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Monoecious and dioecious forms of the submerged aquatic plant hydrilla, Hydrilla verticillata (Hydrocharitaceae) are invasive weeds in the United States. Monoecious hydrilla is expanding northward and the existing biological control agents are not controlling it effectively. Results of fine-scale genetic identification of plants and herbivores matched the invasive US monoecious hydrilla genotype with a leaf-mining fly, Hydrellia sp. in their native range in the Republic of Korea. This fly was imported into quarantine in Australia to conduct preliminary evaluations of its biology, life-history and host range; a pre-screening before considering its importation into US quarantine for complete assessment as a potential biological control agent. The fly completed its development on two non-target test plant species, Elodea canadensis and Potamogeton crispus, the former being native to North America. It was recommended that no further consideration be given to Hydrellia sp. as a biological control agent of monoecious hydrilla. Other Hydrellia species from the Republic of Korea and China found on monoecious hydrilla should instead be evaluated. [ABSTRACT FROM AUTHOR]

• We conducted surveys across China and South Korea for natural enemies of hydrilla. • DNA barcoding identified 14 chironomid midge species. • Four distinct Hydrellia (Ephydridae) clades were identified. • Our results help clarify the species status of the collected material. • They also suggest hypotheses for further testing through additional structured sampling and screening. Biological control is considered the best management option against the aquatic weed Hydrilla verticillata , one of the most damaging invasive plants in the United States. The search for natural enemies is ongoing. Larval flies from two families (Ephydridae and Chironomidae) were collected feeding on H. verticillata during recent surveys across China and Korea (the possible origin of both the monoecious and dioecious forms of this weed found in the United States). DNA barcoding was used to provide initial identifications, using the Barcode of Life database. In addition, we assessed the genetic structuring and geographic distribution of each provisionally identified species. Four distinct Hydrellia (Ephydridae) clades were identified. The species identity of these clades could not be confirmed, although two form part of the Hydrellia pakistanae species group. Significant genetic structuring was observed within the most widespread clade , suggesting it may comprise several species, although further investigation is required. Fourteen distinct chironomid clades were identified, nine of which correspond to previously described species. Genetic diversity within the chironomid species was low, even across a broad spatial scale, though some genetic structuring was observed in Cricotopus sylvestris. The results of this study provide direction for additional sampling of H. verticillata across China and Korea. Specifically, we recommend further surveys in Zhouzhuang (China), Yeonpori, Yangsancheon, and Deokeuwonri (all Korea). These surveys should be complemented with structured population genetics screening of both plant and insect material; this will help resolve the species status and host associations of the discrete mitochondrial lineages identified in this study. [ABSTRACT FROM AUTHOR]