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Efficacy of Biological Control of Leafy Spurge in the Northern Great Plains
Leafy spurge (Euphorbia esula L.) is an exotic plant species introduced into North America in the late 1800s that is now well established throughout much of the western United States and Canadian Provinces. The success of this aggressive perennial is attributed to its ability to tolerate and exploit a variety of habitats and environmental conditions. Heavy infestations of leafy spurge threaten the structural and functional integrity of many natural communities. Classical biological control involves the use of imported natural enemies to suppress or maintain populations of the target pest species below an economically or ecologically relevant threshold. Leafy spurge is considered a good candidate for biological control because of its growth habit, low densities in its native habitat, and abundance of host specific herbivorous insects (natural enemies) associated with this species in its native range. The overall objectives of this biological control assessment effort were to document leafy spurge population dynamics in response to control by the black and brown flea beetles, determine the role of selected site characteristics on establishment and persistence of the beetles, and evaluate the response of resident plants to control of leafy spurge by the two species of flea beetles. This study evaluates the dynamics and trends of flea beetle populations, leafy spurge stands, and resident vegetation within the Montana and South Dakota study sites. Because of the relationships exhibited by the insects and their food source in their native range, it was hypothesized that the insects would reduce the dominance of leafy spurge, as measured from estimates of cover and density, and that the resident vegetation would respond to this reduction in leafy spurge dominance through increased foliar cover.
Successful establishment of black flea beetles occurred on a wide variety of grassland and shrubland sites typical for the region under a variety of land management strategies. Although population growth characteristics of black beetles were highly variable across the range of study plots, the observed patterns of reduced leafy spurge dominance were fairly consistent. Brown beetles were infrequent constituents on the vast majority of plots and appeared to be restricted to locations characterized by drier, coarser (sandy) soils. Reductions in leafy spurge cover appeared to stimulate seedling recruitment, which we observed to correspond with a noticeable drop in beetle abundance. Should insect populations decline below some unknown threshold because of low abundance of mature leafy spurge plants (high numbers of seedlings), long-term control may require re-releases of insects. In contrast, the observed dynamics in leafy spurge and flea beetles may be a short-term phenomenon that is part of a long-term population cycle that ultimately results in equilibrium between the biological control agent and its food source. While graminoid cover increased following reductions in the dominance of leafy spurge, forb cover in Montana on release and non-release plots remained well below levels observed on non-infested plots and changed very little throughout the study. Infestations of leafy spurge appear to have a strong filtering effect on composition and structure of impacted plant communities. The lack of change in forb cover coupled with the significant increases in graminoid cover would suggest that the loss of forbs may be, from a practical perspective, a long-term phenomenon without intensive management. Long-term monitoring is necessary to fully evaluate the population dynamics and inter-relationships between leafy spurge and flea beetles, and the recovery status of the resident vegetation.
More information can be found in: "Efficacy of Flea Beetle Control of Leafy Spurge in Montana and South Dakota." Rangeland Ecology and Management 59 (in press); and "Assessing biological control agents for area-wide control of leafy spurge with foci in Montana and South Dakota." Published abstract presented at the TEAM Leafy Spurge Symposium during the Annual Meeting of the Society of Range Management, Salt Lake City, UT. January, 2004; and Patterns of vegetation Recovery following biological control of leafy spurge." Published abstract presented at the 20th North American Prairie Conference, Kearney NE, July 24, 2006.
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