Genetic characterization of introduced European frogbit and water soldier (Hydrocharis morsus-ranae and Stratiotes aloides) to support management and biological control development

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Genetic characterization of introduced European frogbit and water soldier (Hydrocharis morsus-ranae and Stratiotes aloides) to support management and biological control development: In the US, aquatic invasive plants cost millions of dollars annually in management and mitigation, requiring development of cost-effective strategies for preventing and dealing with their impacts. One species of particular concern is European frogbit (Hydrocharis morsus-ranae) due to the negative effects of infestations on recipient ecosystems. Similarly, water soldier (Stratoites aloides) is an emerging invader in North America with similar predicted impacts as H. morsus-ranae). Herbicide and mechanical management programs may be viable in some cases; however, they can be detrimental to native flora, and cost prohibitive at scale. Biological control represents a potentially cost-effective tool for long-term sustainable management.Hydrocharis morsus-ranae is an invasive nuisance species that forms large free-floating mats. Infestations impede water traffic, reduce water flow, displace native flora, contribute to hypoxic water conditions, and degrade recreational activities like swimming and fishing (Catling et al. 2003). Native to Europe and Northwest Asia, H. morsus-ranae escaped from a botanic garden in Ottawa, Canada in the 1930s, and has since spread throughout the Great Lakes states, as well as Vermont and Washington (Zhu et al. 2017; EDDMaps 2023). Current biological control efforts targeting H. morsus-ranae have been limited to generalist herbivores, including native snails and grass carp, but have only achieved limited success (Zhu et al. 2017). Classical biological control remains a potential tool, and several potential natural enemies have been identified from the native range (Weyl and Hinz 2022).Stratiotes aloides is an emerging invasive weed in North America. The current distribution of S. aloides is limited to the Trent River in Ontario, Canada where it was detected in 2008 (USDA-APHIS 2017x). Impact of S. aloides are similar to those of H. morsus-ranae, with the added detriment of sharp serrated leaves that can cut those who come in contact with it (OFAH/OMNR 2012). Though little is known about the potential for the biological control of S. aloides, it often co-occurs with H. morsus-ranae within its native range, and due to the close phylogenetic relationship between the two species (Family: Hydrocharitaceae), it has been suggested that biological control surveys should include both (Weyl and Hinz 2022).The USDA-APHIS has conducted a Weed Risk Assessment for both species and found a high likelihood that they will become serious invaders, with predicted distributions that include the entire eastern half of the contiguous United States (USDA-APHIS 2017a; 2017b). It is crucial in the early stages of biological control development to determine 1) genetic diversity of introduced populations, and 2) where introduced populations originated (i.e. from which regions were invasive populations introduced).As biological control agents are developed, there are a series of important steps needed to maximize the likelihood that chosen agents will establish and be damaging to target infestations. First, source areas must be identified, through molecular matching, in order to direct exploration for agents. This is needed for both H. morsus-ranae and S. aloides. This step is especially important when plants have large native ranges because agents may be adapted to particular genotypes that can occur regionally. Concurrent with genetic assessments is the determination of genetic structure in the introduced range. Once source populations are identified, exploration for natural enemies can be focused on regions and genotypes that have the greatest potential for success. This process requires detailed surveys of plant populations for damaging insect, pathogen agents, and phenology multiple times per year. Once agents are identified, they are prioritized and imported into quarantine where their biology is studied and evaluated for host-specificity.