Efficacy of single and consecutive early-season diquat treatments on curlyleaf pondweed and associated aquatic macrophytes: A case study

Curlyleaf pondweed (Potamogeton crispus L.) is an invasive aquatic macrophyte that impairs lakes throughout much of North America. Management of curlyleaf pondweed with herbicides is common with long-term control and protection of desirable native vegetation being important goals. The herbicide diquat was applied to Crystal Lake (Middletown, CT) in 2007, 2009, and 2010 to control curlyleaf pondweed prior to turion production. No herbicides were applied in 2006, 2008, or 2011. Two other invasive macrophytes, Eurasian watermilfoil (Myriophyllum spicatum L.) and brittleleaf naiad (Najas minor All.), were also present along with 14 species of native macrophytes. Aquatic vegetation surveys were performed to assess the efficacy of the single (1-yr) and consecutive (2-yr) diquat treatments on the invasive and native plant assemblages. The frequency of occurrence and abundance of curlyleaf pondweed were reduced to negligible levels in the treatment years. In the untreated year after a single treatment (2008), curlyleaf pondweed frequency was reduced slightly but the abundance was greater than in the year prior to treatment (2006). In the untreated year following two consecutive early- season diquat treatments (2011), the frequency of curlyleaf pondweed decreased by 30% and the abundance declined by 55% compared to the year prior to treatment (2006). After the first of the two consecutive years of diquat treatments (2009), Eurasian watermilfoil was eliminated. Both the frequency and abundance of brittleleaf naiad increased in the untreated years of 2008 and 2011. The response of native macrophytes to the diquat treatments was species-specific but overall native species richness was greater in years following the single and consecutive diquat treatments.