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Mesocosm evaluation of three herbicides on Eurasian watermilfoil (Myriophyllum spicatum) and hybrid watermilfoil (Myriophyllum spicatum ×myriophyllum sibiricum): Developing a predictive assay

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... HWM is well documented to pose significant management challenges through the emergence of herbicide tolerance. Specifically, certain strains of HWM are less sensitive than EWM to commonly used herbicides, such as 2,4-D and fluridone (Berger et al. 2012;LaRue et al. 2013b;Netherland and Willey 2017). However, the putative ecological invasiveness of HWM in the field (e.g., the ability to grow fast and large, produce more propagules, and disperse more readily; Van Kleunen et al. 2010;Hovick and Whitney 2014;Moravcová et al. 2015) has largely stemmed from anecdotal observations of high abundance in lakes where the taxa occurs Les 2002, 2007). ...
... Our results demonstrate substantial differences among taxa in traits associated with invasiveness; however, traits of hybrid taxa can vary between generations and lineages (Whitney et al. 2006;Hovick and Whitney 2014). Phenotypic variation among HWM lineages has been documented (Berger et al. 2012;Netherland and Willey 2017;Taylor et al. 2017) and there was underlying genotypic variation in HWM and NWM-both within and among study lakes. While our study did not account for intraspecific variability, laboratory studies that have examined multiple HWM genotypes suggest that increases in HWM growth rate and biomass are general phenomena (LaRue et al. 2013b;Taylor et al. 2017). ...
... EWM is considered highly invasive and is a focus of management throughout the U.S. Our findings suggest that HWM may be more invasive in some respects. Furthermore, management of HWM is made more challenging with the hybridization-driven emergence of herbicide tolerance in this taxon (Berger et al. 2012;LaRue et al. 2013b;Parks et al. 2016;Netherland and Willey 2017;Nault et al. 2018). Our description of HWM phenology could help refine management tactics. ...
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Hybridization has been associated with increased invasiveness in plants. In North America, the hybrid aquatic plant Myriophyllum spicatum × Myriophyllum sibiricum (hybrid watermilfoil, hereafter HWM) is a cross between non-native invasive Eurasian watermilfoil ( M. spicatum , EWM) and native northern watermilfoil ( M. sibiricum , NWM). Lab-based trials have demonstrated higher growth rates in HWM compared to EWM and NWM, but these patterns have not been systematically examined in the field. In this study, we compared the invasiveness of HWM to its parental taxa, EWM and NWM, by examining the amount and timing of: (1) flowering, (2) surface cover, and (3) biomass (using stem counts as a proxy). We conducted repeat surveys of Myriophyllum beds at eight lakes (2–3 lakes/taxon) in the Minneapolis–St. Paul Metropolitan area (Minnesota, USA) between June 2017 and November 2018. HWM produced more flower spikes earlier and overall, and maintained consistently more flower spikes throughout the growing season than EWM and NWM. In addition, surface cover reached greater annual peaks and was higher for longer throughout the growing season for HWM than for both parental taxa. We did not observe a significant difference in stem counts among the three taxa, but HWM did reach a higher maximum number of stems than either parental taxon. This study provides field-based evidence of increased invasiveness associated with hybridization between EWM and NWM; specifically, greater reproductive potential via flowering and greater surface cover may increase HWM spread, have greater impacts on native species, and pose more of a nuisance to lake users.
... It appears that there are inherent differences between Myriophyllum genotypes and exposure to herbicides such as triclopyr, fluridone and 2,4-D are often strain-specific with varied responses Slade et al. 2007;Chorak and Thum 2020;Madsen et al. 2021;Hoff and Thum 2022). Furthermore, it appears that management strategies need to be specific to genotype, as it appears that different hybrid populations respond to management treatments independently (Glomski and Netherland 2010;Berger et al. 2012Berger et al. , 2015Netherland and Willey 2017). These results might suggest that there is a genetically-specific response mechanism taking place across these strains and more research is necessary. ...
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Eurasian watermilfoil (Myriophyllum spicatum) is one of the most challenging invasive aquatic plant species in Minnesota. Eurasian watermilfoil has hybridized with the native Northern watermilfoil (Myriophyllum sibiricum), creating a more complex issue when it comes to invasions. The hybridization of watermilfoil has increased difficulty for management and an underlying biological mechanism explaining this difficulty has yet to be determined. We compared the photosynthetic temperature response of the parental Northern and Eurasian parental genotypes to five hybrid watermilfoils using pulse amplitude modulated (PAM) chlorophyll fluorometry to determine if there are intrinsic physiological differences in photosynthesis between genotypes. We examined both the ratio of variable to maximal fluorescence (Fv/Fm) and light-adapted quantum yield of photosystem II (PSII) electron transfer (ΦPSII) every 5 °C from 0 to 45 °C representing sub- to supraoptimal water temperatures. On average, hybrids had 6% and 8% higher Fv/Fm and ΦPSII values respectively, than parental genotypes across all temperatures. In both parental and hybrid genotypes Fv/Fm values were consistent from 0 to 35 °C and declined 36% as water temperature approached 45 °C. Optimal water temperature for ΦPSII was between 25-35 °C and hybrids had 17% higher values for suboptimal temperatures between 5-15 °C. Our findings indicate that although the many strains of watermilfoil are morphologically similar, they differ in their photosynthetic capacity and may indicate heterosis. Innate differences between parental strains and hybrids could impact overall primary productivity, life history characteristics, and overall invasiveness. These factors could also be contributing to the documented management failures seen in field populations.
... However, a synthesis of the available scientific literature demonstrates that many hybrid genotypes are impacted similarly to pure Eurasian genotypes, and that only specific genotypes exhibit reduced susceptibility (Berger et al. 2012(Berger et al. , 2015Chorak and Thum 2020;Hoff and Thum 2022;Netherland and Willey 2017;Poovey et al. 2007;Slade et al. 2007;Thum et al. 2012). In other words, reduced herbicide response does not appear to be a general property of hybridity, per se. ...
... Cerveira et al. (2019) reported that diquat application decreased dry matter accumulation in Myriophyllum aquaticum and Brachiaria subquadripara macrophytes with increasing doses. Netherland and Willey (2017) reported that the application of diquat reduces biomass in Myriophyllum and exposure almost completely collapses the plant texture. The genotoxic effects of diquat dibromide were analyzed by MN and CAs frequency, MI ratio, and tail length DNA percentage by comet assay in the study. ...
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Diquat dibromide is a comprehensive herbicide commonly used in the cultivation of cotton, soybeans, and other crops to combat unwanted weeds. In this study, the half-maximal effective concentration (EC50) value of diquat dibromide was determined 60 mg/L in the Allium root growth inhibition test. ½ × EC50 (30 mg/L), EC50 (60 mg/L), and 2 × EC50 (120 mg/L) concentrations of diquat dibromide were applied to Allium cepa L. bulbs for 72 h to investigate the dose-dependent toxic effects. To determine the toxic effects cytogenetic, biochemical and physiological parameters were used. Physiological effects were investigated by determination of the percentage of rooting, relative injury rate, root length, and weight gain. Genetic effects were evaluated by the frequency of chromosomal abnormalities (CAs), micronucleus (MN) formation, mitotic index (MI) rate, and comet assay. Biochemical parameters were evaluated with antioxidant enzyme activities and lipid peroxidation by determining malondialdehyde (MDA) level, superoxide dismutase (SOD) activity, catalase (CAT) activity, and glutathione (GSH) level. Also, chlorophyll pigment contents (a, b, and total) in green leaves were calculated to elucidate the effect of diquat dibromide on plants and the biosphere. The findings show that increasing doses of diquat dibromide caused a decrease in all physiological parameters and MI ratio, promoting MN and CAs and tail DNA formation in genetic parameters. It was determined by the increases in MDA level, SOD, and CAT activities and decreases in GSH levels that diquat dibromide administration caused oxidative stress depending on the dose. Also, chlorophyll pigment levels (a, b, and total) measured in leaf tissues decreased with the application dose. Considering that the toxic effects caused by diquat dibromide and that organisms other than unwanted plants will be exposed during the application, its use should be abandoned and biocontrol methods should be used instead. In cases where use is compulsory, doses that will not harm the environment and organisms should be determined and used. Graphical abstract
... Eurasian watermilfoil (Myriophyllum spicatum L.) including its hybrid with native northern watermilfoil (Myriophyllum spicatum × Myriophyllum sibiricum Kom.) are heavily managed with herbicides in the United States (Bartodziej and Ludlow 1998). Experimental studies of Myriophyllum clearly indicate that distinct genotypes can differ in vegetative growth and herbicide response properties (Netherland and Willey 2017;Taylor et al. 2017;Thum et al. 2012). Although Myriophyllum is capable of sexual reproduction, clonal reproduction is common (Hartleb et al. 1993), and the same clones of Myriophyllum have been distinguished using multilocus microsatellite genotyping (MG) within and among regions (Taylor et al. 2017;Thum et al. 2020). ...
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Genetic assays to identify herbicide resistant plants are a promising tool to reduce herbicide failures. However, the genetic basis of herbicide resistance is frequently unknown. In clonal weed species, DNA fingerprinting could be a useful tool to identify known resistant versus susceptible genets (clones) that occur in multiple locations, without an immediate need for understanding the genetic mutation(s) conferring resistance. Eurasian watermilfoil ( Myriophyllum spicatum L.) is a mostly clonal invasive aquatic plant, and the same clones can be found in multiple waterbodies. Previously, a clone was confirmed as resistant to the commonly used herbicide, fluridone, and a recent genetic survey in Michigan identified this genotype (MG-237) in at least seven other lakes. We hypothesized MG-237 collected from different lakes would also exhibit fluridone resistance. However, MG-237 may have accumulated resistance mutations at different times during its spread across Michigan, resulting in fluridone resistant and susceptible MG-237 clones distributed in different lakes. We used an herbicide assay to test the response of several accessions, including MG-237 accessions from multiple lakes, to the Michigan operational rate of 6 µg L ⁻¹ fluridone. We found that all accessions of MG-237 exhibited resistance to 6 µg L ⁻¹ fluridone. A second genotype (MG-377) was also resistant to 6 µg L ⁻¹ fluridone. The rest of the accessions were found to be significantly injured by 6 µg L ⁻¹ fluridone. Our results suggest that 6 µg L ⁻¹ fluridone would not effectively control waterbodies dominated by MG-237 or MG-377, whereas waterbodies dominated by the other genotypes in our study would likely be controlled. Although more studies are needed to identify the variation in sensitivity of the accessions tested here, and the genetic basis of fluridone resistance in watermilfoil, our results suggest that multilocus genotype data may be an effective tool to identify and track herbicide-resistant genotypes of watermilfoil in the short-term.
... Reduced fluridone sensitivity has been observed in both field and laboratory studies with a HWM genotype from Townline Lake in central Michigan, how-510 ever the mechanism contributing to the increased tolerance is not yet well understood , Berger et al. 2015. A recent mesocosm study by Netherland and Willey (2017) demonstrated that HWM strains from Frog and English lakes 515 showed a greater tolerance to 2,4-D when compared to another strain of HWM and EWM. In the future, resource managers should consider conducting genetic pretreatment screening of target populations to better understand among-and within-population variation, 520 as well as changes in population genetic composition after chemical control (Moody et al. 2008). ...
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Nault ME, Barton M, Hauxwell J, Heath E, Hoyman T, Mikulyuk A, Netherland MD, Provost S, Skogerboe J, Van Egeren S. 2017. Evaluation of large-scale low-concentration 2,4-D treatments for Eurasian and hybrid watermilfoil control across multiple Wisconsin lakes. Lake Reserv Manage. 00:00–00. Herbicides have been utilized for decades for nonnative milfoil control; however, limited literature is available examining large-scale herbicide applications, especially for commonly used herbicides such as 2,4-D (2,4-dichlorophenoxy acetic acid). Twenty-three lakes were studied pretreatment and posttreatment to monitor large-scale and low-concentration (lakewide rate: 73–500 μg/L) 2,4-D dissipation and degradation patterns, and determine the efficacy and selectivity of these treatments for Eurasian watermilfoil (Myriophyllum spicatum; EWM) and hybrid watermilfoil (Myriophyllum spicatum × M. sibiricum; HWM) control. Measured mean surface concentrations averaged throughout the initial 2 weeks after treatment ranged from 119 to 544 μg/L. In addition, the threshold for irrigation of plants which are not labeled for direct treatment with 2,4-D (<100 μg/L by 21 d after treatment) was exceeded in 18 of the 28 treatments. Calculated 2,4-D half-lives ranged from 4 to 76 d, and herbicide degradation was generally observed to be slower in oligotrophic seepage lakes. Year of treatment reductions in milfoil frequency ranged from 4 to 100%, with sustained multi-year control observed in some lakes. While good year of treatment control was achieved in all lakes with pure EWM populations, significantly reduced control was observed in the majority of lakes with HWM populations. Several native monocotyledon and dicotyledon species also showed significant declines posttreatment, with variation in recovery observed over time. Although target species control was achieved with some of these treatments, variation in herbicide persistence, reduced control in many HWM populations, and nontarget impacts to certain native plants demonstrate the need for additional research and field studies.
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Crested floating heart [ Nymphoides cristata (Roxb.) Kuntze] is an invasive aquatic plant in the southeastern United States. For clonal plants like N. cristata , clonal diversity may influence response to control tactics and/or evolutionary potential. However, little is known about the diversity of introduced N. cristata . In this study, we used genotyping-by-sequencing to quantify N. cristata diversity in the southeastern U.S. and determine how that diversity is distributed across the invaded range. Our results show that at least three distinct genetic lineages of N. cristata are present in the southeastern U.S. Geographic distribution of the lineages varied, with one widespread lineage identified across several states and others only found in a single waterbody. There is also evidence of extensive asexual reproduction, with invaded waterbodies often host to a single genetic lineage. The genetic diversity reported in this study likely results from multiple introductions of N. cristata to the southeastern U.S. and should be considered by managers when assessing control tactics such as screening for biocontrol agents or herbicide testing. The extent and distribution of genetic diversity should also be considered by researchers studying the potential for invasive spread of N. cristata within the U.S. or hybridization with native Nymphoides species.
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Nymphoides peltata (yellow floating-heart), native to Eurasia, is an invasive plant in the USA, where it grows in relatively isolated but widespread populations. The species is capable of sexual reproduction by seed and asexual reproduction through fragmentation. Although N. peltata is recognized as a noxious weed, little is known about its geographic region of origin or its dispersal mechanisms and relative amount of genetic variation in its adventive range. We conducted a genetic analysis of N. peltata by studying 68 localities across the native range and 47 localities in the adventive range, using microsatellite markers to determine genetic variability within and among populations, and to infer regions in the native range from which invasive plants originated. A large number of sites in the USA were genetically identical to one another, and there were two predominant multilocus allele phenotypes that were distributed in the northern and southern latitudes, respectively. Additional USA sites were similar to one of the predominant genetic profiles, with greater genetic diversity in southern populations. The genetically identical sites are consistent with asexual spread, potentially via anthropogenic mechanisms. Plants across the USA range were observed to produce viable seeds, and some genetic variation could be explained by sexual reproduction. All USA plants were more similar to plants in Europe than they were to plants in Asia, indicating that the plants likely were introduced originally from Europe. The existence of two genetic clusters and their similarity to plants in different parts of Europe constitute evidence for at least two N. peltata introductions into the USA.
Article
Population genetic studies of within- and among-population genetic variability are still lacking for managed submerged aquatic plant species, and such studies could provide important information for managers. For example, the extent of within-population genetic variation may influence the potential for managed populations to locally adapt to environmental conditions and control tactics. Similarly, among-population variation may influence whether specific control tactics work equally effectively in different locations. In the case of invasive Eurasian watermilfoil ( Myriophyllum spicatum ), including interspecific hybrids with native northern watermilfoil ( M. sibiricum ), managers recognize that there is genetic variation for growth and herbicide response. However, it is unclear how much overall genetic variation there is, and how it is structured within and among populations. Here, we studied patterns of within- and among-lake genetic variation in 41 lakes in Michigan and 62 lakes in Minnesota using microsatellite markers. We found that within-lake genetic diversity was generally low, and among-lake genetic diversity was relatively high. However, some lakes were genetically diverse, and some genotypes were shared across multiple lakes. For genetically diverse lakes, managers should explicitly recognize the potential for genotypes to differ in control response, and should account for this in monitoring and efficacy evaluation, and for pre-treatment herbicide screens to predict efficacy. Similarly, managers should consider differences in genetic composition among lakes as a source of variation in the growth and herbicide response of lakes with similar control tactics. Finally, laboratory or field information on control efficacy from one lake may be applied to other lakes where genotypes are shared among lakes.
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