The subtribe Carduinae (Compositae: Cynareae) consists of many thistle species which are becoming an ever increasing economic menace to range and pasture lands. A phytophagous weevil, Ceuthorrhynchidius horridus Panzer has been found to be associated with these weeds in Europe, but cannot be introduced without a comprehensive investigation of its host specificity and biology. First instar host specificity tests on 36 species of economically and aesthetically important plants revealed an approximate host range among some species based on development to only the third instar. Field host range information disclosed that this weevil has not been reported to be a serious pest of any economic plant species, although it was cited to have been found on some important agronomic species. An adult aestival diapause termination experiment indicated that diapause was most efficiently terminated with a gradually reduced long - day photoperiod while it was achieved quickest with a short-day photoperiod. / Master of Science