Hybridization Facilitates the Rapid Evolution of Reduced Herbicide Sensitivity in the Widely-managed Invasive Aquatic Plant, Eurasian Watermilfoil

Eurasian watermilfoil (Myriophyllum spicatum) is one of North American’s most invasive and widely managed aquatic weeds. Eurasian watermilfoil also hybridizes with the native northern watermilfoil (Myriophyllum sibiricum) to produce many invasive and genetically diverse populations of hybrid watermilfoils. Both Eurasian and hybrid watermilfoils are commonly controlled with herbicides, but there has been recent concern that some hybrids exhibit reduced herbicide sensitivity. There is a lack of studies comparing the response of Eurasian and hybrid watermilfoils to commonly used herbicides. To determine if hybrids exhibit reduced herbicide sensitivity, I compared the sensitivity of hybrid and Eurasian watermilfoil populations from the Upper and Lower Peninsulas of Michigan to the commonly used herbicide, 2,4-D, with laboratory studies. I also asked whether hybrids were more abundant in lakes with a history of 2,4-D management in the Menominee River watershed of the Upper Peninsula of Michigan. Hybrids exhibited reduced 2,4-D sensitivity and were more common in 2,4-D treated lakes. To estimate how quickly reduced 2,4-D sensitivity can evolve in hybrids, I estimated whether watermilfoil populations from the Menominee River watershed, including populations with reduced 2,4-D sensitivity, were composed of first generation or advanced generation hybrids. The presence predominance of first generations hybrids in the Menominee River watershed suggests that reduced 2,4-D sensitivity can evolve in a single generation. In conclusion, I discuss management implications of reduced herbicide sensitivity in hybrid watermilfoils