Location: Invasive Species and Pollinator Health
2025 Annual Report
Objectives
The long-term objective of this project is to develop and improve integrated weed management (IWM) and restoration strategies that successfully reduce the abundance of invasive aquatic and wetland weeds, to aid in the protection of water resources and improve environmental quality in aquatic and wetland ecosystems in far western states. This holistic approach, applied through an IWM framework, will increase the efficacy of weed management and reduce weed abundance to restore invasion-resistant vegetation and ecosystem services. Specific objectives to be addressed follow.
Objective 1: Advance basic knowledge of weed biology and invasion ecology and develop improved integrated weed management (IWM) strategies in aquatic and wetland ecosystems.
Sub-objective 1A: Determine the correct taxonomy, systematics and extent of hybridization of invasive weeds.
Sub-objective 1B: Identify key biological and ecological processes influencing growth, invasiveness and IWM of aquatic and wetland weeds.
Sub-objective 1C: Evaluate new herbicides and improve herbicide application techniques to enhance management efficacy of aquatic weed species.
Objective 2: Evaluate the contributions of biological control on aquatic weed population dynamics through the lens of environmental variation, IWM, and ecosystem management.
Sub-objective 2A: Evaluate biological, demographic and ecological factors that affect insect biological control agents, herbivory and weed abundance to improve efficacy of biological control.
Sub-objective 2B: Evaluate impact of biological control of invasive wetland and riparian weeds in the context of integrated weed management.
Objective 3: Develop ecological restoration implementation and monitoring strategies within an IWM framework to overcome invasive plant impacts and achieve restoration of plant communities and ecosystem services.
Sub-objective 3A: Determine plant community and environmental characteristics that contribute to invasion resistance.
Approach
To support Objective 1, field sampling and molecular tools will be used to confirm genotypes of native and alien Phragmites australis and hybrids to elucidate genetic identity and diagnostic morphological traits of invasive taxa specific to the Delta-Suisun Marsh. In a 2-year field study at 3 Delta study sites, we will evaluate phenological development, biomass production and growth rates of South American spongeplant monthly to determine optimal timing of management. In a greenhouse experiment, we will also assess growth of 5 invasive and 3 native aquatic weed species in response to 6 water temperatures to develop predictive models to identify optimal timing for herbicide application. We will field measure plant traits and acclimation of alligator weed along a tidal range and salinity gradient. In a greenhouse, we will evaluate salinity tolerance of alligator weed using 2 growth forms (floating, emergent) X 4 salinity levels X 6 replicates arranged in a nested random block design. Experimental screening tests of new herbicide active ingredients will be conducted under controlled conditions using a hood-enclosed spray table and jar trials. Effective herbicides will then be tested in large replicated outdoor mesocosm experiments to assess weed survival and biomass responses. Dye studies will be performed at replicated Delta sites with low, medium and high water residence times to determine efficacy, optimal concentrations and exposure times of new herbicides to improve management of submersed aquatic plant species.
Under Objective 2, alligator weed biological control agents (A. hygrophila and A. andersoni) will be acquired from domestic and foreign sources. Experiments in controlled temperature incubators will elucidate critical minimum thermal limits and interspecific differences in cold tolerance to discover climatically-compatible biotypes for establishment, over-wintering, and efficacy for IWM in western watersheds. The effect of plant water availability on the establishment and impact of biological control for IWM of arundo will be studied. We hypothesize releases of arundo wasp and arundo armored scale will establish larger populations in release plots with integration of mechanical control than in plots with no pre-treatment. Pre-dawn water potential measurements of plant water status will be correlated with arundo wasp exit hole counts at 50 points across 3 sites. Colonization and impact of both insects on regrowth of arundo following herbicide application will be assessed.
Under Objective 3, we will design revegetation techniques using biotic resistance in an IWM framework to overcome invasive water primrose impacts in wetlands. Plant community composition, species abundance, and environmental variables will be assessed in large replicated field plots. Indicator species analysis, trait–environment filter models, and experiments will be used to identify strongly persistent native plant species resistant to competitive displacement by the invader under varying environmental conditions. Results will provide a foundation for IWM using improved restoration techniques to reduce invader impacts.
Progress Report
This is the final report for project 2030-22300-032-000D, “Integrated Weed Management and Restoration Strategies to Protect Water Resources and Aquatic and Wetland Ecosystems of the Far Western U.S.” which is currently under OSQR review. For additional information, see the new project report once it has completed the OSQR certification process.
In support of Sub-objective 1A, diagnostic traits were identified and applied to differentiate Phragmites across the occupied estuarine range. Phenological research and trait analyses were completed to refine observation-based identification tools to support targeted management of the highly invasive weed, tidal marsh restoration, and conservation of biological communities associated with the native Phragmites subspecies. Presentations were made to transfer results to relevant stakeholder groups, and a manuscript is in preparation.
For Sub-objective 1B, research was conducted to assess salinity tolerance and invasive spread of alligator weed (Alternanthera philoxeroides) invading tidal wetlands in California’s Sacramento-San Joaquin Delta and Suisun Marsh. Results documenting the weed’s sensitivity to salinity stress and stress escape strategies were presented at an international invasive species research conference, and a submitted manuscript is in review for publication. Data was also collected in the final year of a field study to assess environmental conditions relative to the establishment of the weed, and its rate and extent of spread. A manuscript is in preparation to report the results of the field study focusing on plant community dynamics and abiotic factors.
Also, in support of Sub-objective 1B, research efforts continued towards identifying key biological and ecological processes influencing the growth, invasiveness, and IWM of South American spongeplant (Limnobium laevigatum), a noxious weed that has been introduced to the United States and impairs waterways key for irrigation and navigation. Two years of data collection were performed at three study sites in the Sacramento-San Joaquin River Delta to determine the phenology and life history of this weed as a basis for improved management. Statistical analysis of plant traits, biomass allocation, and environmental data over time were performed. Progress is continuing on a manuscript for publication in a scientific journal by the new ARS researcher in Davis, California, who took over management of this research.
Due to a critical vacancy from December 2022 to February 2024, further progress was not able to be made in support of Sub-objective 1C. Not enough data was collected prior to February 2024 to warrant publication of initial herbicide efficacy experiments or tidal dye tests and bubble curtain testing. The new scientist assumed leadership of studies to evaluate new and efficacious herbicides and improved application methods to support IWM of aquatic weeds. Research performed in North Carolina was published concerning growth and herbicide efficacy on several aquatic taxa including Vallisneria australis, one of the focal species in this sub-objective. This research was also presented at two scientific conferences and two stakeholder meetings. Through collaboration with researchers in Vicksburg, Mississippi, the ARS scientist contributed to tidal dye research that was conducted on the Connecticut River to identify residence times and inform management solutions in a different tidal system with a new aquatic weed impairing navigation, hydropower and irrigation delivery. A manuscript on this research is in preparation.
Under Sub-objective 2A, research on expanding biological control of alligator weed into California initially included relocating the beetle from Louisiana to California, but these efforts failed as the insects were unable to overwinter. Scientists subsequently conducted experiments to compare thermal limits of multiple alligator weed flea beetle populations from the insect's native and exotic ranges. Additional research revealed that the host range of the Argentine biotype of the flea beetle is limited to the target weed, with larvae failing to complete development on all other plants tested. Collectively, these data indicate that importing new insects from Argentina is more likely to result in safe and successful control of the exotic weed in contrast to simply moving insects from the southern to the western United States. A petition was submitted to the federal regulatory agency, and the Technical Advisory Group recommended this new biotype be released in California. Additional research determined that the host range of a second species of flea beetle (Disonycha argentinensis) is too broad to be considered suitable for release in the United States.
For Sub-objective 2B, the shoot-tip galling wasp Tetramesa romana and the armored scale Rhizaspidiotus donacis were confirmed as established in the Central Valley of northern California. The wasp established populations at eight sites, and wasps dispersed up to 6.5 km. The wasp showed potential to contribute to integrated management, as exit hole density was 19-fold higher on cut and pruned arundo, and density was 12-fold higher on regrowth shoots that emerged one to two years after chemical control of arundo. The armored scale R. donacis established populations at nine sites. At five of the sites, 87 percent or more of the release plots had established scale populations and across all sites, 47 percent of the side shoots had at least one adult female armored scale, which produced offspring when isolated in the lab. The scale is immobile for most of its lifecycle and typically dispersed 10 m or less from release plots or about 1 m per year. The wasp and armored scale are not consistently reducing live biomass of arundo because wasp densities are relatively low, and the scale is still establishing and spreading. Both agents have the potential to exert impact on arundo in California in the future.
Also, in support of Objective 2, two species in the fungal genus Fusarium, which can act as both generalist decomposers and as plant pathogens, were identified as the primary fungal occupants of leaf sheaths of arundo collected at nine California field sites. Both were recovered from leaf sheaths infested with the arundo leafminer Lasioptera donacis after rearing a population of the leafminer from Europe in quarantine. These results suggest that the leafminer can acquire and feed on California fungi.
Under Objective 3, research was conducted in the Laguna de Santa Rosa wetlands invaded by Uruguayan primrose-willow (Ludwigia hexapetala) to evaluate plant species that may provide invasion resistance and resilience. Data collection was completed after delays due to deep persistent flooding. Vegetation plots were stratified by hydrological subarea and presence of L. hexapetala. Soil cores were analyzed for physico-chemical characteristics. Annual mapping of the distribution of L. hexapetala and five clonal aquatic plant species to assess interannual changes was completed. Analyses to identify species-environment associations and indicator species that persist within Ludwigia-invaded communities were completed to support experimental tests of biotic resistance potential. Results were presented to a large stakeholder group of invasive species management practitioners in the greater Russian River Watershed in January 2025, and a manuscript was initiated for publication.
Also, under Objective 3, ARS researchers in Davis, California, and scientists at the University of Seville, Spain, collaborated in a subordinate project (58-2030-0-038-F) focused on reducing the impact of invasive yellow flag iris (Iris pseudacorus). Results from analyses of environmental variables and traits of I. pseudacorus recorded at population sites along estuarine gradients in the native and invaded regions revealed that functional trait differences of alien iris populations in California have greater adaptive capacity to adjust to physiological stresses imposed by rising salinity and inundation than the native populations and refuted conventional wisdom that the species is limited to freshwater. Analyses of trait variation of I. pseudacorus populations from California and Andalusian estuaries, grown in common garden conditions at Universidad de Sevilla, were completed. In California, a two-year experiment with five California populations of I. pseudacorus evaluated trait variation influencing growth, sexual maturity, and biomass allocation of the plants. Results indicate all populations required two years of growth to reach sexual maturity and pre-reproductive plants should be targeted for management prior to significant investment in belowground carbohydrate storage organs that support resprouting. Field research identified native plant indicator species persisting within estuarine-invaded I. pseudacorus and Ludwigia hexapetala in floodplain wetlands. In this five-year project, the ARS and University of Seville collaborations resulted in eight publications in scientific journals, including two featured with images on science journal covers, and three additional manuscripts are in preparation. The collaborators presented results at six scientific meetings, and the ARS scientist made 10 presentations of the team’s results to stakeholder groups in California (7) and Oregon (3).
Accomplishments
1. Regulators recommend release of alligator weed biological control agent. Alligator weed is a relatively new invasive aquatic plant that threatens water availability and disrupts water distribution in California and other parts of the U.S. The alligator weed flea beetle (Agasicles hygrophila), a native of South America, successfully controls the weed in the southeastern U.S. but does not overwinter in California. ARS scientists in Albany, California, evaluated eight populations of the flea beetle and temperature development tests revealed one biotype of the flea beetle from southern Argentina was better adapted to California’s cooler winter temperatures than others evaluated. Feeding bioassays confirmed the new biotype, like the one already present in the southern U.S., will not attack native or agriculturally important plants. Predation tests indicate that threatened and endangered fish species will not use the flea beetle as a meaningful food source. These studies collectively indicate the new Argentine biotype is environmentally safe and a petition for release was submitted to the USDA’s Animal Plant Health Inspection Service (APHIS). Federal regulators are preparing the release permit and initial introductions are expected in the fall of 2025.
Review Publications
Beets, J.P., Haug, E., Sperry, B., Thum, R., Richardson, R. 2024. Response of four Vallisneria taxa to aquatic herbicides. Invasive Plant Science and Management. 17(4):297-304. https://doi.org/10.1017/inp.2024.33.
Gebhart, M., Schmid, S., Turner, S., Webb, D., Thum, R., Beets, J.P., Turnage, G. 2024. Invasive eelgrass hybrid (Vallisneria × pseudorosulata) in the southeastern United States. Invasive Plant Science and Management. 18. Article e2. https://doi.org/10.1017/inp.2024.37.
Moran, P.J. 2025. Regrowth of arundo (Arundo donax) after herbicide treatment supports a higher density of the shoot tip-galling wasp Roman Tetramesa released for biological control. Southwestern Entomologist. 50(1):104-116. https://doi.org/10.3958/059.050.0133.
Flaminia, M., Pratt, P.D., Massimo, C., Simona, C., Ilgoo, K., Rodrigo, D. 2024. Natural enemies of Lemna minuta in its native range and their potential as biological control agents for Europe. Biocontrol Science and Technology. 34(10):921-941. https://doi.org/10.1080/09583157.2024.2393383.
Sandenbergh, E., Gervazoni, P., Grewell, B.J., Franceschini, C., Minuti, G., McGrannachan, C., Stiers, I., Coetzee, J. 2024. Biology of Invasive Plants 7. Iris pseudacorus L. (Iridaceae). Invasive Plant Science and Management. 18(1-22). Article e6. https://doi.org/10.1017/inp.2024.35.
Reddy, A.M., Juarez, C.M., Moran, P.J., Grewell, B.J., Harms, N.E., Cibils-Stewart, X., Sosa, A., Cabrera Walsh, G., Faltlhauser, A., Pratt, P.D. 2024. Insights to the host range of the flea beetle Disonycha argentinensis Jacoby, 1992 (Coleoptera: Chrysomelidae), a candidate biological control agent of alligator weed in the U.S.A. The Pan-Pacific Entomologist. 100(2):136-147. https://doi.org/10.3956/2024-100.2.136.
Beets, J.P., Sperry, B.P., Thum, R.A., Richardson, R.J. 2025. Mesocosm evaluation of growth differences between Vallisneria taxa. Journal of Aquatic Plant Management. 63(2):41-44. Available: https://apms.org/journal-all-issues/.
Winston, R.L., Schwarzlander, M., Hinz, H.L., Pratt, P.D. 2024. Prioritizing weeds for biological control development in the western USA: Adaption of the biological control target selection system. BioControl. 69:335–351. https://doi.org/10.1007/s10526-024-10243-8.
Wheeler, G.S., Lake, E.C., Mattison, E., Pratt, P.D., Sutton, G. 2025. Host range, biology, and thermal tolerance of Lygomusotima stria, a potential biological control agent of Old World climbing fern (Lygodium microphyllum) in the USA. Biological Control. 204. https://doi.org/10.1016/j.biocontrol.2025.105741.
Moran, P.J., Rogers, D.V., Bitume, E.V., Portman, S.L., Goolsby, J.A. 2025. Establishment and dispersal of the armored scale Rhizaspidiotus donacis for biological control of Arundo donax in northern California. Biocontrol Science and Technology. 35(7):755-776. https://doi.org/10.1080/09583157.2025.2514451.