Cascading ecological impacts of emerald ash borer: Tritrophic interactions between prickly ash, giant swallowtail butterfly larvae, and larval predators

Background/Question/Methods

Extensive ash tree mortality caused by emerald ash borer (EAB, Agrilus planipennis) has generated widespread canopy gaps, resulting in increased 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 in defense, while males allocate more towards growth. Therefore, male and female prickly ash located in canopy gaps may differ in their furanocoumarin concentrations, growth rates, and reproductive effort. Giant swallowtail butterfly larvae (Papilio cresphontes) are specialist herbivores on prickly ash capable of detoxifying furanocoumarins. Energetic costs of furanocoumarin detoxification can slow larval development, and thus increase exposure to natural enemies. In a series of field and lab experiments, we examined the effects of EAB-induced canopy gaps on resource allocation of prickly ash, and growth and survival of P. cresphontes. 

Results/Conclusions

Prickly ash located in canopy gaps had lower specific leaf area, increased growth, and increased thorn densities compared to shaded plants. Male prickly ash grew faster than females, but females produced more flowers and fruits. We hypothesize that the slower growth of females results from higher allocation to reproduction and defense. In laboratory bioassays, P. cresphontes larvae fed foliage from plants in gaps had lower growth rates than larvae feeding on shaded plants. There was no difference in survival of larvae placed on plants in gaps compared to understory plants, with mortality over 48 hours close to 70% in both habitats. We conclude that larval survival may be lower in gaps because decreased growth rates may increase the amount of time larvae are exposed to natural enemies.