Competitive interactions affect larval survival of two root-boring weed biological control candidates of Rumex spp

Abstract In weed biocontrol, the release of multiple candidates has been proposed as a way to increase herbivore load and thus impact on target weed. However, use herbivorous species may reduce overall due interspecific interactions. We assessed whether combined application two Pyropteron spp. (Lepidoptera: Sesiidae) with root-boring larvae can larval infestation rate Rumex weeds (Caryophyllales: Polygonaceae). laboratory assay (‘behavioural experiment’), interference competition was by comparing survival single larva (no competition), four conspecific (intraspecific competition) each (interspecific competition). common garden study (‘infestation outcomes competitive interactions were potted plants mixed applications using replacement design. behavioural experiment, similarly reduced under intra- compared no competition, suggesting strong among independent species. did only scarcely load. Increased found root decay when applied in an inundative approach placing eggs directly plants. joint not impact. propose that detailed studies be conducted assess likely outcome biocontrol programs minimize associated risks introductions.