Aprostocetus bromi (Hymenoptera: Eulophidae) invaded parasitoid guilds of Stenodiplosis (Diptera: Cecidomyiidae) in the northern Great Plains

Aprostocetus nebraskensis (Girault) (Hymenoptera: Eulophidae) was described from Lincoln, Nebraska, and since reported mostly as a “Tetrastichus” species. It is a primary parasitoid of larvae of the gall midge Stenodiplosis wattsi Gagné (Diptera: Cecidomyiidae). Known species of Stenodiplosis are all predators of immature caryopses of grasses, and associated Aprostocetus larvae are ectoparasitoids on these gall midge larvae. Critical morphological and taxonomic evaluation demonstrated that Aprostocetus specimens from the introduced gall midges S. bromicola Marikovskij & Agafonova on Bromus inermis , and an undescribed Stenodiplosis on native Spartina pectinata are actually A. bromi (Kostjukov) and is new to North America. Parasitoid rearing from S. geniculati Reuter on introduced Alopecurus arundinaceous revealed an undescribed Aprostocetus, but A. bromi is not confirmed. Rearing revealed that each gall midge species has a unique guild of primary parasitoids and that A. bromi invaded examined guilds and apparently displaces native parasitoids. These guilds include undescribed and seemingly native species of other Aprostocetus that co-parasitize host gall midges in all generations. Aprostocetus bromi likely came to North America in diapause with S. bromicola, probably beginning from the early 1880’s when B. inermis seed was imported in massive quantities from Hungary and the Penza region of Russia. The plant was intended to provide a drought tolerant and cool-growing season forage for livestock, but has become a major weed in many areas of its North American occurrence. The relative impact of A. bromi on native co-parasitoid diversity, abundance and impact on gall midge populations requires further evaluation.