Occurrence ofTrichosirocalus horridus(Coleoptera: Curculionidae) on NativeCirsium altissimumVersus ExoticC. vulgarein North American Tallgrass Prairie

Abstract

Postrelease studies can provide data with which to evaluate expectations based on prerelease tests of biological control insects. In 2004, we observed Trichosirocalus horridus Panzer (Coleoptera: Curculionidae), the rosette weevil introduced into North America against Eurasian thistles, feeding on native tall thistle, Cirsium altissimum L. Spreng., in tallgrass prairie. In this study, we examined the rosette weevil's use of tall thistle, compared with its use of the co-occurring exotic bull thistle, C. vulgare (Savi) Tenore. For both thistle species, we quantified weevil frequency, abundance, and seasonal variation in incidence, using both timed observations at two sites over two growing seasons (2004, 2005) and dissections of thistle flowering shoots from 13 sites (2005). Based on prerelease information, we expected the Eurasian thistle to be the quantitatively preferred host plant for this Eurasian weevil. Instead, we found that both the frequency of infestation and the mean number of adult rosette weevils per plant were at least as high, and sometimes higher, on the native thistle as on the exotic thistle. Furthermore, adult weevil phenology coincided on the two host species. This study provides new quantitative evidence of nontarget feeding by another weevil released for thistle biological control; and it raises important questions for further research. We conclude that continued new releases, as well as augmentation of existing populations, of T. horridus should wait until more research is done on the impact of the nontarget occurrence now reported for this biological control insect.

No interaction between competition and herbivory in limiting introduced Cirsium vulgare rosette growth and reproduction

“…Both richness and abundance of thistle specialist and generalist insect herbivores on C. vulgare were comparable to the native C. altissimum (Takahashi et al 2009). The phenological synchrony of C. vulgare with C. altissimum has been hypothesized to facilitate herbivore transfer (Jackson 1998;Louda and Rand 2002;Andersen and Louda 2008).…”

“…Although C. vulgare is sparse in eastern Nebraska (Andersen and Louda 2008), it is highly invasive in other regions of the USA (USDA 2009), as well as in Australia (Medd 1981) and New Zealand (Johnson 1982). In eastern Nebraska, C. vulgare is a host plant for many indigenous insect herbivore species (Takahashi et al 2009); and, we found that feeding by these insects on C. vulgare significantly reduced both growth (Figure 2) and seed matured (Figure 3) in its final, flowering year. Contrary to the classic assumption that competition from native plants provides strongest resistance to invasion by introduced weeds, a recent meta-analysis (Levine et al 2004) found that herbivory by native insects can be as important as competition in resisting invasion by introduced plants.…”

“…The large effect of insect herbivores on C. vulgare in eastern Nebraska likely results from the richness and abundance of thistle-feeding insects on Cirsium altissimum (Takahashi et al 2009), the most common native thistle in western tallgrass prairie (Louda and Rand 2002;Kaul et al 2006). Both richness and abundance of thistle specialist and generalist insect herbivores on C. vulgare were comparable to the native C. altissimum (Takahashi et al 2009).…”

First documentation of adultTrichosirocalus horriduson several non-target nativeCirsiumspecies in Tennessee

“…These occurrences are the first reports of T. horridus feeding to any degree on C. carolinianum, C. horridulum, or C. muticum in the US and only the second record of non-target feeding on C. altissimum (Piper and Coombs 2004;Takahashi, Louda, Miller, and O'Brien 2009). Furthermore, the infested populations of C. altissimum in Sevier Co. and C. discolor in Blount Co. are both within the borders of the Great Smoky Mountains National Park (GRSM).…”

Native insect herbivory limits population growth rate of a non-native thistle

“…For example, timed observations conducted throughout the growing season at two sites in Lancaster County, Nebraska, revealed that 97% of the individuals identified on C. vulgare were native species (see ESM 1 in the supporting information). The primary non-native insect herbivore on C. vulgare in eastern Nebraska is Trichosirocalus horridus (ESM 1); specifically, in a survey of 13 sites in Lancaster County, adult T. horridus were found in fewer than 10% of C. vulgare shoots (Takahashi et al 2009). …”