Establishment and early impact of the native biological control candidate Pyropteron chrysidiforme on the native weed Rumex obtusifolius in Europe

Rumex obtusifolius (Caryophyllales: Polygonaceae) is one of the most troublesome weeds in European grasslands and non-chemical control options are largely lacking. In this study, we assessed the potential of the native root-feeding specialist insect Pyropteron chrysidiforme (Lepidoptera: Sesiidae) for inundative biological control of R. obtusifolius. At multiple grassland sites, we applied P. chrysidiforme varying in developmental stage and level of protection onto R. obtusifolius to be examined in the subsequent autumn and spring. We consistently found the highest infestation resulting from the application of eggs (71 % of plants infested in autumn) with concomitantly elevated root decay and fewer rosettes compared to control plants. Moreover, in spring we found trends for associated decreases in biomass and number of shoots. Our findings support P. chrysidiforme as a promising biological control candidate of R. obtusifolius, but further investigations will be required to assess the consistency and long-term impacts of this novel approach.