Needle age-dependent defence of Scots pine against insect herbivory.

This article investigates how needle age in Scots pine (Pinus sylvestris L.) influences plant defence responses to herbivory by the pine sawfly (Diprion pini) and how the insect responds to needles of different ages. The study found that D. pini females prefer to oviposit on old (previous-year) needles, which also support higher egg survival, while larvae shift their feeding preference from young (current-year) needles in early instars to old needles in later instars, gaining more weight on the latter. Phytohormonal analyses revealed that feeding-induced jasmonic acid (JA) and jasmonic acid isoleucine (JA-Ile) levels increased similarly in young and old needles, whereas salicylic acid (SA) and abscisic acid (ABA) responses were stronger in young needles. Proteinase inhibitor (PI) activity was constitutively higher in young needles but was not induced by sawfly feeding in either needle age class. These findings highlight ontogenetic plasticity in conifer needle defences and suggest co-adaptation between Scots pine and its specialist herbivore, with implications for understanding age-dependent plant–insect interactions in evergreen gymnosperms.