Growth Responses of Musk and Plumeless Thistles (Carduus nutansandC. acanthoides) to Damage byTrichosirocalus horridus(Coleoptera: Curculionidae)

Abstract

Plant response studies were conducted from 1980 to 1982 under pasture and field plot conditions to determine the sublethal effects of the thistle rosette weevil [Trichosirocalus (Ceuthorynchidius) horridus(Panzer)] onCarduusthistles. Response to weevil damage was dependent upon thistle size and growing conditions. Damage byT. horridusdestroyed apical dominance, which altered the thistle growth pattern. Infested plants consistently produced more stems and a larger crown than uninfested thistles. Large thistles were stimulated by weevil damage, producing heavier stems and more heads. Small infested thistles developed more quickly than small uninfested thistles. Small and medium infested thistles were shorter and produced fewer seeds and heads than uninfested thistles. The extent of thistle reduction by this weevil will ultimately be determined by the conditions under which thistles grow.