Utilization of non-target native host plants (Cirsium spp.) by adult Trichosirocalus horridus in Tennessee: host expansion or incidental occurrence?

Releases of Trichosirocalus horridus (Coleoptera: Curculionidae), native to Europe, began in Tennessee in 1989 as part of a biological control program against musk thistle (Carduus nutans).  In surveys conducted to investigate non-target feeding of T. horridus on native Cirsium thistle species from 2005 through 2008, adults of T. horridus were observed on all five native Cirsium thistles.  These data represent the first documentation of T. horridus occurring on three of these native species [C. carolinianum, C. horridulum and C. muticum], and the first record of T. horridus occurring in the Great Smoky Mountains National Park.  While C. carolinianum and C. horridulum did not show symptoms of larval feeding in the meristematic tissues, C. altissimum, C. discolor, and C. muticum all had damaged meristems and possible oviposition scars on the midribs of the leaves.  However, the impact of feeding by larvae of T. horridus on the reproductive potential of native plants is uncertain, because even in the target species (musk thistle and other introduced Cirsium species) plant death rarely occurs, and seed production continues.