Temporal and species-dependent variation in foliar monoterpene compositions of conifers under spruce budworm defoliation

Abstract Key message No induced herbivory effect under defoliation by Eastern spruce budworm ( Choristoneura fumiferana Clemens) was detected in young conifer saplings, but a marked difference among species—balsam fir ( Abies balsamea (L.) Mill.), white spruce ( Picea glauca [Moench] Voss), and black spruce ( Picea mariana [Mill.])—in monoterpene compositions. Context Terpenoids are among the most common defensive compounds in conifers, found in foliage, wood, and bark. Induction of terpenoids by chewing folivorous insects is less understood than stem-feeding xylophagous insects. Aims This study examined whether three conifers —balsam fir ( Abies balsamea (L.) Mill.), white spruce ( Picea glauca [Moench] Voss), and black spruce ( Picea mariana [Mill.])— induce terpenoid variation in response to Eastern spruce budworm ( Choristoneura fumiferana Clemens) defoliation. Methods A total of 360 4-years-old saplings from three coniferous species were grown in a greenhouse for a 2-years-long experiment, with half exposed to defoliation and half as controls. Buds and larvae phenology were monitored, and foliage was collected throughout the growing season. Terpenoid compounds from needles were analysed using static headspace gas chromatography (HS-GC). Statistical analyses were performed on the 14 most common compounds that were found. Results A total of 43 terpenoids were identified, of which 67% were monoterpenes. Defoliation did not significantly alter monoterpene composition, except in Picea glauca in 2015, where differences were mainly driven by eucalyptol and β-myrcene. Species identity thus accounted for major variation (26%) in monoterpene profiles. Monoterpene composition also varied with phenological stages, reflecting developmental changes in foliar chemistry during shoot growth. Conclusion Although defoliation by the eastern spruce budworm impacted bud phenology and shoot growth, monoterpene pool composition was not significantly affected by defoliation. Species identity accounted for the largest portion of the variation in monoterpene composition, with specific compounds distinguishing each conifer species.