Divergent responses of cryptic invasive watermilfoil to treatment with auxinic herbicides in a large Michigan lake

Parks SR, McNair JN, Hausler P, Tyning P, Thum RA. 2016. Divergent responses of cryptic invasive watermilfoil to treatment with auxinic herbicides in a large Michigan lake. Lake Reserve Manage. 32:366-372. Invasive plants are a major concern for environmental managers around the world. Cryptic invasive taxa present additional challenges because of their potential to respond differently to management efforts while being difficult to distinguish morphologically. Here, we consider the potential for different responses of Eurasian watermilfoil (Myriophyllum spicatum) and hybrid watermilfoil (Myriophyllum spicatum x Myriophyllum sibiricum), which cannot be reliably distinguished based on morphological characters. Laboratory studies show that hybrid watermilfoil genotypes can grow faster, branch more, and be less responsive to herbicide control methods designed for Eurasian watermilfoil. To date, however, there have been no statistical comparisons of the responses of hybrid versus Eurasian watermilfoil under operational management in the field. In this study, we use genetic methods to distinguish morphologically cryptic Eurasian and hybrid watermilfoil in Houghton Lake, Michigan, and test whether they exhibit divergent responses for distribution and abundance following treatment with 2,4-dichlorophenoxyacetic acid-amine and triclopyr. Our results suggest that treatment reduced the frequency of occurrence and abundance of pure Eurasian watermilfoil more than it did for hybrids in Houghton Lake. These findings highlight the importance of distinguishing these cryptic taxa during management evaluations.