Untangling host specialization in a "double dark taxa" system.

Platygastrine wasps (Hymenoptera: Platygastridae) are parasitoids of gall midges (Diptera: Cecidomyiidae). They and their hosts are exceptionally abundant and speciose, with great relevance to agriculture and biodiversity research. Both groups are also "dark taxa," whose species identification and ecological associations are obscured by a history of taxonomic confusion and neglect. Verified host records are few in number and limited in scope. In order to understand host specialization, more records are needed. However, rearing Cecidomyiidae is challenging, as many species require living host tissue to complete development. There is no universal rearing method for Cecidomyiidae and their parasitoids. The present work applies an exploratory approach to rearing gall midges, with the aim of obtaining accurate host associations and parasitoid identifications. We obtained 5 species of Platygastrinae from reared material, 3 of which are identified and diagnosed. Platygaster demades Walker (= Platygaster marchali Kieffer, syn. nov. = Platygaster ornata Kieffer, syn. nov.) is not host-specific, attacking Cecidomyiidae on Rosaceae worldwide, including Filipendula ulmaria. Synopeas gibberosum Buhl apparently specializes on Dasineura ulmaria (Bremi) on F. ulmaria. Synopeas rhanis (Walker) is known only from galls of D. urticae (Perris), but may attack other midge species on Urtica dioica. Amblyaspis sp. emerged from Hartigiola annulipes (Hartig) galls on Fagus sylvatica, and Synopeas sp. was associated with Mycodiplosis sp. on Rubus sp. Illustrations, DNA barcodes, and distributions are provided. We discuss challenges to understanding "double dark taxa" interactions, implications for biological control, and possible solutions for future research on these important but neglected systems.