Untangling host specialization in a “double dark taxa” system

Platygastrine wasps (Hymenoptera: Platygastridae) are parasitoids of gall midges (Diptera: Cecidomyiidae). Both groups are exceptionally abundant and speciose worldwide, with great relevance to agriculture and biodiversity research. Both groups are also “dark taxa”, whose species identification and ecological interactions 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, with many species requiring living host tissue to complete development. There is no universal rearing method for Cecidomyiidae and their parasitoids. We applied a “trial and error” approach to gall midge rearing, with the aim of obtaining accurate host associations and parasitoid identifications.

We reared five species of Platygastrinae from gall midges, three of which were associated with herbaceous plants in Germany. Integrative taxonomic methods allowed us to identify these parasitoid species and elucidate their biology. Platygaster demades Walker (= Platygaster marchali Kieffer, syn.nov. = Platygaster ornata Kieffer, syn.nov.) is a generalist, attacking Cecidomyiidae on Rosaceae worldwide, including Dasineura ulmaria (Bremi) on Filipendula ulmaria, and possibly also gall midges on other plant families. Synopeas gibberosum Buhl appears to specialize on Dasineura ulmaria. Synopeas rhanis (Walker) appears to specialize on Dasineura urticae (Perris) on Urtica dioica. We discuss challenges to understanding “double dark taxa” interactions, explore implications for agriculture and biodiversity research, and make recommendations for future studies on these important but neglected systems.