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

ABSTRACT.-

In North America host-specific flea beetles (Aphthona spp.; Coleoptera: Chrysomelidae) from Europe have been introduced to suppress invasive populations of the exotic forb, leafy spurge, Euphorbia esula L. (Euphorbiaceae). Long term outcomes of such introduction were examined in 2013 for a spurge infested site (managed as elk winter range) in the mountain foothills of northern Utah where three species of flea beetles had been released two decades earlier, in the 1990s. The abundance of leafy spurge at the site had declined by 2013 to only 4% of its abundance in 1995. The three species of flea beetles (dominated by A. lacertosa Rosenhauer) persisted in low numbers at the site in 2013 [peaking at 7-8 adults (all species combined) per 100 stems and inflicting aboveground feeding damage to all spurge stems by late summer]; flea beetle abundance had declined by 89-97% from high numbers as sampled in 2001. Another biocontrol insect, the stem borer Oberea erythrocephala Schrank (Coleoptera: Cerambycidae), had immigrated on its own to the site (as first detected in 2008) and, by mid-July in 2013, had damaged all spurge stems (typically causing total loss of the inflorescence on flowering stems). With the collapse of the spurge population at the site, the biomass of grasses [dominated by the exotic Bromus inermis Leyss and Arrhenatherum elatius (L.)] and forbs increased by 269% and 507%, respectively, from 1995 to 2013. These increases in grasses and forbs at the site with insect suppression of leafy spurge have resulted in more favorable habitat for elk and other wildlife.

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). …

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