Water and Nitrogen Regulation of Tea Leaf Volatiles Influences Ectropis grisescens Olfaction

Global warming has increased outbreaks of the tea pest Ectropis grisescens. However, how water and nitrogen management modulates tea plant resistance against this pest through induced volatile organic compounds (VOCs) remains unclear. This study aimed to (1) characterize how water–nitrogen interactions alter the composition of VOCs in fresh leaves of Camellia sinensis cv. Rougui, and (2) identify key VOCs that mediate repellence against E. grisescens. Using gas chromatography–mass spectrometry (GC–MS) and olfactometry under three water and three nitrogen levels, we found that nitrogen effects on VOCs were contingent on water status. Four terpenoids—(+)-dihydrocarvone, myrcene, linalool, and β-ocimene—and one green-leaf volatile ((E)-3-hexenoic acid) significantly repelled E. grisescens, whereas hexanoic acid, 3-oxo-, ethyl ester acted as an attractant. Mechanistically, low-water–moderate-nitrogen and high-water–high-nitrogen treatments reduced repellent terpenoids and increased attractant VOCs, thereby elevating pest preference. These results demonstrate that water–nitrogen coupling shifts the balance between repellent and attractant volatiles, providing a physiological basis for manipulating tea plant resistance through agronomic management.