Abundance and impact of egg parasitoids on the pine processionary moth (Thaumetopoea pityocampa) in Bulgaria

We collected 2297 egg batches of the pine processionary moth (Thaumetopoea pityocampa) during the period 1991-2018 from 44 sites in Bulgaria. The sampling sites were classified into three groups according to T. pityocampa phenological form (early, late and both forms) as well as in two groups of its range (historical and newly colonized areas). Seven primary egg parasitoids were identified: Ooencyrtus pityocampae, Baryscapus servadeii, Pediobius bruchicida, Anastatus bifasciatus, Eupelmus (Macroneura) vesicularis, Eupelmus (Macroneura) vladimiri and Trichogramma sp., and one hyperparasitoid, Baryscapus transversalis. The average impact of egg parasitoids (the percentage of parasitized host eggs) on T. pityocampa in Bulgaria was 13.8%. The two main parasitoids, O. pityocampae and B. servadeii, parasitized about 90% of the host eggs. The remaining parasitoids were of insignificant consequence to the parasitism of the T. pityocampa eggs, but in areas recently colonized by the pest, A. bifasciatus and Trichogramma sp. had a noticeable share (up to 33% of the impact). In old habitats of the host (areas colonized more than 10 years), the impact was almost two times higher than in new ones (15.3% vs. 8.6%). This could be attributed to B. servadeii, which was rare in newly colonized areas of T. pityocampa (impact 0.5%), but strongly dominant in old habitats (impact 7.2%). In contrast, O. pityocampae had a significant impact in new habitats (4.9%), which increased only slightly over time, reaching 6.0% in old habitats. There was no significant difference between the percentage of parasitism of the early and late form of the pine processionary moth (14.8% vs. 15.9%). However, there was a significant difference in the share of separate species in the parasitoid complex: in the early form, B. servadeii definitely dominated (63% of the infested eggs), while in the late form O. pityocampae dominated, although not so strongly (52% of the infested eggs). This difference is most likely due to the phenological characteristics of the parasitoids and the two forms of T. pityocampa. B. transversalis secondarily infested about 5% of the eggs of O. pityocampae and B. servadeii. This percentage was slightly lower for new habitats and habitats of the early form of pine processionary moth (3% and 4%, respectively). The impacts of the main parasitoids O. pityocampae and B. servadeii as well as the total impact of the parasitoid complex as a whole decreased with altitude. Conversely, the impacts of A. bifasciatus and Trichogramma sp. slightly increased with altitude probably due to the reduced competition of the main parasitoids.

Automated summary

The average impact of egg parasitoids on the pine processionary moth in Bulgaria was 13.8%, with two main parasitoids (O. pityocampae and B. servadeii) parasitizing about 90% of host eggs. The impact was almost two times higher in old habitats compared to new ones, with O. pityocampae and B. servadeii having significant impacts in new habitats and decreasing impact with altitude.