Physiological Adaptation Strategies of the Interaction Defense Between Larvae of Megastigmus sabinae and the Host Juniperus przewalskii

<i>Juniperus przewalskii</i> Komarov, an endemic conifer in the high-altitude mountains of the Qinghai-Tibet Plateau, develops its cones in a synergistic manner with the oligophagous pest <i>Megastigmus sabinae</i> Xu et He (Hymenoptera: Torymidae), forming a highly specialized interaction system. However, the physiological adaptation mechanisms underlying this interaction remain unclear. Feeding by <i>M. sabinae</i> larvae significantly induced defense responses in <i>J. przewalskii</i> cones. During the early instars (2nd~3rd) of <i>M. sabinae</i> larvae, infested <i>J. przewalskii</i> endosperms upregulated protein content (48.91%; 3rd instar), significantly enhanced peroxidase (POD) activity (71.10%; 2nd instar), and specifically enriched coumarins and cinnamaldehyde derivatives (2nd instar) to increase <i>M. sabinae</i> larvae metabolic costs. In later instars (4th~5th) of <i>M. sabinae</i> larvae, the infested endosperms downregulated starch content (29.69%; 4th instar), increased phenylalanine ammonia-lyase (PAL) activity (57.34%; 4th instar), and accumulated steroid derivatives to suppress larvae development. Conversely, <i>M. sabinae</i> larvae demonstrated unique adaptive strategies: maintaining high levels of glutathione S-transferase (GST) as an antioxidant defense system during early instars and upregulating the level of digestive enzymes in later stages to overcome host multi-layered defenses. <i>Juniperus przewalskii</i> counters pest infestation through dynamic nutrient modulation, temporal activation of protective enzymes, and a multi-layered chemical defense network. The adaptation of <i>M. sabinae</i> larvae appears to involve the developmental regulation of detoxification and digestive enzyme levels. This study provides novel insights that enrich coevolution theory in alpine ecosystems.