Host Specificity Studies on Ceutorhynchus trimaculatus (Col.: Curculionidae), a Potential Biological Control Agent of Musk and Plumeless Thistle

Abstract

Five series of tests (larval development, oviposition, host substitution, oogenesis and host age/oogenesis) were conducted to determine the host specificity of Ceutorhynehus trimaculatus F. Three of 27 host plants used in the larval development test, Carduus nutans L. = Carduus thoermeri Weinmann (musk thistle), C. acanthoides L. (plumeless thistle) and Cirsium vulgare (Savi) Tenore (bull thistle), supported 1st instars to the adult stage. A fourth plant, Cynara scolymus L. (artichoke), yielded one adult out of 225 first instars and is poorly suited to larval development. Oviposition occurred on musk thistle and artichoke but not on Carthamus tinctorius L. (safflower). The number of eggs deposited per female dropped sharply when adults were switched from musk thistle to artichoke. In the oogenesis test, higher numbers of eggs were recorded on mature leaves of Silybum marianum (L.) Gaertn (milk thistle), musk, plumeless and bull thistle than on artichoke, while none was found on safflower. Seedling artichoke was not different from mature leaves for weevil oogenesis. These results show that C. trimaculatus has a high degree of host specificity. It does not attack any cultivated crops in its native area and the risk of the weevil attacking beneficial economic plants in North America is very small.