Foraging efficiency and outcomes of interactions of two pupal parasitoids attacking the invasive spotted wing drosophila

• Pachycrepoideus vindemiae and Trichopria drosophilae readily attack the invasive Drosophila suzukii in much of its invaded regions.
• T. drosophilae was more efficient than P. vindemiae in laboratory tests of D. suzukii parasitism.
• When interspecific competition occurred, the first parasitoid species to have oviposited outcompeted the later parasitoid species.
• Both parasitoid species discriminated against hosts previously parasitized by the other parasitoid species.
• Interspecific competition potentially reduced the overall impact by both parasitoids on host suppression.

Two generalist pupal drosophilid parasitoids, Pachycrepoideus vindemiae (Rondani) (Hymenoptera: Pteromalidae) and Trichopria drosophilae Perkins (Hymenoptera: Diapriidae), are sympatric and among only a few parasitoid species attacking the invasive Drosophila suzukii Matsumura (Diptera: Drosophilidae) in many regions of the world. In this study, we evaluated their foraging efficiency when attacking D. suzukii infesting cherry fruit in a laboratory cage experiment; and we examined their potential interspecific interactions, including outcomes of intrinsic competition, host discrimination, and the impact of their interaction on pest control. We show that both parasitoids readily parasitized D. suzukii pupae located inside fruit or buried in soil. However, T. drosophilae was more efficient than P. vindemiae and that parasitism by either parasitoid species was higher in the fruit than in the soil. Generally, the parasitoid species that oviposited first in the host out-competed the later parasitoid in multi-parasitized hosts, we assume, through physiological suppression. Both parasitoids discriminated against hosts parasitized previously by the other species. In an additive-series design experiment with single (D. suzukii) or two (D. suzukii and Drosophila melanogaster Meigen) host species, T. drosophilae performed similarly regardless of the presence of a competitor, while P. vindemiae achieved a higher parasitism when alone. The observed parasitism when the two parasitoid species were present together was always lower than the expected one, assuming each species acted independently. This indicates a negative effect by P. vindemiae on interspecific competition on host suppression.

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