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Title: Impact of herbivory on performance of Vincetoxicum spp., invasive weeds in North America

item MAGUIRE, D - University Of Toronto
item SFORZA, RENE - European Biological Control Laboratory (EBCL)
item SMITH, S - University Of Toronto

Submitted to: Biological Invasions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/2/2010
Publication Date: 2/18/2011
Citation: Maguire, D., Sforza, R., Smith, S.M. 2011. Impact of herbivory on performance of Vincetoxicum spp., invasive weeds in North America. Biological Invasions. DOI 10.1007/s10530-011-9955-4.

Interpretive Summary: Introduced into North America as ornamentals in late the 18th century, swallow worts, common names for Vincetoxicum rossicum and V. nigrum belonging to the Apocynaceae family, pose a significant threat to native biodiversity by invading thousands hectares in natural ecosystems, including national parks, reserves and urban areas, in more than 20 U.S. and Canadian. They can produce large, dense stands that reduce floral and faunal diversity and possibly redirect succession or interfere with forest regeneration.The aim of our study was to study the impact of a potential biocontrol agent, a chrysomelid beetle on various densities of plants of swallow wort. The herbivory impact was recorded daily and data were analyzed for different beetle densities. Introducing biocontrol agents in North America is the main purpose of our research, but prior to proceed to release many experimental studies have to be conducted to verify specificity and impact of the biocontrol candidate over the target weed.

Technical Abstract: The alien invasive vines Vincetoxicum rossicum and Vincetoxicum nigrum (swallow-wort) are of major concern in eastern North America, where both species invade forested landscapes and threaten faunal and plant diversity. Among the few native natural enemies reported in Eurasia, the specialist chrysomelid, Chrysochus (Eumolpus) asclepiadeus (Coleoptera; Chrysomelidae), feeds on Vincetoxicum both above ground (as adults) and below ground (as larvae). The goal of our study was to assess the potential for using this beetle to manage invasive Vincetoxicum spp. in North America by quantifying the impact of herbivory by C. asclepiadeus on Vincetoxicum and determining whether this effect was influenced by plant density (“Allee effect”). Experimental work was carried out using a split plot design in the field in southern France. Pots of V. nigrum and V. hirundinaria, a substitute for V. rossicum, were planted at high (255 plants/m2), medium (127 plants/m2), and low (32 plants/m2) plant densities, and received treatments of 0 (control), 2 or 4 C. asclepiadeus adult beetles/pot. Leaf damage, root and shoot biomass, and quantity of seeds were measured after four weeks of adult feeding. Densities of 2 and 4 beetles/pot caused similar damage, with significant reductions in plant biomass at low plant density. While V. hirundinaria increased allocation of resources to roots in response to herbivory, V. nigrum did not. Seed production was greatest for both species grown at low plant densities, but only V. nigrum produced fewer seeds in response to herbivory. Our results, based on the effects of herbivory by C. asclepiadeus adults, suggest that if this beetle were to be introduced into North America for the management of Vincetoxicum spp. such as V. rossicum, reductions in plant biomass and spread would be greatest if beetles were released on edges or in newly-established satellite populations at low plant densities. In the case of V. nigrum, beetles could be released irrespective of plant density as reproductive output and seed dispersal would be reduced similarly.