First European record of the Nearctic hyperparasitoid Cheiloneurus swezeyi and impacts on Neodryinus typhlocybae populations

The introduction of the Nearctic parasitoid Neodryinus typhlocybae (Hymenoptera: Dryinidae) in Europe has been considered a successful example of biological control against the invasive pest Metcalfa pruinosa (Hemiptera: Flatidae). Recent reports of increasing M. pruinosa populations prompted research to assess biotic factors that may interfere with the activity of the introduced parasitoid. Among these, hyperparasitoids represent a critical yet unexplored component of this system. In a three-year study, 5,993 cocoons of N. typhlocybae were sampled from 28 semi-natural sites in northeastern Italy. Cocoon fate, hyperparasitoid assemblage, emergence patterns, and the influence of vegetation structure on hyperparasitism were examined. Hyperparasitoids were characterized by an integrative approach combining morphological characters and mitochondrial cytochrome c oxidase subunit I (COI) sequencing. Results showed that adult N. typhlocybae emerged from only about 15% of the cocoons, while 31% yielded hyperparasitoids. The Nearctic hyperparasitoid Cheiloneurus swezeyi (Hymenoptera: Encyrtidae) was detected for the first time in Europe. This species is known as the primary antagonist of N. typhlocybae in its native range. C. swezeyi and the native Pachyneuron muscarum (Hymenoptera: Pteromalidae) represented the two hyperparasitoids with the greatest impact on the emergence of the primary parasitoid. Multiple individuals of C. swezeyi frequently emerged from single cocoons, with an average exceeding five individuals per host. Hyperparasitism by C. swezeyi was high in the lower vegetation layer (<0.5 m above ground), a pattern consistent with the high proportion of wingless females and their limited dispersal ability. Molecular analyses revealed six different COI haplotypes of C. swezeyi. These findings indicate that C. swezeyi represents an emerging component of the M. pruinosa–N. typhlocybae system, potentially reducing the efficacy of the biological control program. Continued monitoring of hyperparasitoids is therefore essential to define the distribution of C. swezeyi across Europe and to assess the long-term success of biological control programs.