Protection from natural enemies in managing rare plant species

Published source details Bevill R.L., Louda S.M. & Stanforth L.M (1999) Protection from natural enemies in managing rare plant species. Conservation Biology, 13, 1323-1331.
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Summary

In order to try and enhance population growth of Pitcher's thistle Cirsium pitcheri a US federally listed, threatened, species and thus improve the probability of long-term persistence, a field study was undertaken to evaluate the potential benefits of excluding insect herbivores from juvenile leaf rosettes, and to assess the value of such insect exclusion as a conservation management tool.

Two techniques of insect exclusion were investigated: caging plants plus insecticide, and insecticide application alone.

Study site: The study was conducted at two sites within Sleeping Bear Dunes National Lakeshore in northwest Michigan, USA.

Insect exclusion: Two experiments were undertaken to evaluate the influence of insect herbivores on the performance of juvenile Pitcher's thistle plants:

i) Cage plus insecticide: Conducted over two growing seasons (n = 60 plants). Insects were excluded using screen cages and insecticide. Individual large rosettes (>6 leaves) with no apparent damage were randomly assigned to two treatments (30 per site). Larger rosettes were selected as insect damage to juvenile Pitcher's thistle has been shown to be concentrated on larger individuals. Mesh cages (40 × 50 cm, 30 cm tall, 1.5 mm mesh) were placed over 15 individual plants, and similar 'partial' cages (south-facing side left open; providing similar shade and wind restriction while allowing insects access) were placed over 15 plants as controls. The experiment was set up at Peterson Beach in September 1994 and Aral Dunes in May 1995. The cages were removed on 8 June 1996. Final measurements were taken on 30 July 1996.

Caged plants were sprayed every 7–10 days (20 May 1995 to 5 August 1995) with a non-systemic contact insecticide in water (Isotox: 8% "Acephate", plus 2% "Hexakis") at the recommended rate of 6.7 mL/L water. Estimates of insect damage and plant measurements were taken on 20 May, 3 June, 1 July and 5 August 1995.

Measures of reproductive performance were recorded for plants that subsequently flowered in 1996, even though treatment had ended: total number of heads initiated (diameter ≥5 mm); diameter of the terminal flower head on each branch measured and converted to area of receptacle to represent the space available for seed production (i.e. a conservative estimate of insect influence on seed production); an estimate of the maximum number of viable seeds produced per head.

ii) Insecticide: Conducted over three growing seasons (n = 198 plants) in two plots in both swale and inland zones within each of the two study sites (n = 8 plots). Most plots contained 25–30 large juveniles, except for an inland zone with 14–18 plants/plot. Methods were as above. On 22 May 1994, those large juveniles with a single rosette and little evidence of insect feeding were identified. Adjacent plants were paired and randomly assigned to either insecticide (sprayed with Isotox in water during the 1994 and 1995 growing seasons) or control (sprayed with water) treatment. Insect damage and plant size were measured monthly May to August in 1994 and 1995.

Insect herbivory effects varied across the study areas. Excluding insects from habitat where thistle insect herbivory was high resulted in obvious direct benefits to these plants, including a 53% decrease in juvenile plant mortality (60% to 7%) and a 10-fold increase in seed production of juveniles that matured and flowered. In other parts of the study areas where herbivore-induced juvenile mortality was relatively low, excluding insects either increased seed production of plants that flowered or had no major effect.

Results from the study also suggest indirect benefits to the thistle population through potential improvement in dispersal among habitat patches.

Note: If using or referring to this published study, please read and quote the original paper, this can be viewed at:

http://www.blackwell-synergy.com/doi/full/10.1046/j.1523-1739.1999.98450.x