Needle age-dependent defence of Scots pine against insect herbivory

Plant defence responses to herbivory vary with foliage age, which in angiosperms lasts a few months, but may be several years in conifers. While age-dependent leaf responses of angiosperms to insect herbivory are well studied, much less is known about anti-herbivore responses of conifer needles that differ in age by a year or more. Similarly, insect responses to conifer foliage that differs in age by years have rarely been studied. Here, we used Pinus sylvestris L. (Scots pine) and the herbivorous sawfly Diprion pini to elucidate (i) the responses of this herbivore to pine needles of different age and (ii) needle age-dependent differences in two anti-herbivore plant defence traits, rarely studied in interactions of conifers and phyllophagous insects, i.e., phytohormones and proteinase inhibitor (PI) activity. The sawfly D. pini preferred old (previous year) over young (current year) pine needles for oviposition and feeding by late larval instars. The insect benefited from these preferences through higher egg survival rates on old needles and more larval weight when feeding upon old needles. Scots pine needles showed needle-age dependent differences in their phytohormonal responses to D. pini larval feeding. Feeding-induced concentrations of salicylic acid and abscisic acid were higher in young than old needles. No such age dependency was detected for herbivory-induced changes in levels of jasmonic acid, in contrast to known, age-dependent jasmonic acid responses to damage in angiosperms. Contrary to angiosperms, PI activities of pine were not induced by sawfly feeding upon young or old needles. However, old needles showed constitutively significantly lower PI activities, which are discussed with respect to the benefits that D. pini gains when preferring old needles. Our results highlight the developmental plasticity of defence traits of conifer foliage as well as the adaptation of a pine herbivore specialist to these defences.