Hybrid watermilfoil lineages are more invasive and less sensitive to a commonly used herbicide than their exotic parent (Eurasian watermilfoil)

Laboratory measurements underestimate persistence of the aquatic herbicide fluridone in lakes

Fluridone is an aquatic herbicide commonly used to treat invasive freshwater plant species such as Eurasian watermilfoil, hydrilla, and curly-leaf pondweed. However, required exposures times are very long and often exceed 100 days. Thus, understanding the mechanisms that determine the fate of fluridone in lakes is critical for supporting effective herbicide treatments and minimizing impacts to non-target species. We use a combination of laboratory and field studies to quantify fluridone photodegradation, as well as sorption and microbial degradation in water and sediment microcosms. Laboratory irradiation studies demonstrate that fluridone is susceptible to direct photodegradation with negligible indirect photodegradation, with predicted half-lives in sunlight ranging from 2.3 days (1 cm path length) to 118 days (integrated over 1 meter). Biodegradation is attributable to microbes in sediment with an observed half-life of 57 days. Lastly, fluridone sorbs to sediments (Koc = 340 ± 28 L kg-1); sorption accounts for 16% of fluridone loss in the microcosm experiments. While the laboratory results indicate that all three loss pathways can influence fluridone fate, these controlled studies oversimplify herbicide behavior due to their inability to replicate field conditions. Fluridone concentration measurements in a lake following commercial application demonstrate a half-life of >150 days, indicating that the herbicide is very persistent in water. This study illustrates why caution should be used when relying on laboratory studies to predict the fate of pesticides and other polar organic compounds in the environment.

Quantifying the Role of Simultaneous Transformation Pathways in the Fate of the Novel Aquatic Herbicide Florpyrauxifen-Benzyl

Predicting the fate of organic compounds in the environment is challenging due to the inability of laboratory studies to replicate field conditions. We used the intentionally applied aquatic herbicide florpyrauxifen-benzyl (FPB) as a model compound to investigate the contribution of multiple transformation pathways to organic compound fate in lakes. FPB persisted in five Wisconsin lakes for 5-7 days with an in-lake half-life of <2 days. FPB formed four transformation products, with the bioactive product florpyrauxifen persisting up to 30 days post-treatment. Parallel laboratory experiments showed that FPB degrades to florpyrauxifen via base-promoted hydrolysis. Hydroxy-FPB and hydroxy-florpyrauxifen were identified as biodegradation products, while dechloro-FPB was identified as a photoproduct. Material balance calculations using both laboratory rates and field product concentrations demonstrated that hydrolysis (∼47% of loss), biodegradation (∼20%), sorption (∼13%), and photodegradation (∼4%) occurred on similar timescales. Furthermore, the combined results demonstrated that abiotic and plant-catalyzed hydrolysis of FPB to florpyrauxifen, followed by biodegradation of florpyrauxifen to hydroxy-florpyrauxifen, was the dominant transformation pathway in lakes. This study demonstrates how combined field and laboratory studies can be used to elucidate the role of simultaneous and interacting pathways in the fate of organic compounds in aquatic environments.

Synthesizing Laboratory and Field Experiments to Quantify Dominant Transformation Mechanisms of 2,4-Dichlorophenoxyacetic Acid (2,4-D) in Aquatic Environments

Laboratory studies used to assess the environmental fate of organic chemicals such as pesticides fail to replicate environmental conditions, resulting in large errors in predicted transformation rates. We combine laboratory and field data to identify the dominant loss processes of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) in lakes for the first time. Microbial and photochemical degradation are individually assessed using laboratory-based microcosms and irradiation studies, respectively. Field campaigns are conducted in six lakes to quantify 2,4-D loss following large-scale herbicide treatments. Irradiation studies show that 2,4-D undergoes direct photodegradation, but modeling efforts demonstrated that this process is negligible under environmental conditions. Microcosms constructed using field inocula show that sediment microbial communities are responsible for degradation of 2,4-D in lakes. Attempts to quantify transformation products are unsuccessful in both laboratory and field studies, suggesting that their persistence is not a major concern. The synthesis of laboratory and field experiments is used to demonstrate best practices in designing laboratory persistence studies and in using those results to mechanistically predict contaminant fate in complex aquatic environments.

2,4-D and 2,4-D butoxyethyl ester behavior in Eurasian and hybrid watermilfoil (Myriophyllum spp.)

BACKGROUND

Hybrid watermilfoil is becoming more prevalent in many lakes where the invasive Eurasian (Myriophyllum spicatum, EWM) and native northern watermilfoil (M. sibiricum) co-occur. These Eurasian and northern watermilfoil hybrids (HWM) grow 30% faster and in many cases are less sensitive to 2,4-dichlorophenoxy acetic acid (2,4-D) than either parent. The mechanism(s) impacting 2,4-D tolerance in these hybrids was investigated by comparing the absorption, translocation, metabolism, and desorption of two 2,4-D formulations in EWM and HWM.

RESULTS

2,4-D absorption in EWM and HWM was 5.7 and 7.9 times the external herbicide concentration determined by the plant concentration factor, a metric used to determine herbicide bioaccumulation, and 2,4-D butoxyethyl ester absorption was 35.6 and 52.1 times the external concentration in EWM and HWM, respectively. Herbicide bioaccumulation was greater in HWM than in EWM. Herbicide translocation to HWM roots was limited at 192 HAT and herbicide desorption in HWM was slightly lower than EWM. No differences were found in herbicide metabolism between the two plant species.

CONCLUSION

2,4-D resistance in HWM is not due to non-target-site resistance as no differences in herbicide absorption, translocation, desorption and/or metabolism were identified; therefore, target-site resistance is the most likely resistance mechanism. More research is needed to identify the molecular basis for the 2,4-D-resistant trait in HWM. © 2021 Society of Chemical Industry.

Hybrid watermilfoil (Myriophyllum spicatum × Myriophyllum sibiricum) exhibits traits associated with greater invasiveness than its introduced and native parental taxa

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.

Evaluation of Juvenile Freshwater Mussel Sensitivity to Multiple Forms of Florpyrauxifen-Benzyl

ProcellaCOR^® (active ingredient [ai], florpyrauxifen-benzyl) is an aquatic herbicide registered for use in 2018 for managing invasive and nuisance macrophyte species. Registration studies evaluating its acute toxicity revealed a favorable environmental profile; however, prior to this study, no information existed on the toxicity of florpyrauxifen-benzyl to native freshwater mussels (Family Unionidae), one of the most sensitive and imperiled faunal groups globally. We followed standard acute (96 h) toxicity test guidelines and exposed juvenile Fatmucket (Lampsilis siliquoidea) and Eastern Lampmussel (Lampsilis radiata) to the following formulations or compounds: ProcellaCOR SC and EC formulations, technical grade active ingredient (TGAI, florpyrauxifen-benzyl), and an analytical-grade sample of the weaker florpyrauxifen acid (FA). In all tests, the estimated median lethal concentrations to produce 50% mortality (LC50) were greater than the highest concentration tested of each formulation or compound. The no observable adverse effect concentrations (NOAEC, based on analytical recoveries measured at the highest concentration tested where no toxicity was observed) were TGAI = 26 µg/L, FA = 100,000 µg/L, ProcellaCOR^® SC = 193 µg ai/L ProcellaCOR^® EC = 585 µg ai/L and the NOAEC values for the registered commercial formulation products (ProcellaCOR^® SC and ProcellaCOR^® EC) were orders of magnitude greater (3.9× and 11.7×, respectively) than the maximum application rate (50 µg/L). Our results show that the herbicide formulations and compounds tested were not acutely toxic to juveniles of these two species of freshwater mussels, indicating minimal risk of short-term exposure from florpyrauxifen-benzyl applications in the environment for aquatic weed control. However, potential chronic or sublethal effects remain uncharacterized and warrant additional investigation.

Factors Influencing the Distribution of Invasive Hybrid (Myriophyllum Spicatum x M. Sibiricum) Watermilfoil and Parental Taxa in Minnesota

Eurasian watermilfoil (Myriophyllum spicatum L.) hybridizes with the native northern watermilfoil (M. sibiricum Kom.), which raises new issues regarding management strategies to control infestations. To determine the distribution of hybrid (and coincidentally Eurasian and northern) watermilfoil in Minnesota, we sampled lakes across the state during 2017–2018 for watermilfoil. A total of 62 lakes were sampled, spanning a range of sizes and duration of invasion. Forty-three lakes contained Eurasian, 28 contained hybrid and 21 contained northern watermilfoil. Eurasian watermilfoil populations were widespread throughout the state. Hybrid populations were more commonly found in lakes in the seven county Twin Cities Metro and northern watermilfoil populations were more commonly found in lakes outside of the Metro area. We found no evidence that hybrid watermilfoil occurred in lakes environmentally different than those with Eurasian and northern watermilfoil, suggesting that hybrid watermilfoil is not associated with a unique niche. Hybrid watermilfoil presence was significantly associated with the Metro area, which may likely be due to spatial and temporal factors associated with hybrid formation and spread. Hybrid watermilfoil presence was also significantly associated with lakes that had more parking spaces and older infestations, but this relationship was not significant when the effect of region was considered. Hybrid watermilfoil populations were the result of both in situ hybridization and clonal spread and continued assessment is needed to determine if particularly invasive or herbicide-resistant genotypes develop.

Effects of Invasive Watermilfoil on Primary Production in Littoral Zones of North-Temperate Lakes

Species invasions are changing aquatic ecosystems worldwide. Submerged aquatic macrophytes control lake ecosystem processes through their direct and indirect interactions with other primary producers, but how these interactions may be altered by macrophyte species invasions in temperate lakes is poorly understood. We addressed whether invasive watermilfoil (IWM) altered standing crops and gross primary production (GPP) of other littoral primary producers (macrophytes, phytoplankton, attached algae, and periphyton) in littoral zones of six Michigan lakes through a paired-plot comparison study of sites with IWM (standardized abundance 7–56%) compared to those with little or no IWM (standardized abundance 0–2%). We found that primary producer standing crops and the GPP of epiphytes, phytoplankton, and benthic periphyton were variable among lakes and not significantly different between paired study plots. Macrophyte standing crops predicted rates of benthic periphyton GPP, and standing crops of all other primary producers across all study plots. Overall, our results suggest that the effects of IWM on other primary producers in littoral zones may be lake-specific, and are likely dependent on the density of IWM, or whether it is functionally similar to other native species that it replaces or co-exists with. Moreover, in lakes where IWM is established but does not dominate macrophyte assemblages, the effects on littoral zone productivity may be minimal. Instead, overall macrophyte biomass is the primary factor controlling the rates of production and biomass of the other littoral zone primary producers, as has long been understood and observed in lake ecosystems.

Competitive Interactions of Flowering Rush (Butomus umbellatus L.) Cytotypes in Submersed and Emergent Experimental Aquatic Plant Communities

The ability to invade communities in a variety of habitats (e.g., along a depth gradient) may facilitate establishment and spread of invasive plants, but how multiple lineages of a species perform under varying conditions is understudied. A series of greenhouse common garden experiments were conducted in which six diploid and four triploid populations of the aquatic invasive plant Butomus umbellatus L. (Butomaceae) were grown in submersed or emergent conditions, in monoculture or in a multispecies community, to compare establishment and productivity of cytotypes under competition. Diploid biomass overall was 12 times higher than triploids in the submersed experiment and three times higher in the emergent experiment. Diploid shoot:root ratio was double that of triploid plants in submersed conditions overall, and double in emergent conditions in monoculture. Relative interaction intensities (RII) indicated that triploid plants were sixteen times more negatively impacted by competition under submersed conditions but diploid plants were twice as impacted under emergent conditions. Recipient communities were similarly negatively impacted by B. umbellatus cytotypes. This study supports the idea that diploid and triploid B. umbellatus plants are equally capable of invading emergent communities, but that diploid plants may be better adapted for invading in submersed habitats. However, consistently lower shoot:root ratios in both monoculture and in communities suggests that triploid plants may be better-adapted competitors in the long term due to increased resource allocation to roots. This represents the first examination into the role of cytotype and habitat on competitive interactions of B. umbellatus.

Is the cure worse than the disease? Comparing the ecological effects of an invasive aquatic plant and the herbicide treatments used to control it

Invasive species are known to have negative ecological effects. However, few studies have evaluated the impacts of invasive species relative to the effects of invasive species control, thereby limiting our ability to make informed decisions considering the benefits and drawbacks of a given management approach. To address this gap, we compared the ecological effects of the invasive aquatic plant Eurasian watermilfoil ( Myriophyllum spicatum L.) with the effects of lake-wide herbicide treatments used for M. spicatum control using aquatic plant data collected from 173 lakes in Wisconsin, USA. First, a pre–post analysis of aquatic plant communities found significant declines in native plant species in response to lake-wide herbicide treatment. Second, multi-level modeling using a large data set revealed a negative association between lake-wide herbicide treatments and native aquatic plants, but no significant negative effect of invasive M. spicatum. Taken together, our results indicate that lake-wide herbicide treatments aimed at controlling M. spicatum had larger effects on native aquatic plants than did the target of control—invasive M. spicatum. Our comparison reveals an important management tradeoff and encourages careful consideration of how we balance the real and perceived impacts of invasive species and the methods used for their control.

Endothall behavior in Myriophyllum spicatum and Hydrilla verticillata

BACKGROUND

Endothall has been used to control submersed aquatic plants since 1960, providing broad-spectrum control of aquatic weeds. Although endothall is considered a contact herbicide, many field observations suggest that it might have systemic activity. The goals of this research were to determine endothall's (i) absorption characteristics, (ii) translocation from shoots to roots, and (iii) potential for desorption in Eurasian watermilfoil (EWM), monoecious and dioecious hydrilla.

RESULTS

Endothall absorption was linear in dioecious hydrilla up to 192 HAT, while in EWM and monoecious hydrilla absorption data best fit an asymptotic rise function. Endothall absorption in EWM, monoecious and dioecious hydrilla was 3.3, 6.6, and 11.0 times the external herbicide concentration determined by the plant concentration factor. Translocation to EWM roots reached 7.9% of total absorbed radioactivity by 192 HAT, while translocation to monoecious and dioecious hydrilla roots reached 17.8% and 16.4% by 192 HAT, respectively. For all three species, no more than 30% of absorbed endothall moved from the plant to clean water 96 HAT.

CONCLUSION

Endothall is a very water soluble compound and yet it accumulated in these three important aquatic weeds at concentrations significantly higher than the external herbicide concentration. These data provide evidence that endothall could have systemic activity in these aquatic species. Following ¹⁴ C-endothall applications, more ¹⁴ C translocated from shoots to roots compared to the translocation of ¹⁴ C for other systemic aquatic herbicides. The final confirmation of endothall's systemic behavior requires that the radioactivity found in the root system of these aquatic plants is ¹⁴ C endothall. © 2019 Society of Chemical Industry.

Effects of chemical management for invasive plants on the performance of Lithobates pipiens tadpoles

Invasive plants impact amphibians by altering habitat, altering species interactions, and releasing potentially toxic secondary chemicals. Despite being costly and having the potential to affect nontarget wildlife, chemical management is commonly used to control invasive plants. Prior research has indicated that individual effects of invasive plants or herbicides can be harmful to aquatic organisms; however, information is lacking on the combined effect of these factors on amphibians. A laboratory experiment was performed to assess the impact of leachates of the invasive plants Eurasian watermilfoil (Myriophyllum spicatum) and European buckthorn (Rhamnus cathartica), the herbicide Renovate^® 3 (triclopyr [3, 5, 6-trichloro-2-pyridinyloxyacetic acid]), and the combined effects of each plant leachate and the herbicide on the growth, morphology, and survival of northern leopard frog (Lithobates pipiens) tadpoles. No effects of treatment on survival were observed. Tadpole exposure to M. spicatum reduced body mass by 17%, exposure to R. cathartica increased body mass by 36%, and exposure to R. cathartica + low herbicide increased body mass by 38% (although only early in the experiment). Exposure to Renovate 3 induced a 16% and 29% decrease in tadpole size in lower (0.22 mg triclopyr active ingredient [a.i.]/L) and higher (0.92 mg triclopyr a.i./L) concentration treatments, respectively. Results from the present study highlight the importance of considering both individual and combined effects of invasive plants and herbicides because they may have different outcomes for tadpole growth and development. Environ Toxicol Chem 2017;36:2958-2964. © 2017 SETAC.

A KASP Genotyping Method to Identify Northern Watermilfoil, Eurasian Watermilfoil, and Their Interspecific Hybrids

The invasive aquatic plant Eurasian watermilfoil (Myriophyllum spicatum L.) can hybridize with the related North American native species northern watermilfoil (M. sibiricum Kom.). Hybrid watermilfoil (M. spicatum × M. sibiricum) populations have higher fitness and reduced sensitivity to some commonly used aquatic herbicides, making management more difficult. There is growing concern that management practices using herbicides in lakes with mixed populations of watermilfoil species may further select for hybrid individuals due to the difference in herbicide sensitivity. Accurate and cost-effective identification of rare hybrid individuals within populations is therefore critical for herbicide management decisions. Here we describe KASP assays for three SNPs in the ITS region to genotype individuals from both parental watermilfoil species and their hybrid, using synthesized plasmids containing the respective sequences as positive controls. Using KASP we genotyped 16 individuals from one lake and 23 individuals from a second lake, giving a highly accurate picture of Myriophyllum species distribution dynamics. We identified one hybrid individual among 16 samples from one lake, a discovery rate of <10%. Discriminant analysis showed that while a single SNP was generally sufficient for genotyping an individual, using multiple SNPs increased the reliability of genotyping. In the future, the ability to genotype many samples will provide the ability to identify the presence of rare individuals, such as a less common parental species or the inter-specific hybrid. Lakes with complex species distribution dynamics, such as a low proportion of hybrids, are where herbicide application must be carefully chosen so as not to select for the more vigorous and less herbicide-sensitive hybrid individuals.

Influence of niche similarity on hybridization between Myriophyllum sibiricum and M. spicatum

The impact of ecological factors on natural hybridization is of widespread interest. Here, we asked whether climate niche influences hybridization between the two closely related plant species Myriophyllum sibiricum and M. spicatum. Eight microsatellite loci and two chloroplast fragments were used to investigate the occurrence of hybridization between these two species in two co-occurring regions: north-east China (NEC) and the Qinghai-Tibetan Plateau (QTP). The climate niches of the species were quantified by principal component analysis with bioclimatic data, and niche comparisons were performed between the two species in each region. Reciprocal hybridization was observed, and M. sibiricum was favoured as the maternal species. Furthermore, hybrids were rare in NEC but common in the QTP. Accordingly, in NEC, the two species were climatically distinct, and hybrids only occurred in the narrow geographical or ecological transition zone, whereas in the QTP, obvious niche overlaps were found for the two species, and hybrids occurred in multiple contact zones. This association between hybridization pattern and climate niche similarity suggests that the level of hybridization was promoted by niche overlap. Compared with the parental species, similar climate niches were found for the hybrid populations in the QTP, indicating that other environmental factors rather than climate were important for hybrid persistence. Our findings highlight the significance of climate niche with respect to hybridization patterns in plants.