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Interactions between oaks and the oak processionary moth, Thaumetopoea processionea L . : from trees to forest

Abstract : Forest ecosystems provide a series of ecosystem services that can be threatened by pest insects. Maintaining forest health is therefore a major challenge of forest management that requires a sound understanding of tree-pest interactions. In this context, we investigated oak resistance (and its drivers) to a pest insect, the oak processionary moth (OPM), Thaumetopoea processionea.The OPM is a specialist herbivore feeding on oaks in Europe. It is responsible for significant defoliation, in addition to serious urtication in forest users and practitioners. Through a series of observations and experiments carried out at several levels of integration (from tree to forest stand levels), we investigated the response of this insect to various ecological factors involved in oak resistance: forest stand species composition, tree neighbourhood effects and leaf traits potentially associated with defence.Forest species composition has well documented effects on tree resistance to herbivores. For a given species, trees are generally less attacked in mixtures than in pure stands (i.e. associational resistance). But the opposite - associational susceptibility - also exists. We tested whether tree species diversity generated associational resistance to OPM, by comparing abundance and damage on the pedunculate (Quercus robur) and sessile oaks (Q. petraea) in pure and mixed stands. We also investigated the underlying mechanisms, by characterizing the leaf traits involved in the variability of OPM performance. Finally, we focused on the genetic and environmental causes of inter-individual variability in the expression of these traits.First, we found that Q. petraea was generally more susceptible to the OPM than Q. robur. Stands dominated by Q. petraea attracted more OPM moths, Q. petraea suffered more defoliation, and OPM larvae had greater survival and weight gain on this species. We also found greater susceptibility of both oak species in pure stands as compared to mixed stands, particularly those associating an oak with another non-host species. Second, we investigated the mechanisms conferring greater resistance to Q. robur and to mixed stands by quantifying leaf chemical and developmental traits involved in oak-OPM interactions. The synchronization between leaf development and larval development also appeared to be a key factor determining the OPM performance. The concentration of leaf chemical defences was positively correlated with OPM consumption rate, suggesting a compensatory feeding response to maintain OPM growth rate on well-defended leaves. Leaf traits involved in oak-OPM interactions were partially determined by the identity of oak neighbours. Third, we found evidence that OPM performance was partially under the genetic control of their host trees in Q. robur.Overall, these results built toward a better understanding of the processes involved in the effect of tree species mixture on the damage caused by pest insects and provide new avenues to guide oak stand management for improved resistance to OPM. Based on the results of this study, it would seem preferable to favour Q. robur in oak stands because it is less attractive and more resistant to OPM attacks, both in the field and in the laboratory. However, this statement goes against current recommendations to use Q. petraea to adapt the oak to the increased risk of water deficit and therefore adds to complexity of managing forests for resistance to multiple-interacting stresses.
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Thomas Damestoy. Interactions between oaks and the oak processionary moth, Thaumetopoea processionea L . : from trees to forest. Biodiversity and Ecology. Université de Bordeaux, 2019. English. ⟨NNT : 2019BORD0220⟩. ⟨tel-02446138v2⟩

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