Effects of leaf and root herbivory by potential insect biological control agents on the performance of invasive Vincetoxicum spp.

The European leaf-feeding moth Abrostola asclepiadis and root-feeding beetle Eumolpus asclepiadeus are promising biological control agents for two European swallow-worts (Vincetoxicum rossicum and Vincetoxicum nigrum) in North America, however, their impact on plant performance is uncertain. Densities of each herbivore were manipulated in a common garden to determine whether leaf and root herbivory affect the performance of these plants. During the second year of the experiment, V. rossicum and V. nigrum unexpectedly became infected with the fungal pathogens Ascochyta sp. and Cercospora sp. (Ascomycota), respectively. Although pathogen infection mainly reduced shoot height and delayed reproduction, herbivore effects on plant growth were still evident. Leaf herbivory by A. asclepiadis had no effect on plant growth 1 year after defoliation. Root herbivory by E. asclepiadeus reduced shoot height and plant biomass and decreased the ability of plants to compensate for pathogen attack. Pathogen infection prevented detection of herbivore effect on reproduction. Due to its substantial impact on plant biomass, E. asclepiadeus should be further evaluated as a biological control agent against Vincetoxicum spp. populations invading open habitats in North America. Further research is needed to evaluate the impact of A. asclepiadis in combination with E. asclepiadeus and plant competition under high and low light conditions.

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► Leaf herbivory by Abrostola asclepiadis had no effect on plant growth.
► Root herbivory by Eumolpus asclepiadeus reduced shoot height and plant biomass.
► The fungal pathogens Ascochyta sp. and Cercospora sp. (Ascomycota) were isolated from Vincetoxicum rossicum and Vincetoxicum nigrum, respectively, but herbivore impact was still evident.
► Due to its substantial impact on plant biomass, E. asclepiadeus should be further evaluated as a biological control agent against Vincetoxicum spp.
► Further work is needed to evaluate in what context leaf herbivory by A. asclepiadis is detrimental to Vincetoxicum spp. performance.