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Hybrid Watermilfoil: A Michigan Lake Case Study

Posted On 03/14/2012 By Mike Jahnke

Goal: To develop an effective and selective treatment strategy for controlling hybrid milfoil that does not respond to typical Eurasian watermilfoil treatments.

Study Site:

Background:
In recent years, Eurasian watermilfoil (EWM) has been found to hybridize with northern watermilfoil, a closely related native plant. This hybrid grows to extreme densities similar to EWM, and is a major management concern. To confound matters, hybrid watermilfoil often does not respond to treatment strategies that effectively control EWM.

In 2008, laboratory analysis confirmed that Runkle Lake milfoil was a hybrid cross of Eurasian and Northern watermilfoil.

In 2009, Cason & Associates mapped and treated 15 acres of hybrid watermilfoil with Navigate® at a rate of 150 lbs/acre. Treatments using this method had great success at controlling EWM on Runkle Lake in years prior to 2008. This treatment was done early in the season before turions formed on the plants. It was thought that this early-season treatment approach would provide better long-term control of the hybrid, but little lasting impact was achieved.

In May 2010, a total of 28.1 acres of hybrid milfoil were found in Runkle Lake.

A New Approach

Based on recommendations from Applied Biochemists, a new treatment was suggested that Involved combining Navigate®, granular 2,4-D, and Navitrol®, granular triclopyr. It was thought that this combination of auxin-mimic herbicides would provide better control of hybrid watermilfoil. On May 26, 2010, Cason & Associates treated 14.7 acres of the 28.1 acres of hybrid milfoil with the herbicide combination.

Pre and post-treatment surveys were done to determine the amount of hybrid milfoil control that was achieved, as well as the impact on native plant species.

Results

The post treatment survey showed a high degree of hybrid watermilfoil control. In addition, native plants had re-colonized the areas where milfoil had previously dominated. Analysis of the data shows that most species exhibited statistically significant increases following treatment; likely as a result of reduced competition from the hybrid watermilfoil.

The combination of Navigate® and Navitrol® herbicides offers a treatment option for difficult-to-control hybrid watermilfoil that is both effective and highly selective