Biological Control of Musk Thistle (Carduus nutans) in Mesa Verde National Park

Kendall, Deborah

Heavy infestations of musk thistle, Carduus nutans Desf. occur primarily in fireburned areas and continuously disturbed sites in Mesa Verde National Park. Musk thistle .. can also invade moist areas with sparse vegetation. The current study investigated the establishment of two biocontrol agents, Rhinocyllus conicus Froel. and Trichosirocalus - horridus (panzer) (both species belong to the order Coleoptera (beetles) and the family Curculionidae (weevils» and a resident herbivore (plant feeder) of thistles, Vanessa cardui L. (order Lepidoptera, family Nymphalidae-the brush-footed butterflies) in a study area 1 km south of the Morefield sewage facility in Morefield Canyon. In light of concern that R. conicus can become established on native thistle (Cirsium sp.), the effects of all three insect herbivores on native thistle and musk thistle were investigated. R. conicus populations have become established from releases made outside Mesa Verde National Park for control of III musk thistle in agricultural areas. T. horridus was released in the study sites on 29 May 1992 and 4 June 1992. V. cardui is a migratory species that overwinters in Mexico and returns to the southwestern United States in the spring. The study indicates the establishment of R. conicus, T. horridus, and V. cardui in the study sites. Total seasonal means for the three species were as follows: (a) R. conicus was present at 1.078 per plant in the musk thistle release site, 0.822 per plant in the musk thistle control (non-release site), and 0.776 per plant in the native thistle site; (b) T. horridus was present at 0.126 per plant in the musk thistle release site, but never present in the musk thistle control site or the native thistle site; (c) V. cardui was present at 0.114 per plant in the musk thistle release site, 0.124 per plant in the musk thistle control site and 0.086 per plant in the native thistle site. Seasonal means are based on bi-monthly records taken from 500 individual plants . Seedhead analysis of musk and native thistle indicated the presence of R. conicus at an average of 16.49 per seedhead (based on 106 seedheads) and 0.05 per seedhead (based on 55 seedheads) in native thistle.