Michigan Department of Environmental Quality Award #481144/09

Eurasian watermilfoil (RAT_holding_milfoilMyriophyllum spicatum; EWM) was introduced into the US in the early 1900’s and has since spread extensively throughout North America.  EWM is one of the top five biggest nuisance aquatic plants in the US, and is particularly common in the Great Lakes region.  As such, a great deal of lake management is dedicated towards controlling EWM.  Control methods include herbicide application (2,4,-D, triclopyr, and fluridone), biological control by the milfoil weevil Euhrychiopsis lecontei, and physical control methods such as hand-pulling, raking, mechanical harvesting, water drawdown, and benthic covering with mats.


EWM is very closely related and morphologically similar to a native North American species, the northern watermilfoil (Myriophyllum sibiricum; NWM). These two species can be told apart with morphological lcharacters, though it can be difficult.  In addition, hybrid EWM x NWM genotypes have been found in large number of populations across North America (see Moody and Les 2002, 2007). Hybrid Eurasian watermilfoils have rapidly spread across the Great Lakes region over the past several years. During this same time, lake managers have noted increased tolerance to several herbicides commonly employed to control Eurasian watermilfoil.

In this study, we surveyed Michigan lakes for the presence of hybrid versus non-hybrid Eurasian watermilfoil using molecular methods, including DNA sequencing of ITS (nrDNA) and Amplified Fragment Length Polymorphisms (AFLPs) to determine 1) the frequency and distribution of hybrid milfoils, and 2) the extent of genetic variation within  EWM and hybrids, specifically whether distinct genetic groups are identifiable.  In addition, we collated existing information regarding treatment efficacy on invasive populations of EWM and hybrids to determine whether any evidence exists that hybrids are less sensitive to chemical herbicides compared to parental taxa.

Summary of Results - We found hybrid milfoils in ~40% of the lakes surveyed in Michigan's Lower Peninsula.  We identified two genetically distinct groups of EWM, which probably reflect independent introductions from different portions of the native range.  In addition, genetic diversity in hybrid watermilfoils is high compared to EWM, and a number of genetically distinct groups are identifiable, indicating that hybrids have likely been formed repeatedly in Michigan lakes.  Finally, we found evidence that some hybrid watermilfoil populations may be less sensitive to one chemical herbicide (fluridone).  However, the sample size available for these preliminary studies was low and there was no statistically significant difference in fluridone sensitivity between hybrid and EWM.

This MDEQ grant formed the foundation for current experimental research comparing herbicide tolerance in laboratory assays and controlled field studies. In the longer term (~5 years), this research program has a high potential to contribute to the development of herbicide permitting procedures and effective, environmentally responsible management of Eurasian watermilfoil.

Conceptual model for the evolution of increased tolerance or resistance to management of invasive milfoils through hybridization.  Note that other models for herbicide tolerance/resistance are also possible.  For example, germline or somatic mutations may arise within local populations that increase tolerance or resistance.  In these latter models, hybridization is not necessary for the evolution of herbicide tolerance.

Papers resulting from this award:

1) Zuellig, M.P. and R. A. Thum. Amplified fragment length polymorphisms reveal multiple introductions of invasive Eurasian watermilfoil and recurrent hybridization with native northern watermilfoil in North America. Accepted to Journal of Aquatic Plant Management (October 2011).

2) Thum et al.  Field and laboratory documentation of reduced fluridone sensitivity by a hybrid watermilfoil biotype (Myriophyllum spicatum x Myriophyllum sibiricum).  Submitted to Journal of Aquatic Plant Management (September 2011)





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