Lessons from a decade of lake management: effects of herbicides on Eurasian watermilfoil and native plant communities ELLEN RUTH KUJAWA ,1, PAUL FRATER,1 ALISON MIKULYUK,1,2 MARTHA BARTON,1 MICHELLE E. NAULT,1,3 SCOTT VAN EGEREN,1,4 AND JENNIFER HAUXWELL1,5 1
Bureau of Science Services, Wisconsin Department of Natural Resources, Madison, Wisconsin 53716 USA 2 Center for Limnology, University of Wisconsin-Madison, Madison, Wisconsin 53703 USA 3 Bureau of Water Quality, Wisconsin Department of Natural Resources, Green Bay, Wisconsin 54313 USA 4 Bureau of Water Quality, Wisconsin Department of Natural Resources, Madison, Wisconsin 53707 USA 5 University of Wisconsin Aquatic Sciences Center, Madison, Wisconsin 53706 USA
Citation: Kujawa, E. R., P. Frater, A. Mikulyuk, M. Barton, M. E. Nault, S. Van Egeren, and J. Hauxwell. 2017. Lessons from a decade of lake management: effects of herbicides on Eurasian watermilfoil and native plant communities. Ecosphere 8(4):e01718. 10.1002/ecs2.1718
Abstract. Eurasian watermilfoil (Myriophyllum spicatum) is a non-native and invasive aquatic macrophyte with a broad North American distribution. It can have significant negative effects on invaded waterbodies, including decreased native macrophyte diversity, formation of recreational nuisances, and lowered lakefront property values. Previous research suggests that M. spicatum decreases in response to herbicide treatment, but most studies are spatially and temporally limited, usually focusing on a single waterbody for a single year. The long-term effects of herbicides remain relatively unknown. Here, we share the results of an 11-yr observational study of aquatic macrophyte diversity, dynamics, and response to herbicide treatment on 28 Wisconsin lakes (15 of which were adaptively managed with herbicide for M. spicatum and 13 of which acted as unmanaged reference lakes). We found that overall, adaptive management decreases M. spicatum abundance over time, but that the efficacy of individual herbicide treatments can vary. We also found that lakes with relatively new M. spicatum populations (discovered within the last decade) treated smaller areas with lower frequency than lakes with established populations, and were able to maintain lower M. spicatum abundance. This suggests that using adaptive, science-based aquatic plant management strategies, including early detection and response, may increase invasive species management success. Finally, we show that the effect of herbicide treatment on native macrophytes is variable and can be significant. Overall, our results suggest that while herbicide treatment can be an effective adaptive management tool, particularly in lakes with relatively recent M. spicatum invasions, the specific effects of individual treatments can be unpredictable. This study allows lake stakeholders to better understand the efficacy of herbicide treatment, in addition to the possible non-target effects on native macrophyte species. Key words: adaptive management; aquatic macrophytes; early detection and response; invasion; long-term observational research. Received 2 August 2016; revised 23 January 2017; accepted 25 January 2017. Corresponding Editor: Debra P. C. Peters. Copyright: © 2017 Kujawa et al. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. E-mail:
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INTRODUCTION
(Pimentel et al. 2005), and many of these have caused extensive ecological and economic damage. While it is difficult to overstate the importance of prevention as a strategy to reduce the damage caused by invasive species, it is widely recognized that management programs should
Nonindigenous invasive species can alter both ecosystems and ecosystem services for society (Ehrenfeld 2010). At least 50,000 non-native species have been introduced to the United States ❖ www.esajournals.org
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temporally and spatially. Typically, these studies have focused on a single waterbody for a single year (Getsinger et al. 1982, Parsons et al. 2001, Wersal et al. 2010), or have been conducted under carefully controlled laboratory conditions (Netherland and Getsinger 1992, Poovey et al. 2007). This observational study is, to our knowledge, unprecedented in its spatial and temporal scale and provides important information on the longterm effects of management on M. spicatum and native macrophyte communities to researchers, managers, and the public. Our investigation of an operational management program over the long term represents a large investment of time and money, with the tradeoff of reduced control of experimental variables and study design. However, despite this reduced experimental control, studying the outcomes in managed lakes provides an excellent opportunity to investigate how management works in “the real world,” and this research represents a valuable and necessary addition to highly controlled mesocosm or laboratory studies. We compensate for the lack of experimental control available to us with a large sample size, extensive statistical analysis, and conservative interpretation of our results.
employ diverse strategies that include the control of existing invasive populations (Mehta et al. 2007, Vander Zanden et al. 2010). However, despite continued investment in long-term management programs that target specific populations, we lack an adequate understanding of both the efficacy and the non-target effects of long-term management efforts (Blossey 1999). Eurasian watermilfoil (Myriophyllum spicatum L.; EWM) is an aquatic invasive plant that has had considerable economic and ecological effects across the United States. The limitation of waterbased recreation, decrease in aesthetic value, and declines in lakefront property value can make invasion by M. spicatum immensely costly (Eiswerth et al. 2000, Horsch and Lewis 2009, Zhang and Boyle 2010). Like many invaders, M. spicatum is assumed to cause decreases in native species richness and diversity, and this assumption has been verified in a handful of lakes with extensive M. spicatum populations that had rapidly colonized (Madsen et al. 1991, Boylen et al. 1999, Madsen 1999). However, additional research suggests that in most lakes, M. spicatum invasion does not correlate with decreased native macrophyte abundance at a landscape scale (Trebitz et al. 1993, Trebitz and Taylor 2007; A. Mikulyuk, E. R. Kujawa, M. Nault, S. Van Egeren, K. Wagner, M. Barton, J. Hauxwell, and J. Vander Zanden, unpublished manuscript). Here, we examine the patterns in macrophyte abundance over 11 yr as part of an aquatic plant management (APM) program designed to control populations of M. spicatum in Wisconsin, United States, predominantly with 2,4-D herbicide, though several lakes utilized other herbicides (i.e., endothall, diquat), hand pulling, and/ or mechanized weed harvesting. We aimed to answer three related questions:
METHODS Site selection From 2005 to 2006, the Wisconsin Department of Natural Resources (WDNR) surveyed aquatic plant communities in over 100 lakes, in an attempt to quantify invasive species presence in the state. In 2007, we used a stratified random approach to select lakes for a long-term M. spicatum study, selecting from this pool of previously surveyed lakes when possible, and supplementing with additional M. spicatum lakes when necessary. The lakes chosen for the long-term study fell into two groups: lakes that had adopted current best management practices, with the overall goal of reducing M. spicatum abundance (i.e., “managed” lakes), and reference lakes, which acted as our control group (i.e., “unmanaged” lakes). Managed lakes met the following criteria: adoption of, and adherence to, a WDNR-approved APM plan, the use of standardized and repeatable aquatic plant surveys in management decision making, and a stated commitment to long-term monitoring and
1. Is herbicide treatment an effective adaptive management technique for long-term control of M. spicatum? 2. Does an early response to invasion increase management efficacy? and 3. Are there non-target effects of management on native macrophytes? Previous research on the effects of herbicide treatment on M. spicatum and native macrophytes has occurred on a relatively small scale, both ❖ www.esajournals.org
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Managed lakes adopted different treatment regimes, each representing the agency’s sciencebased approach to managing M. spicatum populations based on local occurrence and abundance. There was considerable variability in both treatment scale and frequency: Managed lakes were permitted to conduct both whole-lake treatments and spot treatments, and some lakes were chemically treated on an annual or nearly-annual basis, while others were treated only once or twice. We consider treatment scale in our analysis, but it is not the focus of this paper; instead, we investigate the overall effect of adaptive management.
use of best management practices to control M. spicatum. Unmanaged lakes were determined likely to remain unmanaged throughout the study, primarily due to lack of an organized lake association and/or funding. Study lakes were selected based on input from WDNR lake managers, who are responsible for advising and permitting APM activities on lakes in their region. We balanced study lakes to control for major spatial patterns in Wisconsin aquatic plant communities, by attempting to select equal numbers of managed and unmanaged lakes in each of Wisconsin’s lake-rich ecoregions (Omernik et al. 2000): four managed and four unmanaged lakes in the Northern Lakes and Forests (NLF) region, four managed and four unmanaged lakes in the North Central Hardwood Forests (NCHF) region, and three managed and three unmanaged lakes in the Southeastern Wisconsin Till Plains (SWTP; it was particularly difficult to find lakes that utilized best management practices in this region, thus the 3/3 rather than 4/4 grouping) region. When possible, we balanced by time since initial detection of M. spicatum invasion (occurring before or after the year 2000, i.e., more than five years before initial surveys or less than five years before initial surveys), although this part of the study design is imperfect, due to statewide spatial trends in M. spicatum invasion. Specifically, it is difficult to find new M. spicatum populations in the SWTP region, and difficult to find established M. spicatum populations in the NLF region. Operational management decisions on each lake were made entirely by local WDNR lake managers and lake associations (no decisions were made by the authors as part of the Bureau of Science Services), and during the course of the study, several lakes in our original unmanaged pool were permitted to chemically treat. From 2007 to 2011, a total of five lakes moved from the unmanaged to the managed category, so we re-balanced the study design with six additional unmanaged lakes added from 2007 to 2010. This resulted in a final set of 15 managed lakes and 13 unmanaged lakes (Fig. 1, Table 1). The majority of herbicide treatments utilized an early-season (April–June) approach, which is designed to target the early-emerging exotic plants and minimize exposure to many native plants that are still dormant (Skogerboe and Getsinger 2006). ❖ www.esajournals.org
Data collection We surveyed each of the 28 lakes annually during the summer growing season (approximately June 1–August 31), using a point-intercept method for sampling aquatic plants (Hauxwell et al. 2010). This method consisted of systematically sampling a grid of points, the resolution of which was determined by the size of a lake, its shoreline complexity, and the area of the littoral zone, defined as all points equal to or less than the depth of maximum plant colonization (Mikulyuk et al. 2010). At each point, we recorded water depth and used a double-sided rake head on a 4.6-m pole to retrieve plants by placing the rake head on the bottom and rotating twice. Points deeper than 4.6 m were sampled using a double-sided rake head attached to a rope. The rake head was lowered to the bottom and dragged approximately 1 m before being pulled back up. All plants on the rake were identified to species, using Crow and Hellquist (2000a, b), except for macroalgae species, which were identified only to genus (i.e., Chara and Nitella). In total, results represent 91,924 individual processed vegetated sites throughout the course of the study: on average, over 8000 sampling sites per year and over 300 sites annually per lake. We used littoral frequency of occurrence as a metric of species abundance, calculated with the formula: Psp Fsp ¼ Plittoral where Fsp is the percent frequency of occurrence of a given species in a particular lake, Psp is the number of points within that lake where that species was present on the rake, and Plittoral is the number of points sampled in the littoral zone. 3
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Fig. 1. Site map of managed (black circle) and unmanaged (hollow square) lakes and Wisconsin’s level 3 ecoregions: Driftless Area (DA), Northern Lakes and Forests (NLF), North Central Hardwood Forests (NCHF), and Southeastern Wisconsin Till Plains (SWTP). There are relatively few lakes in the DA, so this region was not included in our study design.
Data analysis
robust to sample non-independence because they calculate the similarity in species responses within lakes over time to estimate regression parameters. The full GEE equation is:
The key advantage of collecting longitudinal data is the resulting ability to observe trends over time. However, temporal autocorrelation among samples violates the assumption of independence present in many methods of analysis. For this reason, we employed several statistical methods that are relatively robust to temporal autocorrelation. We used a generalized estimating equation (GEE) to reveal patterns in macrophyte abundance over time while accounting for nonindependence of samples within lakes (Zeger et al. 1988, Diggle et al. 2002, Hanley 2003, Ballinger 2004). We used GEEs to predict an outcome for a species i at a time t, as a function of covariate(s) x (Zeger et al. 1988). The models are ❖ www.esajournals.org
UðbÞ ¼
N ol X ij i¼1
obk
Vi fYi li ðbÞg
where mean model lij for subject i and time j depends on regression parameters bk and variance structure Vi (Diggle et al. 2002). We used the R package geepack (Yan 2002, Yan and Fine 2004, Højsgaard et al. 2006) to build a GEE of M. spicatum and each native macrophyte species as a function of management (managed or unmanaged) and sampling year. 4
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KUJAWA ET AL. Table 1. Lake summary characteristics for 28 Wisconsin lakes monitored for M. spicatum and native macrophytes.
Management Managed
Ecoregion NLF
NCHF
SWTP
Unmanaged
NLF
NCHF
SWTP
Lake name
County
Silver Vilas Arrowhead Vilas Sand Bar Bayfield Tomahawk Bayfield Seven Island Lincoln Kathan Oneida Connors Sawyer Lulu Shawano Underwood Oconto Loon Shawano Round Burnett Berry Oconto Turtle Walworth Little Green Green Lake Kettle Moraine Fond du Lac Weber Iron Little Bearskin Oneida Manson Oneida Hancock Oneida Boot Vilas Bear Paw Oconto Crooked Adams Crystal Marquette Montana Marinette Storrs Rock Ivanhoe Walworth Gibbs Rock Wingra Dane
Lake Max M. Hybrid area depth spicatum milfoil (ha) (m) reported present 23.1 38.8 41.3 46.1 54.6 86.6 165 6.07 18.2 53.4 84.2 84.6 57.1 75.7 84.6 26.3 74.5 95.5 105 116 19.1 19.4 48.2 57.8 8.09 18.6 29.5 136
5.79 13.1 14.9 12.8 9.45 4.57 25.0 3.96 11.3 6.71 8.23 8.23 9.14 8.53 9.14 11.9 8.23 16.5 6.71 4.57 6.10 17.1 18.3 8.53 7.62 3.05 7.01 4.27
2005 2005 2004 2004 2004 2004 2002 2001 2002 1995 2003 2007 1994 1993 1995 2006 2008 1989 2006 2000 2007 2005 2004 2002 1995 1995 1968 1969
Yes
Yes Yes Yes Yes
Yes
Yes Yes Yes Yes Yes Yes
Years sampled
Public access
2005, 2007–2015 2006–2015 2007–2015 2006–2015 2005, 2007–2015 2007–2015 2005, 2007–2015 2005, 2007–2015 2006–2015 2006–2015 2006–2015 2007–2015 2009–2015 2005–2015 2007–2015 2006, 2010–2015 2009–2015 2005, 2009–2015 2006, 2008–2015 2005–2008, 2010–2015 2007–2015 2006–2015 2005, 2007–2015 2007–2015 2005, 2007–2015 2005–2015 2007–2015 2005, 2007–2015
Yes Yes No Yes Yes No Yes Yes Yes Yes Yes Yes Yes Yes No Yes Yes Yes Yes Yes Yes Yes No Yes Yes Yes Yes Yes
Note: NCHF, North Central Hardwood Forests; NLF, Northern Lakes and Forests; SWTP, Southeastern Wisconsin Till Plains.
and the response of M. spicatum to individual herbicide treatments, we used McNemar’s test for count data, which compares the presence of M. spicatum at each individual point at two different times (e.g., before/after each herbicide treatment). We also used the Kolmogorov-Smirnov test to assess the differences in distribution of year-to-year abundance changes for native species between managed and unmanaged lakes. There is no ideal test for differences between distributions, as our data are temporally autocorrelated and therefore not independent; however, the Kolmogorov-Smirnov test is relatively robust to temporal autocorrelation (Durilleul and Legendre 1992) and is therefore the best suited for our purposes. To assess the effect of treatment magnitude on both M. spicatum and native species, we used linear models of changes in post-treatment species
The general form that we used for producing GEEs for each species is as follows: abundancesp management year As we were interested in the effects of herbicide treatment on native species, we also constructed GEEs for the 15 native macrophyte species that were present in a minimum of eight managed and eight unmanaged lakes during the study period (Chara spp., Ceratophyllum demersum, Elodea canadensis, Heteranthera dubia, Myriophyllum sibiricum, Najas flexilis, Nitella spp., Nuphar variegata, Nymphaea odorata, Potamogeton amplifolius, P. gramineus, P. pusillus, P. zosteriformis, Utricularia vulgaris, and Vallisneria americana). A Wald statistic test was used to assess significance of these GEEs. To evaluate both the change in M. spicatum abundance over the entire course of the study, ❖ www.esajournals.org
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Fig. 2. M. spicatum abundance as a function of year in managed (left) and unmanaged (right) lakes. Lakes are represented by individual colors. The thick black line represents the yearly mean M. spicatum abundance; the gray shaded band represents yearly standard error.
comprehensive, if imperfect, measure of treatment magnitude. R packages plyr (Wickham 2011), reshape2 (Wickham 2007), and rgeos (Bivand and Rundel 2015) were used to manipulate and organize data; ggplot2 (Wickham 2009) and gridExtra (Auguie 2016) were used to make figures in R (R Core Team 2014).
abundance as a function of percent of lake surface area treated, time, and ecoregion. Note that the percent of lake surface area treated is not the most comprehensive measure of treatment magnitude, and in recent years, the WDNR has stressed the importance of “3-D” measurements (including water volume and thermal stratification) to estimate likely herbicide concentration and exposure time. However, as this study began over a decade ago, the data utilized to calculate these 3-D measurements were not available for all lakes, and so we are using percent of lake surface treated (a “2-D” metric) as a proxy measure of treatment magnitude. To ensure the validity of this metric, we compared linear models using percent of lake treated (information we had for all treatments) to models using whole-lake herbicide concentration (information we had for only some treatments), and found that the results were very similar. For this reason, we are confident in the value of this 2-D metric as a more ❖ www.esajournals.org
RESULTS A Wald test of our GEE model reveals average M. spicatum abundance to be lower overall in managed lakes than in unmanaged lakes (P = 0.0034). From beginning to end of the study, the average percent decrease in M. spicatum in managed lakes was greater than that observed in unmanaged lakes (14.6–2.0% mean abundance in managed lakes, compared to 29.4–15.2% in unmanaged lakes, Fig. 2). McNemar’s tests indicated that of 15 managed lakes, nine had statistically significant
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Fig. 3. Changes in lakewide Myriophyllum spicatum percent abundance between each lake’s initial survey and 2015. Gray bars represent managed lakes (top); white bars represent unmanaged lakes (bottom). Asterisks represent a statistically significant difference, using McNemar’s tests ( P < 0.05, P < 0.01, P < 0.001). Due to a change in sampling point resolution for Round, Storrs, and Wingra, McNemar’s tests are not possible; however, we can say that M. spicatum abundance decreased by 22% in Round Lake, decreased by 37% in Storrs Lake, and increased by 30% in Lake Wingra from initial survey to 2015 (these lakes are noted with ‡).
coincided with significant increases in M. spicatum). The full details of these herbicide treatments are summarized in Appendix S1: Table S1. Additional analyses show no significant relationship between time and treatment magnitude overall: Treatment magnitude (measured by the percent of lake treated) does not decrease over time. We also divided lakes by ecoregion, to determine whether geographic location was related to treatment magnitude over time, and found no significant effect. However, long-standing M. spicatum populations were associated with larger and more frequent chemical treatments, while lakes that were invaded only a few years before the study began (after the year 2000) typically experienced one large-scale treatment followed by zero
decreases in M. spicatum abundance from the beginning to the end of the study, one had a significant increase, while five did not significantly change. Of the 13 unmanaged lakes, four had a significant decrease, two had a significant increase, and the remaining seven lakes did not significantly change (Fig. 3). We also used McNemar’s tests to assess changes in M. spicatum abundance related to individual chemical treatments. During the study period, 61 treatments occurred in a statistically testable year (having both a previous and subsequent survey). Of these 61 treatments, 23 coincided with a statistically significant (P < 0.05) decrease in M. spicatum abundance. Of the remaining treatments, 33 had no effect and five ❖ www.esajournals.org
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Fig. 4. Percent of lake area treated (top) and Myriophyllum spicatum abundance (bottom) as a function of the number of years since a lake’s initial treatment (in a few cases, lakes were treated before 2005, but we only show treatments during the 2005–2015 study period). Lakes with established M. spicatum populations (discovered pre2000) are shown at left; lakes with relatively new M. spicatum populations (discovered post-2000) are shown at right.
year-to-year changes are more positive and less variable, i.e., standard error is smaller, in managed lakes than in unmanaged lakes, P = 0.007, Fig. 5). Linear models of the relationship between treatment magnitude (percent of lake area treated) and percent change in lakewide abundance for M. spicatum and each native macrophyte are shown in Figs. 6 and 7. A linear model of the change in M. spicatum abundance as a function of treatment magnitude reveals a highly significant negative relationship: Larger treatments correspond to larger decreases in M. spicatum (P < 0.001). For most native species, this relationship is not statistically significant; the exceptions to this are N. flexilis (P = 0.02), P. pusillus
to only a few small-scale treatments (Fig. 4, Table 1). Of the 15 native species analyzed, Wald tests revealed that the abundance of four species differed significantly between managed and unmanaged lakes (P < 0.05). Abundance was lower in managed vs. unmanaged lakes for C. demersum (P = 0.018) and N. variegata (P = 0.005); abundance was higher in managed than in unmanaged lakes for E. canadensis (P = 0.044) and Nitella spp. (P = 0.031). We used the Kolmogorov-Smirnov test to assess whether the distribution of year-to-year changes between managed and unmanaged lakes was similar. Only one species had significantly different distributions between managed and unmanaged lakes: N. variegata (on average, ❖ www.esajournals.org
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Fig. 5. Distribution of year-to-year changes in abundance for each native species. Managed lakes are gray; unmanaged lakes are white. Kolmogorov-Smirnov tests suggest that few species differ in distribution of yearto-year changes between managed and unmanaged lakes; this means that on a broad scale, management does not appear to exert a significant directional effect on native species. P < 0.01.
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Fig. 6. Change in Myriophyllum spicatum abundance as a function of percent of lake area treated. The x-axis is square-root-scaled, due to the large number of treatments under 25% of a lake’s surface.
(P < 0.001), and V. americana (P = 0.01), all of which have a negative relationship between percent of lake area treated and change in species abundance.
Larger treatments typically corresponded to larger decreases in M. spicatum abundance (Fig. 6). While this relationship was statistically significant, there was substantial variation in M. spicatum response to treatment. Myriophyllum spicatum response was most variable when treatments were applied to 50% of a lake’s surface resulted in a significant decrease in M. spicatum abundance, suggesting that large-scale treatments may be particularly effective at M. spicatum control (Nault et al. 2012, 2014). Our repeated surveys of reference lakes suggest that M. spicatum population dynamics are
DISCUSSION Herbicide treatment as an adaptive management tool Myriophyllum spicatum abundance decreased overall in managed lakes (Fig. 2), and McNemar’s tests revealed significant decreases in M. spicatum in nine out of 15 managed lakes from the beginning to the end of the study period, compared to four of 13 unmanaged lakes (Fig. 3). McNemar’s tests of individual treatments suggest a slightly more nuanced but still compelling narrative: About a third of herbicide treatments (23 of 61) coincided with statistically significant decreases in M. spicatum at the whole-lake scale (Table 1). These results suggest that herbicide treatment can be an effective tool for adaptive management of M. spicatum (Parsons et al. 2001, Wersal et al. 2010, Nault et al. 2014). ❖ www.esajournals.org
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Fig. 7. Change in native species abundance as a function of percent of lake area treated. Each line represents that species’ linear model. The x-axis is square-root-scaled, due to the large number of treatments under 25% of a lake’s surface. P < 0.05, P < 0.001
populations appear to be far more stochastic and unpredictable on an annual basis.
more variable in unmanaged systems: Both the largest increases and decreases in M. spicatum over the course of the study period occurred in unmanaged lakes. Our data strongly suggest that a lack of active management does not necessarily condemn a lake to complete M. spicatum invasion. In fact, M. spicatum abundance decreased or did not exhibit a statistical change in 11 of the 13 unmanaged lakes over the course of the study period, and increased significantly in only two lakes (Fig. 3). However, unmanaged M. spicatum ❖ www.esajournals.org
Implementing adaptive aquatic plant management strategies We found that lakes with established M. spicatum populations (i.e., where M. spicatum was discovered prior to 2000) received maintenance treatments that varied widely in size among lakes nearly every year, while lakes with relatively new M. spicatum populations (discovered 11
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applied as necessary based on yearly M. spicatum abundance. While we have limited access to lake management histories prior to 2005 (Appendix S1: Table S2), and little to no vegetation composition data, we do know that the majority of lakes with established M. spicatum populations began treating with herbicide within five years of discovery: mostly small-scale spot treatments designed to reduce nuisance vegetation. This suggests that while early detection and response (EDR) can be a helpful management tool, EDR alone is not enough to properly manage a lake’s M. spicatum. The established populations are mostly located in the southern part of the state, while the new M. spicatum populations are almost all located farther north. During the course of the study, it was discovered that the majority of the lakes in southern Wisconsin have genetically confirmed hybrid watermilfoil (M. spicatum 9 sibiricum, HWM), while those in the north predominantly have genetically pure M. spicatum (Table 1). Concern over HWM’s invasiveness has been present in APM for over a decade (Moody and Les 2002, Les and Moody 2007). HWM can be more invasive than its parent M. spicatum: The development of HWM appears to be linked to frequent 2,4-D herbicide application (Larue et al. 2013), and 2,4-D has been shown to be less effective in controlling HWM than M. spicatum (Parks et al. 2016). The early and frequent chemical treatment of M. spicatum in southern Wisconsin lakes may have facilitated the presence of HWM or selected for the most tolerant milfoil strains. While we do not believe that this changes our conclusion that adaptive management decreases M. spicatum abundance, it complicates recommended management strategies for lakes with established populations, and the costs and benefits of herbicide application should be carefully weighed against other management options.
post-2000) typically received one relatively largescale treatment and then were not treated again, or treated comparatively small percentages later in the study period (Fig. 4). Lakes with newly established M. spicatum populations also maintained lower populations of M. spicatum, suggesting that their herbicide treatments may have been more effective. On average, lakes that discovered M. spicatum prior to 2000 treated about 20% of the lake’s area every year, and treated an average of 8.8 times over the 11-yr study. By contrast, lakes that discovered M. spicatum after 2000 treated an average of
