Damestoy, T. | Search bioRxiv

bioRxiv2020

Search for top-down and bottom-up drivers of latitudinal trends in insect herbivory in oak trees in Europe

AimThe strength of species interactions is traditionally expected to become stronger toward the Equator. However, recent studies have reported opposite or inconsistent latitudinal trends in the bottom-up (plant quality) and top-down (natural enemies) forces driving insect herbivory, possibly because these forces have rarely been studied concomitantly. This makes previous attempts to understand the effect of large scale climatic gradients on insect herbivory unsuccessful. LocationEurope Time period2018-2019 Major taxa studiedQuercus robur MethodsWe used scholar-based citizen science to simultaneously test for latitudinal variation in plant-herbivore-natural enemy interactions. We further investigated the underlying climatic factors associated with variation in herbivory, leaf chemistry and attack rates in Quercus robur across its complete latitudinal range in Europe. We quantified insect herbivory and the occurrence of specialist herbivores as well as leaf chemistry and bird attack rates on dummy caterpillars on 261 oak trees. ResultsClimatic factors rather than latitude per se were the best predictors of the large-scale (geographical) variation in the incidence of gall-inducers and leaf-miners as well as of leaf nutritional quality. However, insect herbivory, plant chemical defences (leaf phenolics) and bird attack rates were not influenced by latitude or climatic factors. The incidence of leaf-miners increased with increasing concentrations of hydrolysable tannins and decreased with those of condensed tannins, whereas the incidence of gall-inducers increased with increasing leaf soluble sugar concentration and decreased with increasing leaf C:N ratios. However, neither other traits nor bird attack rates varied with insect herbivory. Main conclusionsThese findings help to refine our understanding of the bottom-up and top-down mechanisms driving geographical variation in plant-herbivore interactions, and urge for further examination of the drivers of insect herbivory on trees.

ecology↗

bioRxiv2019

Growth and mortality of the oak processionary moth, Thaumetopoea processionea L., on two oak species: direct and trait-mediated effects of host and neighbour species identity

The presence of heterospecific neighbours can affect damage caused by pest insects on focal plants. However, how plant neighbours influence herbivore performance is poorly understood. We tested the independent and interactive effects of tree species identity and tree neighbour type (conspecific vs. heterospecific) on the performance of a major oak pest, the oak processionary moth larvae (OPM, Thaumetopoea processionea) fed on Quercus robur and Q. petraea. We performed a factorial greenhouse experiment in which we grew two oak saplings per pot, either from the same species or from both species. We quantified growth and mortality of OPM larvae, leaf phenolic compounds, C:N ratio and bud phenology. OPM larvae performed significantly better on Q. petraea than on Q. robur, regardless of plant neighbour type. Phenolic compounds and C:N, but not phenology, differed between oak species and neighbour species identity. Only bud phenology had a significant effect on OPM performance, which was better when young larvae had access to recently unfolded leaves, regardless of oak species and neighbour identity. Although oak neighbour identity altered the expression of leaf traits, this effect had no measurable consequences on OPM performance. However, further studies should consider the effect of oak species neighbour on OPM preferences for either Q. robur or Q. petraea, in pure and mixed stands, before translating current results into recommendations for forest management. Author ContributionTD and BC conceived the study and acquired the data. TD performed experiment and analysed the data. EV, TD and XM performed the chemical analyses. TD and BC drafted the first version of the manuscript and all authors wrote the final version of the manuscript. TD, BC conceived and designed research. TD performed experiment and analysed data. EV, TD, XM performed the chemical analyses. TD, BC wrote the manuscript. All authors read and approved the manuscript. AM and DB conceived and designed research. AM and BB conducted experiments. GR contributed new reagents or analytical tools. AM, BB and GR analysed data. AM wrote the manuscript. All authors read and approved the manuscript.

ecology↗