After being defoliated by Clostera anachoreta larvae, exposure to volatiles induced by C. anachoreta larvae, and five added exogenous volatiles, including methyl jasmonate (MeJA), methyl salicylate (MeSA), cis-hexenal, trans-hexenal, and benzothiazole, the activities of polyphenol oxidase (PPO) in Populus simonii P. pyramidalis ‘Opera 8277’ leaves was measured. A 2-4-fold increase in PPO activities in herbivore-wounded leaves and upper intact leaves showed that the systemic resistance of poplar cuttings was activated. A 1-fold increase in PPO levels in poplar leaves exposed to herbivore-induced volatiles was also found, implying that the volatiles emitted from poplar leaves after insect herbivory acted as signaling molecules triggering the defense response of the neighbouring individuals. Exposure to five exogenous volatiles caused an increase in PPO activity, suggesting that these five volatiles were the potential airborne signals between P. simonii P. pyramidalis ‘Opera 8277’ plants. Moreover, the combined inducible effects of mixed volatiles were significantly stronger than single volatile. The PPO activities in herbivore wounding leaves, upper intact leaves, and leaves exposed to herbivore-induced volatiles were obviously inhibited by EGTA (calcium chelator) and LaCl3 (calcium channel inhibitor), showing that calcium signal played a key role in the increase of PPO activities in P. simonii P. pyramidalis ‘Opera 8277’ leaves in response to insect herbivory and volatiles exposure.