7967

• UCREFRP Catalog Number: 7967
• Author: Charbonneau, C. S., R. D. Drobney and C. F. Rabeni
• Title: Effects Of
• USFW Year: 1994
• Copyright Material: YES

270

C.S. CIlARBONNEAU ET AL.

igating toxicity of Bti to Chironomidae. The various toxicity
tests are summarized in Table I. Procedures from Peltier and
Weber [42] were followed. except where noted.
Glass test chambers contained 200 ml test water, 25 ml
sediment, and 10 chironomid larvae each, unless otherwise
noted. Sediment used in the test chambers to simulate more
natural habitat conditions was either a control sediment or
a sediment from the Refuge. The control sediment was a fine
silt-clay substrate collected in Florissant, Missouri, from an
alluvial floodplain. Refuge sediment was collected with an
Ekman dredge from under the ice from three sites on each
pond (LBP and Big Bass Pond), cleaned of large debris,
placed in a clean sample container, and transported to the
laboratory. Sediment was transferred into the test chambers.
Dilution water (280 mg/L CaC03 hardness, 255 mg/L
CaC03 alkalinity) was added. After 24 h chironomid larvae
were placed in the chambers and allowed to acclimate for
24 h. Third- to fourth-instar larvae were used, unless other-
wise noted. All test chambers were placed in a water bath at
200 :!: 10C.
There is no standard method for measuring Bt; concen-
trations in a water sample. Application rates are expressed
in terms of specific activity, which is a function of toxicity
of a specific formulation to the target organisms [50]. The
activity is usually determined by bioassay techniques in which
the toxicity of a commercial formulation is compared to that
of a reference formulation using a specified test organism
[50,51). Viable spore counts were originally used in earlier
studies as an estimation of the toxicity of Bti. Dulmage [52]
conducted studies with another strain of B. thuringiensis and
demonstrated no correlation between toxicity and spore
count. The relationship between potency (lTU/mg) of the
formulation and activity against larvae is not consistent [53].
The delta-endotoxin contained within the crystal produced
by the bacteria is the principal killing agent. The number of
crystals per preparation of Bti varies with bacillum strain and
its mode of production. Formulation of the control agent
plays an important role in activating the product [53]. so a
realistic way to express dosages may be in terms of milligrams
of formulation per liter. Therefore, the concentration of
Vectobac-G was measured in parts per million (ppm) or milli-
grams of corncob granule per liter. This measure relates to
actual operational applications. The larvicide was in the form
of corncob granules that were ground in a blender so that
small amounts could be weighed to derive a stock solution.
The stock solution was then diluted to the desired exposure
concentrations. It was then determined that once corncob
granules were placed in dilution water, the toxic components
were released from the granule and became particulates in
the water column. It takes an average of 30 min to release
the majority of the pesticide from the granules (personal ob-
servation). A concentrated stock solution of 100 g/L
Vectobac-G was made. The solution was mixed for 20 min
with a mixing bar on a stir plate. As the stock solution was
stirred to suspend the pesticide, the appropriate amount of
the mixture was removed with a pipette to make a given con-
centration when added to the test chambers. No test cham-
bers contained whole corncob granules.
Range tests. An initial series of four range-finding 48-h

static acute toxicity tests were conducted to determine the
general range of toxicities of Yectobac-G concentrations to
C. riparius. Range test I included control sediment and the
following concentrations: control, 0.001,0.01,0.1, 1.0, and
10.0 ppm of Vectobac-G added to the overlying water. Lar-
vae were not acclimated to the sediment before Vectobac-G
was applied. Range test 2 was identical to the first, except the
larvae were allowed 24 h to acclimate to the sediment before
pesticide application. Range test 3 contained the same con-
centrations of Vectobac-G as the first test, but sediment from
LBP was used. Range test 4 was identical to the third, except
that sediment from Big Bass Pond of the Long Meadow Lake
Unit on the Refuge was used.
Mitigation tests. The influence of temperature on the ef-
ficacy of Vectobac-G was evaluat~d at 10 :!: 1, 20 :!: I, and
27 :!: 10C in 48-h water-only exposures.
Tests of the influence of water depth on the efficacy of
Vectobac-G to chironomids were conducted. Water depth test
I compared the water depth of 6.7 em, using 250-ml beakers,
and 40.6 em, using I,OOO-ml graduated cylinders, in water-
only exposures. Vectobac-G was applied at the surface rate
for the beaker and graduated cylinder surface area.
For water depth test 2, one each of control and treatment
6O-L plastic containers were set up in a water bath. The 250-
ml beakers (test chambers) were hung with two pieces of
monofilament line. The water depth of the shallow chambers
was 9.1 cm. The deep replicates (40.6 em depth of water) were
250-ml beakers placed on the bottom of the test containers
and arranged to one side of the shallow replicates. Nytex@
was placed over the top of the test chambers to retain the lar-
vae. Vectobac-G was applied at the recommended surface ap-
plication rate for the surface area of the test container. To
assure proper application to the large test container, the mix-
ture was placed in a spray bottle. The test container was vi-
sually divided into fourths, and 1.2 L dilution water (control)
or pesticide mixture was equally divided and sprayed onto
each quarter. At the end ofthe test a few larvae were found
to have escaped from the beakers. Only animals remaining
in the beakers were considered in the analysis.
The influence of depth on Vectobac-G efficacy was also
evaluated in MP at the Refuge (field depth test). Mean depths
were 14.3 and 37.1 cm.and control water was from MP. lar-
vae from the MP (Micropsectra [Tanytarsus] and Endochi-
ronomus spp.) were added and test chambers were covered
with Nytex. Test chambers were placed as they were for the
laboratory experiment but were hung with monoftlament line
from the tops of enclosures used in the field. After the cham-
bers were in place, the pesticide was applied as previously de-
scribed using the concentrations RAR (5.6 kg/ha) and 5 x
RAR (28.1 kg/halo Treated water was collected immediately
after application, and 500 ml of each concentration and con-
trol was placed in a separate test chamber with 10 mosquito
larvae (Culex and Culiseta spp.). These chambers were
floated in an additional nondosed enclosure in MP.
Two laboratory toxicity tests were conducted to determine
if macrophyte coverage affected the efficacy of Vectobac-G
to C. riparius. Macrophyte test 1 was conducted using Vol-
lisneria sp. (water celery). A range of surface area coverage
was tested. The treatments included no plant material, 0.25 g