Biocontrol by a native hemiparasite: Rapid suppression of noxious invasive plants in a field experiment

Abstract Alien plant invasions are a recognised threat to biodiversity. Although experimental evidence remains limited, their success may be reduced by parasitic plants, which have been proposed as promising biocontrol agents. We tested the effects of hemiparasitic Melampyrum arvense on Soligago gigantea and Symphyotrichum lanceolatum , clonal herbaceous species from North America, which are noxious invaders of European vegetation. In a multi‐site, 4‐year field experiment, we compared the effects of the hemiparasite presence combined with mowing and mowing alone to the unmown controls. The experimental treatments significantly affected invasive hosts' cover, height and below‐ground biomass. Mowing alone significantly reduced host growth; however, hemiparasites had a stronger effect, reducing median host cover from 90% and 78% to 8% and 12% after 3 years in Solidago and Symphyotrichum , respectively. Growth reduction was faster in Solidago than in Symphyotrichum under the same treatments. Hemiparasites also severely reduced the host's below‐ground biomass by more than 80% compared to the unmown treatment. Mowing increased community diversity and species turnover, with significantly stronger effects when combined with hemiparasites. However, the increase in native grassland species was limited, likely due to a restricted local species pool. Synthesis and applications . Melampyrum arvense was demonstrated as a biocontrol tool suitable for containing two harmful invasive species. After the suppression of the invaders, an additional introduction of native species by seeding or green‐hay transfer could facilitate community restoration.