Academic journal article The American Midland Naturalist

Long Term Outcomes of Population Suppression of Leafy Spurge by Insects in the Mountain Foothills of Northern Utah

Academic journal article The American Midland Naturalist

Long Term Outcomes of Population Suppression of Leafy Spurge by Insects in the Mountain Foothills of Northern Utah

Article excerpt

INTRODUCTION

Invasive weeds often cause much ecological damage to natural areas as well as economic loss to agricultural areas. One such weed in North America is leafy spurge, Euphorbia esula L. (Euphorbiaceae), a perennial forb that has invaded many habitats since its introduction from Europe in the 1800s (Bourchier et ai, 2006). Leafy spurge is an aggressive competitor in North America due to its asexual reproduction via rhizomes, widespread seed dispersal, drought tolerance, extensive taproot system, and lack of native natural enemies (Messersmith et al., 1985). Efforts to control spurge have included herbicides, grazing, controlled fires, biological control using insect herbivores, and Integrated Pest Management programs that combine these methods (Lym, 2005). Using herbicides alone for control of large spurge populations is not cost effective, but biocontrol can reduce large populations with minimal effort (Larson et ai, 2007).

Several insect biological control agents targeting leafy spurge have been introduced from Europe to North America (Bourchier et ai, 2006). Of these flea beetles (Aphthona spp.; Coleóptera: Chrysomelidae) have been especially successful in reducing spurge populations (Larson and Grace, 2004; Lesica and Hanna, 2004; Butler et ai, 2006; Samuel et ai, 2008). The peak in flea beetle abundance and associated reduction of spurge populations may occur as early as 2-3 y after beetle release (Butler et al., 2006). Overwintered beetle larvae cause most damage to the weed by feeding on the roots, while adult beetles often cause only minor damage by feeding on aboveground tissue (Kalischuk et ai, 2004).

Flea beetle populations persist even at low spurge densities for up to 16 y after their release (Joshi and Olson, 2009). Although rare such documentation of long term biocontrol outcomes is necessary to assess the success of this approach. Measuring vegetation responses (of both native and exotic species) as spurge declines provides important information for habitat restoration (Samuel et ai, 2008; Butler and Wacker, 2010). These vegetation responses are often influenced by soil properties. In addition individual species of flea beetles vary in abundance at a given location in their native European range depending on soil properties (Nowierski et al., 1996). Therefore, it is useful to consider the role of soil both in the responses of plant communities and in the relative success of individual species of flea beetles released as biocontrol agents (Mundal and Carlson, 1999).

The present study was conducted as a long term evaluation of biocontrol of leafy spurge in northern Utah. The results presented here were obtained primarily in 2013 at a site where three flea beetle species had been released in the early 1990s in a dense stand of leafy spurge. The site is an open meadow of herbaceous vegetation in the foothills of the western slope of the Bear River Range near Richmond, Utah. The soil is silt clay loam (i.e., 60-70% silt, 25-40% clay, and 15-35% sand; USDA soil texture triangle [Schoeneberger et al., 2002]), with pH of 6.8 and 6.5% organic matter (top 10 cm; as determined in June 2013). The meadow lies within an area owned and managed as elk winter range by the Utah Division of Wildlife Resources. We (EWE and student research assistants) had marked plots at the site in 1995, in which we measured the abundance of flea beetles, spurge, and other vegetation. We sampled these plots again in 2001 and 2012. In spring and summer 2013, we sampled the biocontrol insects and vegetation intensively to address three objectives: (1) to document the degree to which leafy spurge abundance declined from 1995 to 2013, (2) to determine the abundance and damage to spurge by flea beetles still remaining at the site in 2013, and (3) to assess the degree to which other forbs and grasses (both native and exotic species) increased in abundance from 1995 to 2013 as biocontrol of spurge proceeded.

A fourth objective was added based on initial field sampling in 2013: (4) to measure the abundance and aboveground damage to leafy spurge by another biological control agent, the red-headed stem borer, Oberea erythrocephala Schrank (Coleóptera: Cerambycidae). …

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