)C for 1 min, 8 \({}^{\circ}\)C/min increase to 180 \({}^{\circ}\)C, maintained for 1 min, then 20 \({}^{\circ}\)C/min to 200 \({}^{\circ}\)C, and maintained for 2 min. Other GC-MS conditions were consistent with the identification conditions of host volatile components shown in Section 2.1. Choices Made by ALB Adults for Branches and Leaves of PaP After Treatment with Different Concentrations of MeJA Aqueous Solutions  
One-year-old healthy laboratory-cultivated PaP seedlings were each sprayed with a different concentration of MeJA aqueous solution (10\({}^{-3}\), 10\({}^{-4}\), 10\({}^{-5}\), 10\({}^{-6}\), 0 mol/L) at a rate of 200 mL per seedling; additional trees were mechanically damaged. Seedlings were labelled to indicate which treatment they had received. Two arenas (each with a 2 m diameter) were established, and one PaP seedling treated with each MeJA concentration and one mechanically injured PaP seedling were evenly arranged at six points around the circumference of each arena (Figure S2). Either ALB males (36 individuals) or ALB females (36 individuals) were released at the center of each arena. A steel screen cage (2 m \(\times\) 2 m \(\times\) 1.5 m) was placed over each arena to prevent the beetles from escaping. After 24 h, the number of beetles perched on each PaP seedling, the number of bites on branches and leaves, the feeding area, and the amount of feeding were recorded for each seedling. There were three replicate arenas for males and three for females, and the position of each treatment seedling in every test was rotated 120\({}^{\circ}\) amongst the three replicates to avoid directional bias in adults. The damage level (\(D\)) of the ALB was calculated as follows: \(D\) = number of beetles selecting a particular treatment \(\times\) 0.25 + number of bite wounds \(\times\) 0.25 + food intake \(\times\) 0.25 + amount of insect feces produced in 48 h \(\times\) 0.25. The smaller the damage value was, the lower the damage level. Food intake was calculated by weighing branches and leaves before and after ALB feeding. The amount of insect feces was calculated by weighing every beetle's fresh feces produced in 24 h. The feces was held individually in plastic cages for 24 h after the arena tests were finished. Oviposition of ALB Females on Trees Treated with Different Concentrations of MeJA Aqueous Solutions in the Field  
Individual five-year-old healthy PaP trees (\(n=25\)) were selected in the field, numbered, and divided into five groups. Trunks of replicate (\(n=5\)) PaP trees in each group were each evenly sprayed with 200 mL of MeJA aqueous solutions (either \(10^{-3}\), \(10^{-4}\), \(10^{-5}\), \(10^{-6}\) or 0 mol/L) at 60-150 cm above the ground (Figure S3).