Exploiting jasmonate-induced responses for field protection of conifer seedlings against a major forest pest, Hylobius abietis

Herbivore damage commonly initiates an increased synthesis of chemical defensive compounds in attacked plants. Such induced defences are a vital part of plant defence systems, but when herbivore pressure is high, as frequently occurs in man-made ecosystems such as agricultural and forest plantations, plants may suffer considerable damage before adequate induced defences build up. To prepare the plants for such conditions their induced defence may be artificially triggered by simulated herbivory, e.g. by application of a chemical elicitor. This method is already widely employed in agriculture but within forestry systems it has so far been restricted to promising laboratory results. The pine weevil, Hylobius abietis, causes damage by feeding on the bark of young conifer plants and it is one of the main threats to successful regeneration in the Palaearctic region. Here we present results from a large scale field experiment where we triggered the induced defences of conifer seedlings using exogenous application of the chemical elicitor methyl jasmonate. To enhance the generality of the results different species were planted under extremely different environmental conditions; Maritime pine and Monterrey pine in Spain, and Scots pine and Norway spruce in Sweden. Weevil damage, chemical defences, and seedling growth were studied during the two growing periods following planting. In general, treated plants showed increased quantitative defences, and were less attacked, less wounded, less girdled and showed lower mortality rates than their untreated counterparts. Effects were mostly dose dependent, although some interactive effects with tree species were observed. The treatment initially caused a growth reduction but it was later compensated by the benefit, in terms of growth, of being less damaged. The measures that are currently taken to protect forest plantations against this harmful pest all around Europe have enormous economic costs and cause important environmental hazards. Elicitation of inducible defences in seedlings in the nursery appears to be a cost-effective and environmentally-friendly alternative to these measures. To our knowledge, this is the first field study that explores the applicability of chemical elicitors of induced defences as a way to protect forest plantations against biotic threats