Effects of <i>Hylurgus ligniperda</i> (Coleoptera: Curculionidae)-microorganism symbiosis complex damage severity on physiological and defensive responses in <i>Pinus thunbergii</i>

To explore the defense response of Pinus thunbergii to different damage levels of Hylurgus ligniperda, 4-yr-old P. thunbergii saplings were transplanted in pots in the field. After enclosing the base of the saplings with insect-proof nets, adult H. ligniperda were introduced for infestation. Terminal and lateral shoot growth of P. thunbergii were measured at 0, 20, 40, and 60 d post-release (dpr) of adult beetles, while samples were collected at 7, 17, 37, 57, and 67 dpr. At each sampling time point, the damage area inflicted by H. ligniperda on P. thunbergii and the pest population density were recorded. Based on the damage area, the trees were classified into five severity grades: healthy pines (no pest released), and those with damage areas of <1%, 1% to 5%, 5% to 20%, and >20%. Laboratory analyses assessed changes in defense-related indices, root vitality, and nutrient content across damage grades. Additionally, H. ligniperda adults were reared on bark from differently damaged trees to evaluate the impact of P. thunbergii defensive responses. Results revealed that H. ligniperda infestation significantly inhibited shoot growth in P. thunbergii, impairing tree development and ultimately causing death. An overall trend of initial increase followed by a decrease was observed in the tree's root vitality and defense indices with increasing damage, which ultimately failed to prevent H. ligniperda colonization. Furthermore, these defensive responses suppressed weight gain and reduced reserves of glycogen, protein, and free fatty acids in adult H. ligniperda, adversely affecting their development.