Individual study: The effect of Canada thistle bud weevil Larinus planus introduced as a biocontrol agent of creeping thistle Cirsium arvense, on the native Tracy’s thistle C. undulatum at Gunnison National Forest, 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 to be 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 creepong thistle biocontrol agent thistle. However, some biocontrol agents can become pests themselves if they target native species, as well as the target species. In this study, the effect of Canada thistle bud weevil on the native Tracy's thistle Cirsium undulatum var. tracyi 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. During the first, 200 weevils were released in 1992, and during the second 300 were released in 1993.
Tracy's thistle density: Two stands of Tracy's thistle Cirsium undulatum var. tracyi were selected. The first stand was 0.3 km southwest of the release site, and the second site was 1.6 km to the northeast. For each stand, the density of flowering plants was recorded by measuring habitat area and counting the number of bolting plants.
Tracy's thistle performance: About 60% of the flowering thistles 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 weevil 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 Tracy’s thistle by Canada thistle bud weevil: Site 1 Tracy’s thistle was recorded at a density of 1.8 flowering plants and 3.1 ramets per 100 m². Ramet height was 75 (± 5) cm and there were 61 (± 14) flower heads per ramet, of which 16 (± 3) were terminal heads of branches. On average, 74% of terminal heads (n = 100) had unambiguous external evidence of insect damage. Dissection revealed that of these heads, 76% (n = 75) had been fed upon by Canada thistle bud weevil. Furthermore, 59% of axillary heads showed evidence of insect feeding and damaged heads, of which 60% were damaged by Canada thistle bud weevil. Finally, seed production was reduced significantly in weevil infected (1.4 ± 0.4 viable seeds) compared with uninfected (44.5 ± 3) thistles, and over half of this loss was directly attributable to the weevils.
Site 2: Results were similar to Site 1. Tracy’s thistle was at a density of 1.7 flowering plants and 2.5 ramets per 100 m². Ramet height was 72 (± 7) cm and there were 33 (± 7) flower heads initiated per ramet, of which 12 (± 2) were terminal heads of branches. On average, 78% of terminal heads had unambiguous external evidence of insect damage: total seed production of terminal heads was reduced 98% to only 1.3 viable seeds per head. Finally, total seed production was reduced by 59% by weevils.
Conclusions: In this study, Canada thistle bud weevil, introduced as a biocontrol agent of creeping thistle, severely and negatively impacted the reproduction of the native, non-target Tracy’s thistle. The authors suggest that because of this, all releases of Canada thistle bud weevil should be halted. (See Case 430 for the lack of effect of Canada thistle bud weevil as a biocontrol agent of creeping thistle).
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