- Elodea densa Planch. was exposed to diquat (6, 7 dihydrodipyrido [1,2-a: 2', 1'-c] pyrazinediium dibromide), alone and in combination
with disodium endothall (disodium salt of 7-oxabicyclo [2.2.1]
heptane-2, 3-dicarboxyclic acid), and copper sulfate pentahydrate.
Studies were conducted in coastal lakes of Oregon infested with E.
densa. Dichlobenil (2, 6 dichlorobenzonitrile) was applied following
diquat treatments in an attempt to extend length of the control period.
Plant samples were collected from trials in Siltcoos Lake for diquat
analysis. Laboratory studies were conducted to determine the effects
of water temperature, exposure time, light quality, and time of application
on the activity of diquat and diquat combinations against E.
densa. Weights of stem, length of stem, and visual control evaluations
were used to measure effectiveness of herbicide application.
In field trials, effective seasonal control of E. densa was
achieved by application of 0.25 ppmw diquat. Addition of 0.25 ppmw
endothall or 0.5 ppmw copper sulfate pentahydrate did not enhance the
activity of the 0.25 ppmw rate of diquat. The optimum elodea control
from 1.0 ppmw diquat did not prevent extensive regrowth the following
season. Dichlobenil at 10 and 20 pounds active per acre suppressed
but did not completely prevent regrowth in previously diquat-treated
plots. Levels of diquat in treated plants were not increased by addition
of endothall or copper sulfate. The influence of water movement
in Siltcoos Lake from wind and flood water resulted in drift of chemical
from treated areas and consequent control of exposed E. densa
over significant untreated areas.
In the laboratory, preliminary trials demonstrated that submersed
plants native to the coastal lakes, Elodea canadensis Michx.,
Ceratophyllum demersum L., and Myriophyllum verticillatum L.
were effectively controlled by 0.25 ppmw diquat as was E. densa.
Addition of 0.25 ppmw disodium endothall or 0.5 ppmw copper sulfate
pentahydrate did not improve E. densa control with 0.25 ppmw diquat
except at 30 C where the diquat-endothall combination was superior.
As the water temperature was reduced from 30 C to 20 C to 10 C,
expression of the phytotoxicity of diquat was delayed but not prevented.
When E. densa was exposed to 0.25 ppmw diquat alone or in combination
with 0.25 ppmw endothall, six hours at 30 C and 24 hours at 20 C and 10 C were required for optimum control. At 10 C, the diquatendothall
combination and, at 20 and 30 C, the diquat-copper sulfate
combination was less effective than 0.25 ppmw diquat alone.
When E. densa was grown under the visible light spectrum (360-
740 mμ), the red band (620-740 mμ), the green band (440-620 mμ), and
the blue band (360-560 mμ) phytotoxicity of diquat, alone or in combination,
was not affected. Application of 0.25 ppmw diquat, alone or in
combination with 0.5 ppmw copper sulfate, was most effective for control
of E. densa at the beginning or middle of the 12 hour photoperiod
rather than at the beginning or middle of the 12 hour dark period.