Abstract
Understanding pine weevil [Hylobius abietis (L.)] feeding preferences and their relations to chemical defenses of pines may lead to improved protection of conifer seedlings across Europe and Asia. Previous studies showed reduced weevil damage in methyl jasmonate (MeJA) treated seedlings and that weevil preference for Monterey pine (Pinus radiata Ait.) or Maritime pine (Pinus pinaster D. Don) depended on feeding conditions. The present study explored whether volatile terpenes in plant tissues of control and MeJA-treated seedlings of these two pine species could explain differences in weevil feeding and seedling growth. Volatile terpenes of hexane extracts of needles and phloem were separated and identified by gas chromatography–mass spectrometry (GC–MS). The results confirmed that MeJA application reduced pine weevil feeding in both pine species, but the increase in resistance was not explained by any general change in the concentration of mono- and sesquiterpenes after MeJA application. MeJA effects on volatile terpenes differed between the two pine species. Responses to MeJA application were more intense in Monterey pine, for which total mono- and sesquiterpenes were induced and growth rates affected, even at the lowest concentration of MeJA (5 mM). In Maritime pine, seedling growth was affected only at the highest MeJA concentration (25 mM), and no effect was observed on total volatile terpenes. Contrasting effects of MeJA on key monoterpenes were also observed. In needles, the content of (−)-β-pinene, a monoterpene with deterrent properties against the pine weevil, increased in MeJA-treated seedlings of Monterey pine, but not in Maritime pine. In the phloem of Maritime pine, the MeJA treatment reduced the concentration of the pine weevil attractant (+)-α-pinene, whereas it was increased in Monterey pine. Altogether, results indicated that weevil preferences between the two species could be altered if resistance of seedlings were previously induced with MeJA. The differential inducibility of key volatile terpenes between the two pine species may be part of the explanation as to why pine weevil preferences to the two pine species differ between field and lab conditions.
Highlights
Conifers are long lived species that have existed on Earth for millions of years (Schulman 1954)
The Maritime pine seedlings were slightly taller than the Monterey pine seedlings, but seedlings of the two pine species did not differ in diameter (Table S1)
Our results confirmed that methyl jasmonate (MeJA) application reduced pine weevil feeding on seedlings of the two pine species as measured in non-choice bioassays, evidencing increased effective resistance
Summary
Conifers are long lived species that have existed on Earth for millions of years (Schulman 1954). Single trees can live for hundreds or even thousands of years (Schulman 1954), and during their long life span, they cope with multiple herbivores and pathogens that feed on their tissues. Conifers have evolved a wide array of physical and chemical defenses (Franceschi et al 2005). Volatile terpenes are known to largely influence insect orientation and herbivory, including feeding by the pine weevil, Hylobius abietis (L.) (Björklund et al 2005; Heijari et al 2005, 2011; Klepzig and Schlyter 1999), a major pest of young conifer seedlings across Europe and Asia (Långström and Day 2004)
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