Survival and vegetative regrowth of Eurasian and hybrid watermilfoil following operational treatment with auxinic herbicides in Gun Lake, Michigan

Abstract

Eurasian watermilfoil (Myriophyllum spicatum L.; EWM) is an invasive aquatic plant that is widespread in inland lakes of the northern tier of the United States. It is commonly managed to alleviate negative economic and environmental impacts, including the formation of dense stands of submerged vegetation that extend from the lake bed to the surface, impairing recreational uses; affecting water movement, nutrient cycling, and sedimentation; reducing property values; and potentially altering habitats used by native species (Smith and Barko 1990, Zhang and Boyle 2010). Management costs of invasive aquatic plants in the United States can exceed $100 million annually (Rockwell 2003, Pimentel et al. 2005). In Michigan, where the present study was conducted, roughly $24 million are spent annually on herbicidal control of aquatic invasive plants, much of which is focused on EWM (MDEQ 2013). The auxinic herbicides 2,4-D and triclopyr have been used extensively to selectively control EWM (e.g., Getsinger et al. 1982, 1997, Parsons et al. 2001, Wersal et al. 2010). However, herbicide treatments do not typically eradicate EWM from lakes. Even when control efforts result in undetectable levels of EWM for months to years, reestablishment of EWM commonly occurs (Netherland 2014). The source(s) of regrowth following auxinic herbicide treatments are unclear. It is possible that regrowth occurs via reestablishment from untreated area(s) of the lake or from other lakes. Similarly, it is possible that regrowth occurs via recruitment from a viable seedbank, as EWM can flower profusely and produce viable seeds (Hartleb et al. 1993, Xiao et al. 2010, LaRue et al. 2013a). Alternatively, regrowth may occur from treated plants that are not completely killed, such as sprouting from root crowns or axillary meristems on surviving shoots. Distinguishing among sources of regrowth when assessing herbicide treatment efficacy could be important for informing decisions about management strategies and/or site-specific tactics. For example, if regrowth occurs from seed, management strategies should include tactics to reduce seed production and/or exhaust the seed bank to improve long-term control. If regrowth occurs via fragment recolonization from untreated areas, different spatial management strategies may be required. And, if regrowth arises from incomplete kill of treated plants, then different herbicides and/or application patterns may be required. However, in our experience, monitoring of the source(s) of regrowth is not routinely included in herbicide evaluations of EWM. In this case study, we monitored quadrats established in treated areas of Gun Lake, Michigan, following treatment by auxinic herbicides. Our goal was to document the source(s) of regrowth throughout the remainder of the summer season following herbicide treatment, and into the following spring before the next management cycle began.