Individual study: Biological control of the invasive leafy spurge Euphorbia esula with the black dot spurge flea beetle Aphthona nigriscutis at sites without herbicide use and with poor soil, Montana, USA
Lesica P. & Hanna D. (2004) Indirect effects of biological control on plant diversity vary across sites in Montana grasslands. Conservation Biology, 20, 460-469
The exotic plant species leafy spurge Euphorbia esula has invaded over 1 million ha of North America, primarily in grasslands, woodlands, and riparian areas. Introducing a second species as a biological control agent may be a way of controlling such invasives. In this study, the effects of an introduced root herbivore, the black dot spurge flea beetle Aphthona nigriscutis, on the target leafy spurge and other native plants at sites with poor soil and no application of herbicide is investigated.
Study sites: Two sites with high densities of leafy spurge were selected at the east end of the Pine Butte Swamp Reserve, Montana, and at which there was no herbicide use and poor soil (low levels of extractable phosphorous and potassium). At each site two permanent circular macroplots (530 m³) were created. Plots were chosen that resembled each other in vegetation composition, soil, slope and aspect. One plot at each site was randomly assigned as the treatment plot and the other was left as the control. From the centre of the plot, four 13 m transects fanned outwards at 90° angles from each other.
Beetle release: In July 1994, 1,500 adult black dot spurge flea beetles Aphthona nigriscutis were released in the centre of each of the two treatment plots. Consquently, the centre of the plots were exposed to beetles for 4-5 years, but because beetles had to move outwards from the centre of the plot to colonise the peripheries, outer areas were only exposed for 1-3 years.
Monitoring: In June 1994 (before beetle release) and July 1999 (after beetle release), the density of leafy spurge flowering stems was sampled in twelve 0.2 m² (Site 1) or 0.4 m² Site 2) quadrats and of leafy spurge vegetative stems was sampled in twelve 0.1 m² (both Sites) quadrats. Quadrats were spaced equidistantly along the transects. Also, in each quadrat one representative flowering stem and vegetative stem was measured (no details given of selection method).
Leafy spurge growth: In the absence of biocontrol herbivory, mean biomass of leafy spurge and the total stem density declined by 54% and there was a 24% increase in total stem density between 1994 and 1999, whereas in the presence of biocontrol herbivory, the mean biomass of leaf spurge decreased by 39% and total stem density by 44% compared to the control plots (see Table 1, numbers estimated from original figure). Furthermore, the number of flowering stems declined by 29% without biocontrol, and the decline with biocontrol was not different compared to the control plots.
Species diversity: In the absence of biocontol herbivory, mean macroplot species richness declined by 19% between 1994 and 1999, comprising a 18% decrease in native and 25% decrease in exotic species. The presence of biocontrol herbivory reversed this trend, however, with a 26% increase in species richness compared with controls. This consisted of a 19% increase in native species and 70% increase in exotic species richness compared to controls.
Conclusions: The introduction of black dot spurge flea beetle reduced the biomass and density of vegetative stems of leafy spurge. Furthermore, biocontrol increased native species richness, but also the species richness of exotic species. Overall, the authors suggest that biocontrol has been successful in its primary objective of reducing leafy spurge densities, but that a further study is required to ascertain the longer term effects of biocontrol on the community (See also Case 410 for the effectiveness of black dot spurge flea beetle at sites with good soil and herbicide use).
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