PARENT SESSION
Poster Session #58: Invasive Species Control.
Thursday, August 8. Presentation from 8:00 AM to 9:30 AM. Exhibit Hall B & C, TCC

Influence by native thistle herbivores can potentially affect the spread of a non-native thistle.

Young, Lauren^*,1, Louda, Svata¹, ¹ University of Nebraska- Lincoln, Lincoln

ABSTRACT- Biotic resistance is a key hypothesis in explaining unsuccessful invasions as it asserts that introduced species will fail to penetrate communities if they encounter native species that reduce performance and prevent spread. Bull thistle (Cirsium vulgare), is an invasive weed throughout much of North America but is not significantly so in Nebraska. Native inflorescence-feeding insects harbored by tall thistle (Cirsium altissimum) could be providing resistance to invasion by bull thistle. In eastern Nebraska, native thistle herbivores severely reduce viable seed production for bull thistle by at least 71%. To evaluate the influence of floral and predispersal seed predation on the spread of bull thistle we asked if seed supply determines the number of seedlings that establish. During the spring of 2001 we planted 0, 30, or 60 bull thistle seeds in plots that were either burned or highly, moderately, or not disturbed. We found seedling establishment is proportional to seed supply. The number of bull thistle seedlings establishing during the summer of 2001 doubled when the number of seeds added were doubled. More seedlings also established with greater disturbance. Thus, the results to date suggest that seed limitation by native herbivores could play a significant role in preventing establishment and spread of bull thistle.

KEY WORDS: biotic resistance, Cirsium, insect herbiviory