Gypsy moth herbivory induced volatiles and reduced parasite attachment to cranberry hosts

Interactions between species can have cascading effects that shape subsequent interactions. For example, herbivory can induce plant defenses that affect subsequent interactions with herbivores, pathogens, mycorrhizae, and pollinators. Parasitic plants are present in most ecosystems, and play important roles in structuring communities. However, the effects of host herbivory on parasitic plants, and the potential mechanisms underlying such effects, are not well known. We conducted a greenhouse study to ask whether gypsy moth (Lymantria dispar) damage, host cultivar, and their interaction affected preference of the stem parasite dodder (Cuscuta spp.) on cranberry hosts (Vaccinium macrocarpum). We then assessed the mechanisms that could underlie such effects by measuring induced changes in phytohormones and secondary compounds. We found that damage by gypsy moths delayed dodder attachment by approximately 0.3 days when dodder stems were added 2 days after damage, and reduced attachment by more than 50% when dodder stems were added 1 week after host plant damage. Gypsy moth damage significantly increased jasmonic acid (JA) levels, total volatile emissions, and the flavonol, quercetin aglycone, suggesting possible mechanisms underlying variation in dodder ability to locate or attach to hosts. Dodder preference also differed between cranberry cultivars, with the highest attachment on the cultivar that had significantly lower levels of total volatile emissions and total phenolic acids, suggesting that volatile composition and phenolics may mediate dodder preference. Our results indicate that herbivory can reduce subsequent attachment by a highly damaging parasitic plant, demonstrating the potential importance of early damage for shaping subsequent species interactions.