An integrative field and modelling study of the bottom-up, top-down and indirect effects of native species on invasive insects and their biological control : the case of the worldwide chestnut tree pest, Dryocosmus kuriphilus, in the French Eastern Pyrenees

Insect pests invading an ecosystem typically face various forces regulating their population growth and spread. While the resources they feed on and their natural enemies have ′bottom-up′ and ′top-down′ effects on insect invasion success, other native species, more distant in the local trophic network, can have indirect effects on the ecosystem′s susceptibility to invasion. Resolving the mechanisms underlying these naturally occurring indirect interactions and their impact on invasion dynamics is a key challenge toward risk assessment and environmental management. In this contribution, we investigated how indirect interactions with several native species contribute to variations in Dryocosmus kuriphilus infestation level in 24 natural chestnut tree populations of the French Eastern Pyrenees. This invasive gall-forming hymenopteran parasites cultivated and wild Castanea sativa stands, and its hymenopteran parasitoid, Torymus sinensis , is world widely used as a control agent. We combined ecological, molecular and statistical approaches to quantify the effects of Quercus pubescens , Fagus sylvatica and the parasitic fungus Cryphonectria parasitica on the pest oviposition rate ( effect i ), its host detection capacity ( effect ii ), the production of native parasitoids ( effect iii ) and in providing alternative hosts for the control agent ( effect iv ). The integration of these effects into a specifically designed D. kuriphilus – T. sinensis dynamical model, provided clear evidence of the quantitative impacts of Q. pubescens and C. parasitica on the invasion potential of the pest and its biological control.