Leaf toughness as a mechanism of defence against spruce budworm

Abstract

Toughness of leaves is an important defence mechanism of plants against insect herbivores and is generally linked to leaf fibre content. We explored the anatomical basis and the ecological role of needle toughness as a mechanism of defence against feeding initiation by second-instar spruce budworm (Choristoneura fumiferana) emerging from diapause. We first aimed at determining relationships between needle physico-chemical traits and toughness [specific leaf area (SLA), nitrogen, carbon, fibre, and water content, and C/N ratio]. Second, we tested the hypothesis that differences in needle toughness between balsam fir (Abies balsamea) and black spruce (Picea mariana) explain differences in the ability of young budworm to mine into a needle, asking whether nitrogen content or toughness is a better predictor of budworm mining success. Our results show first that needle toughness correlates with high cellulose content and that SLA is a good proxy. Second, needle toughness is a stronger predictor of mining success (defined as the number of larvae that entered a needle) of young budworm than nitrogen content. Our results suggest that needle toughness can be an important defensive trait in conifers, as suggested by the low mining rate exhibited by second-instar spruce budworm larvae feeding on 1-year-old black spruce foliage. The foliage of this host is tougher and has higher fibre content than balsam fir foliage. Differences in needle toughness, and associated differences in resistance to folivores, might be an important component in adaptive suites of traits constituting the different life-history strategies of these two sympatric species.