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Long‐term effects of exogenous methyl jasmonate application on Scots pine (Pinus sylvestris) needle chemical defence and diprionid sawfly performance

Scots pine [Pinus sylvestris L. (Pinaceae)] trees with four different seed origins were exposed to exogenous applications of the elicitor, methyl jasmonate (MeJA), for three consecutive years. We studied the effects of MeJA on needle chemistry (including monoterpenes, sesquiterpenes, and tricyclic resin acids), plant growth, and the performance of two diprionid sawflies, the European pine sawfly (Neodiprion sertifer Geoffr.) and the common pine sawfly (Diprion pini L.) (both Hymenoptera: Diprionidae). In general, foliar MeJA application affected the whole range of needle secondary chemistry with significantly higher concentrations of two monoterpenes, β‐pinene and limonene, in particular. Furthermore, for some seed origins the growth rates of N. sertifer and D. pini larvae were lower on needles of MeJA‐treated plants with either high total terpene or high resin acid concentrations. However, inconsistencies in diprionid sawfly performance within each studied Scots pine origin suggest genetic variance in needle secondary chemistry. The differences between selected seed origins and notably variable responses to MeJA application imply that adaptation of the seed to new conditions may have had an impact on secondary chemistry and, thus, on insect performance. Finally, our results suggest that modification of Scots pine defence by a low‐concentration exogenous elicitor affects the production of terpenoids in the newly growing needles, leading to poorer pine sawfly performance in origins with high terpenoid content, while not harming the growth of Scots pine trees.