STATE k. -N f BOARD
May 1949 ET-266
United States Department of Agriculture
Agricultural Research Administration
Bureau of Entomology and Plant Quarantine
A METHOD FOR TESTING RESIDUAL DEPOSITS
P. G. Piquett, R. H. Nelson, and E. R. McGovran i
r Division of Control Investigations
With the discovery of the new insecticides that leave long-
lasting deposits, it became necessary to find methods for testing
the effectiveness of the residues. The laboratory method presented
herein for testing the effectiveness of the residue of DDT can be
used for other similar materials.
Twenty-six-gage galvanized iron was used to construct the frame
shown in figure 1, A. The two sides of this frame fitted the fronts
of two 9-inch cubical, screened fly cages with sliding glass doors
(figs. 2 and 3). A slot 1/A inch high and l inches wide was cut in
the middle of the lower edge of each of the metal strips which fitted
across the bottom of the glass doors. These slots were connected by
two 6-inch, L-shaped metal strips, one on each side of the slots
(fig. 1). These connecting strips were 1- inches high, with an
additional 1/I inch turned inward on the bottom.
Two 6-inch lengths of 1/4-inch bar steel (fig. 1, C) and two
pieces of double-strength window glass li by 6 inches (fig. 1, B)
were cut. One piece of glass was placed between the L-shaped metal
strips connecting the slot openings, and a steel bar was placed on
each side of the glass. The other piece of glass was laid on top
of the bars. When the glass fronts of the cages were lifted above
the slot openings, a tunnel connecting the two cages was formed
(fig. 3). Flies from one cage could then walk over to the other,
the dimensions of the tunnel prohibiting flying across.
One side of each glass piece was treated with an acetone-refined
kerosene solution containing 5 grams of DDT per 100 ml. Only enough
acetone to dissolve the DDT was used. This solution was applied from
a pipette. One inch at each end of each piece of glass was not
treated. The remaining 6 square inches received 0.05 ml. of solution,
or 2.5 mg. of DDT. This deposit is equivalent to 60 mg. of DDT per
square foot. The application was allowed to dry overnight before use.
/ Now with the Office of Experiment Stations.
One empty cage and one well stocked with 2- to 3-day-old flies
were joined by the apparatus. The tunnel was assembled as previously
described, the treated surface of the bottom glass facing up and
that of the top piece facing down. The glass fronts of the cages
were then lifted long enough to allow approximately 100 flies to
cross over. A fly required about 2 seconds to walk across.
Numerous tests were made with the apparatus. In one series of
six tests when the DDT dosage specified above was used, an average
of 100.7-+ 8.6 flies per test crossed the treated glass. The average
number of males per test was 48.8 4.8 and of females 51.8 8.0.
This is quite close to a 50-50 sex ratio. The mean mortality for
these six tests was 46.1 6.3 percent. The mean mortality for males
was 43.0 7.5 percent and for females 45.7 t 7.3 percent.
Relative toxicities of residues of various formulations can be
determined by this method. Studies of the effect of storage on
various insecticides can also be made by comparison with freshly
iriuu iin niin r
Figure l.--A, Metal frame for connecting two
B, Glass slides used in making the tunnel.
to hold the glass slides apart and form the
cubical fly cages.
C, Metal bars used
Figure 2.-Metal frame with the two cages attached. Looking down
on the tunnel.
Figure 3.--Metal frame with the two cages attached, showing com-
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