Keeping an eye on hybrid watermilfoils in Michigan inland lakes

Grand Valley State scientists warn that native and invasive watermilfoils are hybridizing. Offer genetic testing of plants to inform management decisions.

Leaves from northern watermilfoil (top), hybrid watermilfoil (middle) and Eurasian watermilfoil (bottom). GVSU scientists say genetic analysis is more reliable than visual ID methods. Photo credit: Ryan Thum l MSU Extension

Leaves from northern watermilfoil (top), hybrid watermilfoil (middle) and Eurasian watermilfoil (bottom). GVSU scientists say genetic analysis is more reliable than visual ID methods. Photo credit: Ryan Thum l MSU Extension

Most lakefront property owners are all too familiar with the invasive aquatic plant, Eurasian watermilfoil (Myriophyllum spicatum), since it causes multiple problems in inland lakes. They may be less familiar with Northern Watermilfoil (Myriophyllum sibiricum) – a plant native to Michigan that provides benefits to inland lake ecosystems. In recent years, it has become evident that these two members of the genus Myriophyllum have begun to hybridize to produce strains of watermilfoil that are more vigorous and more resistant to some of the usual chemical control methods.

In addition, hybridization has made it difficult, if not impossible, to reliably distinguish one watermilfoil from another using visual observations alone. Scientists at Grand Valley State University (GVSU) point out that what we commonly refer to as Eurasian watermilfoil is actually a complex of genetically distinct plants that can grow and respond to management differently. In particular, hybrid watermilfoil can differ significantly from pure Eurasian or pure northern watermilfoil in patterns of nuisance growth. It can also differ in its response to management. However, due to variability in form and structure, many hybrids are difficult to distinguish from either Eurasian or northern watermilfoil – even for those with aquatic plant identification training.

A newer method of identification, using molecular genetics, has proven more reliable than visual observations for watermilfoils. GVSU scientists estimate that approximately 30 percent of watermilfoil identifications based on visual observations are incorrect. While most aquatic vegetation mapping and identification programs have yet to use molecular genetics, the method is gaining interest as awareness increases and costs decrease. GVSU suggests incorporating genetic monitoring of watermilfoils into existing aquatic vegetation mapping to assist in the prescription and evaluation of management actions for Eurasian and hybrid watermilfoil.

It is possible that past failures to accurately discriminate between Eurasian, northern and hybrid watermilfoils will have limited the ability to develop, implement and evaluate management actions intended to provide control of Eurasian watermilfoil. Genetic monitoring could help identify which species are present. In turn, this information could be used to ‘adapt’ future management strategies.

A companion Michigan State University Extension article provides more on aquatic vegetation mapping as a tool for ‘adaptive management’ and how volunteers can make it happen. Also, for information and guidelines on genetic monitoring and identifications of watermilfoils, visit the website of GVSU’s Annis Water Resources Institute.

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