Saline–Alkali Stress Exacerbates Bark Beetle Infestation by Impairing Plant Defense Mechanisms

Abiotic stresses such as high salinity or alkalinity usually alter plant secondary metabolism, thereby affecting their defense against herbivorous insects. This study investigated the effects of changes in terpenoids and phenolics of black pine ( Pinus thunbergii ) induced by salt, alkali, and mixed saline–alkali stresses on the host selection, host fitness, and laboratory bioassays of red-haired bark beetle ( Hylurgus ligniperda ). Field experiments showed that saline–alkali stress resulted in a decreased proportion of (1R)-(+)-α-pinene, an increased proportion of β-pinene and sabinene, and reduced contents of tannins, total flavonoids, and total phenols in the phloem of P. thunbergii , and promoted the damage of H. ligniperda to P. thunbergii . Laboratory bioassays showed that changes in terpenoids had a significant impact on the electroantennogram (EAG) and behavioral responses of H. ligniperda , while changes in phenolics had little effect on its growth and development. Based on Partial Least Squares Path Model (PLS-PM) comparison and laboratory bioassays results, terpenoids were identified as the main factor promoting damage of H. ligniperda to P. thunbergii . Specifically, saline–alkali stress enhances the host-locating ability of H. ligniperda , leading to an increase in its population density and thus promoting damage to P. thunbergii . This study provides insights into the complex interactions between plant physiology and insect behavior under abiotic stress, delivering more precise strategies for pest management in salinized–alkaline forest environments.