2.1. Insects and plants

Plutella xylostella pupae were initially collected from Brassica fields in July 2004 in the Fuzhou suburban area (26.08°N, 119.28°E) and reared in the laboratory on “Nanpanzhou” cultivar radish (Raphanus sativus) for more than 200 generations (referred to as PxR population). To establish a PxP population, PxR eggs were artificially placed on “Purple Flower‐Soft Pod” cultivar pea (Pisum sativum) leaves. We named the 1st generation of P. xylostella after the host shift as PxP‐1, and so on in a similar fashion (PxP‐n). Upper generations of the PxP population feeding on radish were named PxP‐n_R. Larvae were left to feed on radish or pea under controlled conditions of 23 ± 1°C, 65 ± 5% relative humidity (RH), and 16‐hr light: 8‐hr dark photoperiod. Adults were provided with a 10% w/v honey–water mixture.

Radish was cultivated in rectangular plastic trays (420 × 320 × 100 mm) with nutrition soil, and pea was cultivated in disposable nutrition bags (16 × 14 mm) with peat soil. Plants were kept in fine transparent cages with 0.1 mm mesh screens in a walk‐in growth chamber at 23 ± 1°C and 65 ± 5% RH, under a 16‐hr light: 8‐hr dark photoperiod. Leaves of radish at 1 week and pea at 4 weeks were used to feed P. xylostella larvae.