English laurel (Rosaceae: Prunus laurocerasus L.) is an evergreen shrub to small tree that is native to Asia but widely used in landscaping in temperate zones. In the Pacific Northwest this plant has become an important invasive species. However, surprisingly little is known about its ecology—in particular, its chemical defenses and interaction with herbivores and organisms of higher trophic levels. In the present field study, we quantitatively measured cyanogenesis (a constitutive direct chemical defense) and secretion of extrafloral nectar (EFN; an inducible indirect defense), and analyzed the effects of these defenses on insect herbivores (black vine weevil [Otiorhynchus sulcatus]) and predators (sugar ants [Tapinoma sessile]). To induce EFN production, English laurel leaves were mechanically damaged at different intensities and in a way to mimic feeding damage by the black vine weevil, which represents the only observed insect herbivore feeding on this plant at our study site near Portland, OR. While cyanogenesis was expressed at homogenously high levels (> 80 µmol CN- g-1 leaf fresh weight) among all experimental plants and was not affected by leaf damage, we found a significant positive correlation between damage, EFN production, and presence of ants. The number of vine weevils observed on experimental plants was significantly negatively correlated with EFN secretion and ant attraction. Our findings suggest that English laurel—beyond expression of cyanogenesis—efficiently utilizes indirect defense through ants in its invasive range. This protective mutualism may significantly contribute to the success of English laurel as an invasive species in native ecosystems of the Pacific Northwest.
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Vol. 91 • No. 2