One explanation for why invasive species are successful is that they escape natural enemies from their native range or experience lower attack from natural enemies in the introduced range relative to native species (i.e., the enemy-release hypothesis). However, little is known about how invasive plants interact with co-introduced herbivores or natural enemies of the introduced herbivores. We focus on Phragmites australis, a wetland grass native to Europe (EU) and North America (NA). Within the past 100–150 years, invasive European genotypes of P. australis and several species of specialist Lipara gall flies have spread within NA. On both continents we surveyed P. australis patches for Lipara infestation (proportion of stems infested) and Lipara mortality from natural enemies. Our objectives were to assess evidence for enemy-release in the invaded (NA) versus native (EU) range and whether Lipara infestation or mortality differed between invasive and native P. australis genotypes in NA. Enemy-release varied regionally; Lipara were absent throughout most of NA, supporting enemy-release of Phragmites. However, where Lipara were present, the proportion of invasive P. australis stems infested with Lipara was higher in the introduced (11 %) than native range (<1 %). This difference may be explained by the absence of Lipara parasitoids in our NA survey, strongly supporting enemy-release of Lipara. In NA, native P. australis genotypes exhibited higher Lipara infestation (32 %) than invasive genotypes (11 %), largely driven by L. rufitarsis. We attribute genotypic differences in infestation to a combination of Lipara exhibiting 34 % greater performance (gall diameter) and suffering four times less vertebrate predation on native than invasive genotypes. Our study suggests that complex interactions can result from the cointroduction of plants and their herbivores, and that a multitrophic perspective is required for investigating how biotic interactions influence invasion success.
Keywords: Biotic resistance, Chloropidae, Enemy release, Invasive plants, Invasional meltdown, Lipara spp., natural enemies Ele