Enemy release of Callery pear: implications for an emerging invasive plant pest

Background/Question/Methods

Callery pear (Pyrus calleryana) is a non-native invasive tree from Asia, brought to the United States as a potential way to combat the devastating fire blight disease of the American pear industry. Since the 1950s, however, certain P. calleryana cultivars have been widely planted as landscape and ornamental trees, valued for the showy, white flowers that bloom early in the spring, thick crown, and bright red fall foliage. It has since become invasive into forested lands, abandoned fields, roadsides, and other disturbed areas. Despite its long history in the United States, little is known about its spread dynamics or impacts on native flora and fauna. To investigate potential driving mechanisms behind P. calleryana’s recent rapid spread across the eastern U.S., we conducted choice and no-choice feeding trials with a native specialist herbivore (eastern tent caterpillar, ETC; Malacosoma americanum), a native generalist herbivore (fall webworm, FWW; Hyphantria cunea) and a non-native generalist herbivore (Japanese beetle, JB; Popilia japonica). To assess the impact on herbivores in field scenarios, we also conducted field herbivory surveys on P. calleryana as well as three native, disturbance-associated species (black cherry, mockernut hickory, and persimmon).

Results/Conclusions

The native specialist herbivore ETC fed copiously on P. calleryana in choice assays but fed significantly less on P. calleryana in no-choice assays compared with native tree species. Both generalist herbivores exhibited low to moderate feeding on P. calleryana in choice and no-choice assays compared to native tree species. Reduced feeding by specialists in no-choice scenarios suggests a potentially novel secondary metabolite which native specialists are not accustomed to processing. Low to moderate feeding by generalists, both native and non-native, may result in increased growth and spread of P. calleryana rather than control. Additionally, field herbivory surveys showed significantly less feeding on P. calleryana compared with natives in the same environments. Combined, these results provide evidence for the Enemy Release Hypothesis as a potential driving mechanism behind the rapid population growth and spread of P. calleryana in the eastern United States. Future research will focus on interactions among P. calleryana and early spring pollinators, and how these interactions may impact seed production, native flora, and pollinator communities.