Eurasian watermilfoil (Myriophyllum spicatum - EWM) is an invasive aquatic macrophyte in North America that can reduce the biodiversity of littoral zone communities in lakes and interfere with recreational activities. Since the 1960s herbicides have routinely been used to combat EWM invasions but lately there have been multiple documented cases of herbicide resistance in EWM possibly as result of hybridization with native Northern watermilfoil (Myriophyllum sibericum - NWM). Our research aims to understand whether hybridization, genetic diversity, and/or selection from prior exposure to herbicide treatments best predict EWM sensitivity to herbicides. We collected roughly 180 EWM samples from ten lakes with different histories of herbicide treatment; samples were collected pre and post herbicide treatments in 2015 and 2016. From each sample (i.e., individual), healthy plant material was tested for herbicide susceptibility to three commonly used herbicides (2,4-D, triclopyr, and fluridone testing by SePro Corp. Carmel, IN), and genetic analysis. For each sample we extracted DNA, amplified 12 polymorphic microsatellite loci (Wu et al 2013), combined data across loci to determine multilocus genotypes, examined genotypic and population level genetic variation, and tested for correlations between sensitivity to herbicides and patterns of admixture or herbicide treatment histories.
Preliminary analyses from pre-treatment samples indicate that lakes with a history of herbicide treatment have greater genotypic diversity and evenness than lakes with no known recent history of herbicide application, although surprisingly some lakes were dominated by a single genotype. Furthermore, hybridization between native NWM and invasive EWM appears to be more common in some lakes than in others. Neither genotypic diversity nor history of herbicide application explained patterns of herbicide sensitivity, rather less sensitive genotypes were more often found within lakes containing hybrid zones, and the majority of less sensitive plants appear to be hybrid watermilfoil (EWM x NWM). Hybridization, as opposed to genotypic diversity or prior selection resulting from histories of herbicide treatment, appears to be playing an important role in the evolution of traits associated with reduced sensitivity to herbicides in invasive watermilfoil. These preliminary results might suggest that hybridization creates novel invasive genotypes that are less susceptible to herbicide applications. Findings from this research could prioritize future research and management efforts into better understanding and combating watermilfoil invasions.