Effect of forest stand type on host plant quality and direct and indirect effects on pine sawfly performance

Davide Bellone,Maartje J Klapwijk,Christer Björkman,Axel Schmidt,Jonathan Gershenzon

doi:10.1111/afe.12415

Abstract

Abstract Observed lower levels of herbivory in mixed compared with monoculture stands have been hypothesized to depend on top‐down forces, through higher predation pressure by natural enemies or through bottom‐up mechanisms through plant quality effects on herbivore performance. In this study, we compared the performance measured as host plant induced mortality, cocoon weight, and predation mortality of the European pine sawfly Neodiprion sertifer (Geoffroy) (Hymenoptera, Diprionidae) in mixed and monoculture forest stands. We did not observe a difference in host plant induced mortality, cocoon weight, or predation mortality between mixed and monoculture forest stands. We did find an effect of local conditions around each experimental tree on pine sawfly performance. For example, the nitrogen content of pine needles is negatively affected by the proportion of pine around the experimental tree, which in turn increases the survival of sawfly larvae. The results suggest that local conditions around individual trees are more important for the performance of the European pine sawfly than stand type, i.e. mixed or monoculture plant stands. We conclude that the ongoing trend for diversification within commercial forestry calls for more research where the effects of both bottom‐up and top‐down effects are studied at several spatial scales.

Highlights

Do herbivorous insects cause less plant damage in diverse compared with simplified habitats? The current body of literature provides different answers depending on what ecological system is considered (Letourneau et al, 2011)
We modelled a direct relationship between nitrogen and total resin acid content with host plant induced mortality, predation mortality, and cocoon weight
Cocoon weight was related to a negative interaction between nitrogen content and total resin acid content

Summary

Introduction

Do herbivorous insects cause less plant damage in diverse compared with simplified habitats? The current body of literature provides different answers depending on what ecological system is considered (Letourneau et al, 2011). The other hypothesis considers plant apparency (Sholes, 2008) and resource availability (Abdala-Roberts et al, 2015) as drivers of herbivore populations and plant damage from specialist herbivores. One less explored mechanism is the effect of plant diversity on the quality of the host plant as resource for herbivorous insects (Mraja et al, 2011; Moreira et al, 2014; Kostenko et al, 2017), potentially with indirect consequences for herbivore performance. The majority of studies investigating the effects of stand diversification on specialist herbivores have focused on the effect on natural enemies (Muiruri et al, 2015; Leles et al, 2017) or plant apparency (Jactel et al, 2011; Dulaurent et al, 2012). The potential effects of stand diversification on host plant quality and on herbivorous insect performance have so far been underemphasized

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