Combinations of Endothall with 2,4-D and Triclopyr for Eurasian Watermilfoil Control

BACKGROUND: Eurasian watermilfoil (Myriophyllum spicatum L.) is a widespread submersed aquatic plant that causes nuisance problems across the continental United States. Eurasian watermilfoil is an herbaceous perennial submersed aquatic plant that typically grows in water depths of 1 to 3 m (Aiken et al. 1979). Vegetative propagation is either by direct stem fragments or by autofragmentation through the development of an abscission layer in stem segments (Madsen et al. 1988). The production of these stem fragments, either by external forces or by autofragmentation, allows for widespread plant dispersal. Eurasian watermilfoil can form a dense surface canopy, which is the cause of both ecological harm and nuisance impacts. Eurasian watermilfoil is often responsible for reductions in oxygen exchange, depletions in dissolved oxygen, increases in water temperatures, and internal nutrient loading (Madsen 1998). Eurasian watermilfoil has been directly associated with declines in native plant species richness and diversity (Madsen et al. 1991a, 1991b); and indirectly with reductions in habitat complexity resulting in declines in macroinvertebrate abundance (Krull 1970, Keast 1984), reductions in fish growth (Lillie and Budd 1992), and an overall reduction in habitat value for invertebrates and fish (Dibble and Harrel 1997). Eurasian watermilfoil also poses nuisance problems to humans in the form of increasing flood frequency and intensity, restricting water intakes, impeding navigation, and limiting recreation opportunities (Madsen et al. 1991a, 1991b). Once Eurasian watermilfoil has become established in a waterbody, it is generally persistent and difficult to control. Management techniques such as biological, mechanical, and physical methods are available and have been instituted alone, or in integrated management regimes; but adequate control with these techniques has been highly variable. To date, chemical techniques have offered the most consistent and effective control of Eurasian watermilfoil at both small and large spatial scales.