Host utilization of field-caged native and introduced thistle species by Rhinocyllus conicus

Area of Science:

• Ecology
• Biological Control
• Entomology

Background:

• Rhinocyllus conicus (weevil) introduced from Europe for biological control of Carduus nutans (nodding thistle).
• Concerns exist regarding R. conicus feeding on over 25 native Cirsium thistle species.
• Previous observations in the region showed R. conicus only on introduced thistles.

Purpose of the Study:

• To test if R. conicus can reproduce on native Tennessee thistle species.
• To assess the impact of R. conicus larval feeding on native thistle seed production.
• To document the first instance of R. conicus feeding and reproduction on C. carolinianum and C. horridulum.

Main Methods:

• Cage studies conducted in 2008 isolating adult R. conicus on buds/flower heads of eight native and introduced thistle species.
• Monitoring of R. conicus reproduction, larval development, and impact on seed numbers.
• Comparison of R. conicus density and body length between native and introduced thistle hosts.

Main Results:

• R. conicus larvae completed development in C. carolinianum and C. horridulum heads.
• Significant reductions in seed numbers of both native species were observed.
• R. conicus oviposition and larval density were similar on native thistles compared to target and introduced hosts.

Conclusions:

• This study provides the first documentation of R. conicus reproduction on native C. carolinianum and C. horridulum.
• Controlled conditions show potential for R. conicus to utilize native thistles, impacting seed production.
• Continued field monitoring and research into factors influencing non-target feeding are warranted.