Unexpected Ecological Effects of Distributing the Exotic Weevil, Larinus planus (F.), for the Biological Control of Canada Thistle

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Abstract
Rhinocyllus conicus, a weevil introduced for biological control of exotic weeds, has had major nontarget ecological effects on native thistles. Some practitioners have argued that this is an isolated case. We report, however, that another Eurasian weevil ( Larinus planus), currently being distributed in North America for the control of Canada thistle (Cirsium arvense), is significantly reducing seed production by a native thistle ( Tracy's thistle, Cirsium undulatum var. tracyi) in Colorado. In 1999 we discovered L. planus feeding in flower heads of Tracy's thistle near a 1992–1993 biocontrol release site. Of the heads collected, 74% had evidence of L. planus, and these heads produced only 1.1 viable seeds on average, compared with 45.9 in heads without this weevil. In 2000 we investigated whether L. planus feeding persisted on Tracy's thistle and whether this feeding affected seed production significantly. Feeding by L. planus occurred on 80% of the plants and in 76% of all the main heads of Tracy's thistle. Flower heads with L. planus averaged 1.4 viable seeds, compared with 44.5 in uninfested heads. Feeding by L. planus decreased the average number of viable seeds produced per plant by over 51%. In contrast, L. planus had less effect on its targeted exotic host, Canada thistle than it did on Tracy's thistle. The high‐frequency and high‐level feeding of L. planus on the native species, coupled with the lack of evidence of its effectively limiting the seed production or density of Canada thistle suggest that the deliberate distribution of this weevil entails a high risk‐to‐benefit ratio and should be discontinued. Our findings challenge the general assumption that biological control with exotic insects is consistent with conservation goals of weed management in natural areas. Effective, a priori quantitative evaluation of the potential effects on both target and nontarget species and better regulatory oversight are required.