Impact of hyperparasitism and predation on Neodryinus typhlocybae populations at the original release site 35 years after the introduction of the biocontrol agent

The effectiveness of parasitoids used in classical biological control can be affected by various factors, including predation and hyperparasitism. Hyperparasitoids are fourth-trophic-level insects that develop by parasitizing the juvenile instars of primary parasitoids, potentially decreasing the biological control of phytophagous pests. Classical biological control has been successfully carried out in the Veneto Region to manage infestations of Metcalfa pruinosa, a Nearctic flatid accidentally introduced in 1979. This highly polyphagous species causes significant aesthetic and economic damage to crops and ornamental plants. In 1987, the Nearctic dryinid Neodryinus typhlocybae was introduced as a biological control agent of M. pruinosa and, since then, it has spread both naturally and through multiple releases across various European countries. Recently, reports on an increasing of M. pruinosa infestations in north-eastern Italy have prompted a two-year study on the population dynamics of N. typhlocybae, with a particular focus on mortality caused by predation and hyperparasitism. A total of 4857 N. typhlocybae cocoons were collected at 14 sites in north-eastern Italy during the summer and autumn of 2022 and 2023. In the laboratory, the cocoons were examined and classified into four categories: (i) undamaged, (ii) predated, (iii) with emergence hole of hyperparasitoids, and (iv) with emergence hole of N. typhlocybae. Undamaged cocoons were monitored until the end of the winter season, to verify the emergence of N. typhlocybae or any potential hyperparasitoids. Emerged hyperparasitoids were preserved for both morphological and genetic identification, which were performed using dichotomous keys and mitochondrial DNA (COI) sequencing. N. typhlocybae cocoons collected over the two seasons showed high cumulative mortality rates, consistently exceeding 70%. Mortality was primarily due to hyperparasitism, which was significantly higher than predation. Overall mortality did not differ significantly between the two years (p = 0.206), but the difference between mortality induced by predation and hyperparasitism was highly significant (p < 0.001). Furthermore, laboratory rearing of undamaged cocoons resulted in the emergence of 1346 hyperparasitoids and only 201 N. typhlocybae individuals, highlighting the severe impact of hyperparasitism. Given the strong negative effect of hyperparasitoids on N. typhlocybae populations, it is crucial to clarify the taxonomic identity and ecological role of the hyperparasitoid species involved. This two-year study, conducted in the initial introduction region of N. typhlocybae over 35 years after its release in Europe, provides valuable insights about the long-term effectiveness of classical biological control and the complex interactions between the introduced biocontrol agent and the local ecosystem.