Michael G. Cripps, Graeme W. Bourdōt, Karen L. Bailey
Lincoln University, John Burton Building, Christchurch 7647, New Zealand.
Recently, Müller and Nentwig (2011) reviewed the plant pathogens that have been considered for biological control of the weed Cirsium arvense (L.) Scop. (Canada thistle, Californian thistle, creeping thistle), and concluded that the prospects have been largely overestimated. The premise of their conclusion is that no bioherbicide products have achieved marketability, which they surmise is due to lack of host specificity, effectiveness, and issues with application. While it is true that no microbial products have achieved marketability for this weed, we believe their reasoning for this is erroneous, and likely due to lack of distinction between two biocontrol approaches, specifically classical biocontrol, and innundative biocontrol (often referred to as the biopesticide approach). These two different types of biocontrol have different goals, and are applied in different ways. Generally, in classical biocontrol, coevolved insects or pathogens from the native range of a weed are imported and released in regions where the weed has been introduced, and has become invasive (McFadyen 1998; Watson 1991). Classical biocontrol is permanent, and when successful, requires little or no continued management input. The goal is not eradication, but rather to suppress weed populations to a level where they are no longer problematic. When importing natural enemies (insects or microbes), assurance of safety to non-target plants is paramount, and often requires a high degree of host specificity (Barton 2004; Berner and Bruckart 2005).