Although spatial variation in climate can directly affect the survival and reproduction of forest insects and the tree species compositions of forests, little is known about the indirect effects of climate on outbreaks of forest insects through its effects on forest composition. In this study, we use structural equation modeling to examine the direct and indirect effects of climate, water capacity of the soil, host tree density, and non-host density on the spatial extent of Lymantria dispar outbreaks in the Eastern USA over a period of 44 years (1975-2018). Host species were subdivided into four taxonomic and ecologically distinct groups: red oaks (Lobatae), white oaks (Lepidobalanus), other preferred hosts, and intermediate (less preferred) hosts. We found that mean annual temperature had stronger effects than mean annual precipitation on the spatial extent of outbreaks, and that indirect effects of temperature (via its effects on oak density) on defoliation were stronger than direct effects. The density of non-host trees increased with increasing precipitation and, consistent with the 'associational resistance hypothesis', defoliation decreased with increasing density of non-host trees. This study offers quantitative evidence that geographic variation in climate can indirectly affect outbreaks of a forest insect through its effects on tree species composition.

Department of Wildlife Fisheries and Aquaculture Mississippi State University Mail Stop 9690 Mississippi State MS 39762 USA

Faculty of Forestry and Wood Sciences Czech University of Life Sciences Prague 165 00 Praha 6 Suchdol Czech Republic

Tennenbaum Marine Observatories Network MarineGEO Smithsonian Environmental Research Center Edgewater MD 21037 USA

The Blandy Experimental Farm University of Virginia Boyce VA 22620 USA

USDA Forest Service Northern Research Station 180 Canfield Street Morgantown WV 26505 USA

USDA Forest Service Northern Research Station 3460 Industrial Drive York PA 17402 USA

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