Submitted to: California Invasive Plant Council
Publication Type: Popular Publication
Publication Acceptance Date: January 5, 2007
Publication Date: December 1, 2008
Citation: Smith, L. 2008. Classical biological control of weeds. California Invasive Plant Council. Newsletter 17(4):4-7 Interpretive Summary: Classical biological control of weeds is an important tool for managing invasive alien plants that have become too widespread to control by conventional methods. It involves the discovery and release of naturally occurring species of natural enemies (insects, mites or pathogens) to control a pest (plant or animal). It is only safe to use natural enemies that are very host-specific, otherwise they might damage other plants. Prospective biological control agents are carefully studied to determine if they can damage any other plants under no-choice conditions. Any nontarget plants that are at risk are further evaluated under more realistic conditions where the agent has a choice between the target weed and the nontarget plant. Agents that pass safety tests must be approved by both federal and state agencies before being permitted to be released. The decision to release a biological control agent is a governmental decision that weighs potential risks against benefits, which is aided by scientific information and analysis.
Technical Abstract: Ceratapion basicorne (Coleoptera: Apionidae) is a univoltine weevil native to Eurasia whose larvae develop in root-crowns of Centaurea solstitialis (yellow starthistle, Asteraceae). This insect was "rejected" as a prospective biological control agent about 15 years ago after preliminary evaluation of its host plant specificity showed that it could develop on safflower (Clement et al., 1989). However, the insect is known to attack very few plant species in the field and has never been reported from safflower. We conducted a series of no-choice, choice and field experiments to measure the risk that this insect would pose to nontarget plants. Larval development occurred on nine species, including Carthamus tinctorius (safflower) and Centaurea cyanus (bachelor's button, cornflower). All these host plants are within a small monophyletic clade within the Centaureinae. Three years of field studies conducted in eastern Turkey, at three sites with natural populations of the insect, demonstrated that the weevil does not damage safflower plants despite attack rates of 48-98% on Ce. solstitialis. A combination of taxonomic analyses and a hierarchy of host specificity experiments were required to determine that this insect would be safe to introduce.