PARENT SESSION
Oral Session # 36: Rare, Threatened, and Endangered Species I: Plants.
Presiding: H Balbach
Tuesday, August 5. 1:30 PM to 5:00 PM, SITCC Meeting Room 204.

Non-target effects of biocontrol releases on native Colorado thistle species.

Dodge, Gary^{*,1, 2}, Inouye, David^{1, 2}, Louda, Svata^{2, 3}, ¹ University of Maryland, College Park, MD² Rocky Mountain Biological Laboratory, Crested Butte, CO³ University of Nebraska, Lincoln, NE

ABSTRACT- A primary risk associated with introduction of non-native phytophagous species as biocontrol agents is herbivory of non-target species. Despite this recognized risk, the practice is still broadly employed even by agencies and organizations with management goals to protect native plant species. We conducted an experimental exclusion of weevil oviposition on developing thistle capitula and found a major decrease in seed production due to feeding by two Eurasian weevils released as biocontrol agents, Larinus planus and Rhinocyllus conicus, on native Colorado thistle species (Cirsium undulatum var. tracyi, C. eatonii, and C. perplexans). Observed rates of oviposition by either weevil species on plant populations was high (70-90% of all capitula). Initial studies of population limitation suggest that seed predation by the weevils and native tephritid flies results in population-level effects on the native thistles. The release of L. planus and R. conicus is cause for additional concern in Colorado because of adverse impacts on two native Cirsium thistles recognized as rare and threatened (C. perplexans and C. ownbeyi). Studies of the effects of introduced species on native species will help ecologists and environmental managers understand the risk probabilities of biocontrol applications as well as population regulation theory and invasion dynamics.

Key words: thistles, seed-predation, Cirsium, Biocontrol