Individual study: Biological control of the invasive leafy spurge Euphorbia esula with the black dot spurge flea beetle Aphthona nigriscutis at sites with good soil and treated with herbicide, Pine Butte Swamp Reserve, 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, including 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 good soil and herbicide use is investigated.
Study sites: Two sites were selected with high densities of leafy spurge at the east end of the Pine Butte Swamp Reserve, Montana, and at which there was good soil (high levels of extractable phosphorous and potassium) and herbicide treatment (broad-leaf herbicide picloram and 2,4D in 1988-1991). At each site two permanent circular plots (Site 1, 530 m³; Site 2, 1960 m³) were created. Plots were chosen that resembled each other in vegetation composition, soil characteristics, slope and aspect. One plot within 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 (Site 1) or 25 m (Site 2) transects fanned outwards at 90° angles from each other.
Beetle release: In July 1994, 1,500 adult black dot spurge flea beetles Aphthona nigriscutis (beetle origin was not clarified) were released in the centre of 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 quadrats and of leafy spurge vegetative stems was sampled in twelve 0.05 m² quadrats. Quadrats were spaced equidistantly along the transects. Also, in each quadrat the height of 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 did not change significantly between 1994 and 1999, whereas in the presence of biocontrol herbivory, the mean biomass of leaf spurge decreased by 33% and total stem density by 17% compared to the control plots (see Table 1, numbers estimated from original figure). Furthermore, the number of flowering stems did not change significantly wihout biocontrol, compared to a decrease of 13% with biocontrol.
Species diversity: In the absence of biocontol herbivory, mean macroplot species richness increased by 33% between 1994 and 1999, comprising a 32% increase in native and 36% increase in exotic species. The presence of biocontrol herbivory reversed this trend, however, with a 14% decline in species richness compared with controls. This comprised a 15% decline in native species and no significant difference in exotic species richness.
Conclusions: The introduction of black dot spurge flea beetle reduced the biomass and growth predictors (vegetation and reproductive stems) of leafy spurge. However, biocontrol did not have the desired effect of increasing native species richness. The authors suggest that the biocontrol was successful in its primary objective of reducing leafy spruge densities, but that further work and a longer study is necessary to ascertain the longer term effects on the community (See also Case 411 for the effectiveness of black dot spurge flea beetle at sites with poor soil and without herbicide use).
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