- Author: Robert J Keiffer
Italian Thistle (Carduus pycnocephalus) is a Mediterranean annual weed and is an invasive plant throughout California since the introduction in the 1930s. In 1968 the Thistle-head Weevil (Rhinocyllus conicus) was introduced to Canada and the United States as a biological control agent for musk or nodding thistle. The weevil also has a liking for Italian thistle, and the weevil was purposely relocated in many problem areas through Northern California by UCCE and USDA County Agricultural commissioner agents throughout the 1970s and early 1980s. The weevil has persisted at the UC Hopland Research & Extension Center where the species follows through with its annual cycle of adults mating and laying eggs onto Italian thistle flower clusters where the larva burrow into the forming seed heads and devour the immature seeds. The unfortunate part of this story is that the weevil also attacks many of the native thistles belonging to the Carduus, Cirsium and Silybum groups.