Variable watermilfoil (Myriophyllum heterophyllum Michx.)
has recently become a problem in Bashan Lake, East Haddam,
CT, USA. By 1998, approximately 4 ha of the 110 ha lake
was covered with variable watermilfoil. In 1999, the milfoil
was spot treated with Aquacide®, an 18% active ingredient of
the sodium salt of 2,4-D [(2,4-dichlorophenoxy) acetic acid],
applied at a rate of 114 kg/ha. Aquacide® was used because
labeling regarding domestic water intakes and irrigation limitations
prevented the use of Navigate® or AquaKleen®, a
19% active ingredient of the butoxyethyl ester of 2,4-D. Variable
watermilfoil was partially controlled in shallow protected
coves but little control occurred in deeper more exposed
locations. 2,4-D levels in the treatment sites were lower than
desired and offsite dilution was rapid. In 2000, the United
States Environmental Protection Agency (USEPA) issued a
special local need (SLN) registration to allow the use of Navigate
® or AquaKleen® in lakes with potable and irrigation
water intakes. Navigate® was applied at a rate of 227 kg/ha
to the same areas as treated in 1999. An additional 2 ha of
variable watermilfoil was treated with Navigate® in 2001, and
0.4 ha was treated in mid-September. Dilution of the 2,4-D ester
formulation to untreated areas was slower than with the
salt formulation. Concentrations of 2,4-D exceeded 1000 μg/
L in several lake water samples in 2000 but not 2001. Nearly
all of the treated variable watermilfoil was controlled in both
years. The mid-September treatment appeared as effective as
the spring and early summer treatments. Testing of homeowner
wells in all 3 years found no detectable levels of 2,4-D.(PDF contains 8 pages.