Study

Unexpected ecological effects of distributing the exotic weevil, Larinus planus (F.), for the biological control of Canada thistle

  • Published source details 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.

Summary

Study 1

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).

 

Study 2

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).

Note: If using or referring to this published study please read and quote the original paper. This is available from

http://www.blackwellpublishing.com/journal.asp?ref=0888-8892 Please do not quote as a www.conservationevidence.com case as this is for previously unpublished work only.

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