No escape: Non-target host utilization of native thistles by Rhinocyllus conicus on field-caged plants

The exotic weed Carduus nutans has been the subject of a biological control program in North America that included the release of the European weevil species Rhinocyllus conicus Fröelich across much of the U.S. Concern now exists over the non-target feeding of this weevil on at least 25 species of native Cirsium thistles. Beginning in 2008, no-choice cage studies isolating adults of R. conicus on buds and flower heads of all eight thistle species recorded from Tennessee were conducted to test if R. conicus could utilize these species for reproduction and what impacts larval feeding of R. conicus may have on seed production. Larvae of R. conicus completed development in heads of C. carolinianum and C. horridulum, and significant reductions in seed numbers of both of these native species occurred during 2008. Rhinocyllus conicus oviposited on both C. carolinianum and C. horridulum at significantly greater levels than C. arvense and C. vulgare (introduced species). Both native species contained numbers of R. conicus per cm of plant head width similar to C. arvense and C. vulgare in 2009. Body length was similar between R. conicus reared on native thistles and its target host Ca. nutans. Although R. conicus only has been observed on introduced thistles in naturally-occurring populations in this region, this weevil is able to utilize the native thistles C. carolinianum and C. horridulum as host species.