The arundo wasp, Tetramesa romana, does not control giant river reed, Arundo donax, in Texas, USA

Giant river reed, Arundo donax L. (Poaceae), native to the Mediterranean basin and east to the Indus River region of Afghanistan and Pakistan, has become widespread in temperate and subtropical regions worldwide. It is regarded as being deleterious for clogging waterways and for choking out natural riparian plant communities. In the USA efforts to control the pest have included the release of exotic herbivorous insects. One of them, the arundo wasp, Tetramesa romana Walker (Hymenoptera: Eurytomidae), has been reported to utilize primary A. donax stalks for oviposition, stunting and killing the plant. In our study, stands of A. donax in natural habitats of Kerr County (TX, USA) were sampled at monthly intervals for 2 years and small plots of A. donax at annual intervals for five consecutive years. Parasitism of stalks, regardless of plant age, was rare or non‐existent. Arundo donax in natural habitats and freshly planted A. donax grew in dense stands and proliferated regardless of being infested by T. romana. We determined that parasitized culms can serve as plant propagules by growing roots and stems, hence, the wasp does not impede the ability of A. donax to propagate and proliferate from broken off plant parts. The wasp utilizes A. donax culm stems growing from hardening ≥1st‐year stalks for development in the winter and early summer when the larvae pupate and emerge as adults. We found that appreciable T. romana parasitism of an A. donax plant only occurs for 8.3–16.7% of the plant's 2‐year lifespan. Adult exit holes remain on the culms during summer. Under field conditions T. romana is a parasite that is well‐adapted to A. donax by not killing stalks, reducing stand density, and hindering propagation. Hence, the exotic wasp is ineffective as a biological control agent against A. donax.