Rumex obtusifolius is a problematic weed in temperate grasslands worldwide as it decreases yield and nutritional value of forage. Because the species can recruit from the seed bank, we determined the effect of management and soil properties on the soil seed bank of R. obtusifolius in intensively managed, permanent grasslands in Switzerland (CH), Slovenia (SI), and United Kingdom (UK). Following a paired case-control design, soil cores were taken from the topsoil of grassland with a high density of R. obtusifolius plants (cases) and from nearby parcels with very low R. obtusifolius density (controls). Data on grassland management, soil nutrients, pH, soil texture, and density of R. obtusifolius plants were also collected. Seeds in the soil were germinated under optimal conditions in a glasshouse. The number of germinated seeds of R. obtusifolius in case parcels was 866 ±152 m-2 (CH, mean ±SE), 628 ±183 m-2 (SI), and 752 ±183 m-2 (UK), with no significant difference among countries. Densities in individual case parcels ranged from 0 up to approximately 3000 seeds m-2 (each country). Control parcels had significantly fewer seeds, with a mean of 51 ±18, 75 ±52, and 98 ±52 seeds m-2 in CH, SI, and UK, respectively, and a range between 0 and up to 1000 seeds m-2. Across countries, variables explaining variation in the soil seed bank of R. obtusifolius in case parcels were soil pH (negative relation), silt content (negative), land-use intensity (negative), and aboveground R. obtusifolius plant density (positive). Because a large soil seed bank can sustain grassland infestation with R. obtusifolius, management strategies to control the species should target the reduction in the density of mature plants, prevention of the species’ seed production and dispersal, as well as the regulation of the soil pH to a range optimal for forage production.
In weed biocontrol, the release of multiple candidates has been proposed as a way to increase herbivore load and thus impact on the target weed. However, the use of multiple herbivorous species may reduce overall herbivore load due to interspecific interactions. We assessed whether a combined application of two Pyropteron spp. (Lepidoptera: Sesiidae) with root-boring larvae can increase larval infestation rate and impact on two Rumex weeds (Caryophyllales: Polygonaceae). In a laboratory assay (‘behavioural experiment’), interference competition was assessed by comparing the survival of a single larva (no competition), of four conspecific larvae (intraspecific competition) and of two larvae of each species (interspecific competition). In a common garden study (‘infestation and impact experiment’), the outcomes of competitive interactions were assessed on potted plants with single and mixed species applications of the two Pyropteron species using a replacement design. In the behavioural experiment, larval survival was similarly reduced under intra- and interspecific competition compared to no competition, suggesting strong interference competition among larvae independent of the Pyropteron species. In the infestation and impact experiment, mixed application did only scarcely increase herbivore load. Increased herbivore load was found to increase root decay when applied in an inundative approach by placing eggs directly on Rumex plants. However, a joint application of two Pyropteron species did not increase the impact. We propose that detailed behavioural and impact studies be conducted to assess the likely outcome of using multiple candidates in biocontrol programs to minimize associated risks with multiple species introductions.
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