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14
Periodical maintenance for the control of rats in the City sewers continues at quarterly
intervals. The March and June controls were the completion of a series of four treatments
started in September, 1954, comprising direct poisoning with Zinc Phosphide at a concentration
of 10% by weight. These control measures were applied with the object of ascertaining, as far
as possible, if the low level of infestation achieved by direct poisoning with Sodium Fluoroacetate
at quarterly intervals during the three years 1951-1954 could be maintained with Zinc Phosphide,
which is far less toxic, and consequently much safer to dispense and apply.
In the September, 1954 control, 402 sewer entrances were baited, each bait comprising
damp sausage rusk evenly mixed with Zinc Phosphide. The baits were left down for three days
and then picked up and destroyed. Examination showed 7 "takes" (1 x ¾, 2 x ½ and 4 x ¼ or
less). Of a total of 102 lbs. put down approximately 8 ozs. only was "taken." Heavy rain
during the treatment resulted in 16 baits being washed away.
For the second control in December, 1954, 400 entrances were baited and 22 "takes" (2 x ¾,
7 x ½ and 13 x ¼ or less) were recorded and approximately 33 ozs. was taken.
In the third control, March, 1955, 414 entrances were baited, and the recorded result was
15 "takes" (1 full, 4 x ½ and 10 x ¼ or less) and the total weight taken was approximately 22 ozs.
The final control of the series was applied in June, and 405 entrances were baited, and
only 3 "takes" (2 x ½ and 1 x ¼) were obtained, the weight being about 5 ozs. In these treatments
28 bodies were recovered from the base of the shafts or nearly in the flow at the time of
examination.
Subsequent to the final control, the annual test baiting of all available entrances was applied
after a lapse of three weeks, as during the Sodium Fluoroacetate series of controls.
A total of 405 entrances was test baited, each bait being 4 ozs., comprising damp bread
rusk, and left down for three days. The recorded result was 63 "takes" (33 full, 5 x ¾, 19 x ½
and 6 x ¼ or less), and in this instance 101 lbs. of test-bait was put down, and approximately
181 ozs. plus was taken. The test bait "take" subsequent to the final Sodium Fluoroacetate
control was 26 "takes" (15 full, 5 x ¾, 3 x ½ and 3 x ¼ or less) approximately 82 ozs. plus.
Analysis of the sector recordings and the overall result indicated that the low level of
infestation previously achieved had not been maintained. The upward trend and distribution
indicated by the acceptance of the non-poison test baits, while not startling, was significant,
and coincided with the observations of the sewermen, who had noticed an increase in the
number of rats during their routine duties in the sewers.
The increase may be due to several factors, and foremost is poison prejudice produced by the
continued use of Zinc Phosphide at a 10% concentration. In addition, the vast re-building
operations now widespread in the City may be contributary by reason of disturbance of old
drainage, which might tend to scatter the residual rat population into the active lengths of
the sewers.
Following on the results obtained in the Zinc Phosphide series, it was decided that a return
to Sodium Fluoroacetate was desirable, and the September and December controls were applied
on the pattern of the 1951-1954 series.
In the September control 397 sewer entrances were baited. Each bait was 4 ozs. and
comprised bread rusk and water in equal parts by weight evenly mixed with Sodium Fluoroacetate
at a concentration of 1% by weight. A total of 397 entrances were baited and the baits
left down for three days.
On examination, 38 "takes" were recorded (1 full, 9 x ½ and 28 x ¼ or less) and 52 bodies
were recovered from the base of the shafts. This number compares favourably with the Zinc
Phosphide series when the total number found was 28 bodies. As in the previous controls no
doubt others were carried away by the flows. The poison acceptance and number of bodies
recovered does in some measure confirm the sewermens' observations.
The December control result was 12 "takes" of 398 entrances baited (2 x ½ and 10 x ¼ or less)
and 13 bodies were recovered. As far as can be ascertained at this juncture, it appears that
the present control measures are again producing the desired result, and two further controls
will be applied in March and September, 1956.
In conclusion, the graph illustrates the progress made, and the result obtained, in this
phase of Rodent Control."