Revisiting biological control of Hydrilla verticillata: historical perspective and new opportunities

Authors: HARMS, NATHAN - Environmental Laboratory, Us Army Engineer Research And Development Center, Waterways Experiment St; FOLEY, JEREMIAH - Connecticut Agricultural Experiment Station; BROOKES, DEAN - Commonwealth Scientific And Industrial Research Organisation (CSIRO); PURCELL, MATT - Commonwealth Scientific And Industrial Research Organisation (CSIRO); Wheeler, Gregory

Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/9/2026
Publication Date: 7/10/2026
Citation: Harms, N., Foley, J., Brookes, D., Purcell, M., Wheeler, G.S. 2026. Revisiting biological control of Hydrilla verticillata: historical perspective and new opportunities. Biological Control. 219.Article e106105. https://doi.org/10.1016/j.biocontrol.2026.106105.
DOI: https://doi.org/10.1016/j.biocontrol.2026.106105

Interpretive Summary: This article explains that an invasive water plant called Hydrilla verticillata is continuing to damage lakes, rivers, and other freshwater systems in the United States. A newer strain, called H. verticillata subspecies lithuanica, has recently spread into northeastern states, where conditions are cooler and many waterways are tidal. Older biological control programs were designed for different hydrilla strains found in the southern U.S., so they may not work well against this newer strain. The authors explain that the new subspecies produces large numbers of underground structures called turions, which help it spread and survive harsh conditions, making it especially difficult to control. The paper reviews four insect species that were previously approved to fight hydrilla: Hydrellia pakistanae, Hydrellia balciunasi, Bagous affinis, and Bagous hydrillae. Among these, Hydrellia pakistanae appears most promising for further testing because it may survive cooler northeastern climates. The authors also suggest searching for new insects and plant diseases from the hydrilla plant’s native region that may better match this new subspecies. Modern genetic tools could help scientists identify more effective biological control agents. Overall, the article argues that hydrilla management needs a fresh, updated strategy specifically designed for this newly introduced strain and the unique environmental conditions of the northeastern U.S.

Technical Abstract: The invasive aquatic plant Hydrilla verticillata continues to pose a significant threat to freshwater systems in the United States. Management efforts are complicated by the presence of three genetically distinct subspecies, of which H. verticillata ssp. lithuanica is the most recent introduction in the northeastern USA. Historical biological control programs, focused on subspecies in southern states, may not be effective against the new lineage or in the cooler, tidal river systems where it occurs. Here, we review the history of hydrilla biological control in the USA through the lens of the new introduction and discuss opportunities to address this challenge. We summarize the current distribution and distinct biological traits of hydrilla lineages, including the prolific turion production of H. v. ssp. lithuanica that may enhance its invasiveness. We re-evaluate the potential of four previously approved insect agents (Hydrellia pakistanae, H. balciunasi, Bagous affinis, and B. hydrillae) and discuss their suitability in novel environmental conditions of the Northeast and compatibility with H. verticillata ssp. lithuanica. The leaf-mining fly H. pakistanae is a promising candidate for immediate re-evaluation due to its availability and potential tolerance for cooler climates. Furthermore, there is a longer-term opportunity in development of microbial pathogens and discovery of new arthropod agents through targeted exploration in the native range of H. v. ssp. lithuanica, aided by modern molecular techniques. We conclude by outlining key research priorities to build an integrated and adaptive research strategy tailored to the unique biological and ecological context of the H. v. ssp. lithuanica invasion, emphasizing the need for a renewed, lineage-matched approach to the biological control of hydrilla.