Legacy of the emerald ash borer invasion: indirect effects of canopy gaps on interactions between prickly ash and giant swallowtail butterfly larvae

Since its accidental introduction, emerald ash borer, Agrilus planipennis, (EAB) has killed millions of ash trees, Fraxinus spp. in North America. Extensive tree mortality in forests has generated widespread canopy gaps, increasing light penetration to the understory. Foliage of the native shrub prickly ash (Zanthoxylum americanum) contains furanocoumarins, which are secondary metabolites that deter most herbivores, especially as they become more toxic when photoactivated by UV light. Furthermore, furanocoumarin biosynthesis is energy intensive, and their concentration increases when photosynthesis is enhanced by increased light availability. Female plants typically invest more resources towards defense, while males allocate more towards growth. Therefore, male and female prickly ash located in canopy gaps may differ in their furanocoumarin concentrations and growth rates. The giant swallowtail caterpillar, Papilio cresphontes, is a specialist herbivore on prickly ash that is capable of detoxifying furanocoumarins with cytochrome P450’s. Previous studies have documented a fitness cost (delayed development) associated with furanocoumarin detoxification in caterpillars. We compared the growth of male and female prickly ash in canopy gaps and shaded understory. Males grew faster in gaps than in shade, while females grew slower in gaps than in shade, perhaps because they allocated more resources to fruit production. Swallowtail larvae grew slower on foliage from plants located in sun compared to larvae that consumed foliage from shaded plants. This suggests P. cresphontes development is negatively affected by canopy gap formation, perhaps because plants in sun are better defended.