This study was conducted to determine if field evaluations could be used to select insects for biological control of musk thistle. Host specificity and larval development of a weevil, Hadroplonthus trimaculatus, and a fleabeetle, Psylliodes chalcomera, were studied in field trials near Rome, Italy, in which insects were allowed free choice of several hosts. Natural populations of these two insects, which do not occur in North America, were exposed to North American species of Cirsium, Carduus, and selected crops. Adult insects and larvae on host plants were identified and counted on test plants from North America and native attraction plants. In addition to infesting musk thistle, weevil adults and larvae were recorded on flodman thistle, wavyleaf thistle, and spinosissimum thistle. Consequently, this insect was not suitable for introduction into North America. The fleabeetle would be satisfactory for biological control since no adults or larvae were recorded on Cirsium spp. or economic plants. These studies show that field trials are a valid method for identifying specific and nonspecific candidate insects for biological control of weeds.