Individual study: The exotic Canada thistle bud weevil Larinus planus in not a successful biocontrol agent of creeping thistle Cirsium arvense in Gunnison National Forest, Almont, Colorado, USA
Louda S.M. & O’Brien C.W. (2002) Unexpected ecological effects of distributing the exotic weevil, Larinus planus (F.), for the biological control of Canada thistle. Conservation Biology, 15, 717-727
Creeping (or Canada) thistle Cirsium arvense is a Eurasian species that has spread across North American grasslands and the Rocky Mountains and is now considered as invasive. Throughout the 1990's, many states (inc., Colorado, Nebraska, South Dakota, Wyoming, Idaho and Oregon) released Canada thistle bud weevil Larinus planus as a biocontrol agent of this thistle. In this study, the effectiveness of Canada thistle bud weevil at controlling creeping thistle is investigated.
Study site: In Gunnison National Forest, Almont, Colorado, the U.S. Forestry Service made two releases of Canada thistle bud weevil Larinus planus with a view to controling the non-native and highly invasive creeping thistle Cirsium arvense. During the first, 200 weevils were released in 1992, and during the second 300 individuals were released in 1993.
Thistle density: Three stands of creeping thistle were selected. Two stands were 0.1 km from the weevil release site, and the third occurred along State Highway 50, 22 km to the southwest. For each stand, the density of flowering thistle plants was recorded by measuring habitat area and counting the number of bolting plants.
Thistle performance: About 60% of the flowering plants were chosen randomly for measurement of plant performance. For each, height, reproductive effort (number of flower head >4 mm in diameter per ramet and per plant), flowering success, and amount of floral herbivory by insects (total number of heads damaged by insect feeding and of heads with external evidence of feeding by thistle bud weevil per ramet and per plant) was recorded.
Additionally, to obtain an estimate of the effect of thistle bud weevils on reproduction, floral diameter, flower-head development (small bud to flowered and mature), number of florets initiated, number of viable seeds, damage score for insect feeding (0-6: (0) none; (1) slight, <1% area; (2) small, 1-5% area; (3) medium, 5-25%; (4) severe, >25% area; (5) stem mining within 1 cm of head; and (6) hole bored though phyllaries of the head), and number of insects present were recorded.
Use of creeping thistle by Canada thistle bud weevil: Site 1 Creeping thistle was recorded at a density of 119 ramets per 100 m². Ramet height was 72 (± 4) cm and there were 43 (± 5) flower heads initiated per ramet. Of the 215 flower heads dissected, 5% had evidence of insect damage. However, no flower had internal evidence of feeding or development of Canada thistle bud weevil.
Use of creeping thistle by Canada thistle bud weevil: Site 2: Ramet height was 37 (± 5) cm and there were 8 (± 1) flower heads initiated per ramet. Of the 23 flower heads dissected, no insect damage was found. Again therefore, no flower had internal evidence of feeding or development of Canada thistle bud weevil.
Use of creeping thistle by Canada thistle bud weevil: Site 3: Canada thistle was at a density of 45 ramets per 100 m². Ramet height was 53 (± 6) cm and there were 27 (± 7) flower heads initiated per ramet. Of the 137 flower heads dissected, 33% had evidence of insect damage. However, no flower had internal evidence of feeding or development of Canada thistle bud weevil.
Conclusions: At only 100 m from the release area and with Canada thistle bud weevil still present in the local area, these weevils did not negatively affect creeping thistles (also mirrored in a population at site 3 further from the weevil reslease location). This suggests that Canada thistle bud weevil is not a suitable biocontrol agent of this thistle. As a consequence the authors suggest that no further Canada thistle bud weevil releases should be made. (See Case 431 for the severe negative effect of Canada thistle bud weevil on the native Tracy’s thistle C.undulatum).
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