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10
Periodical maintenance treatment for the control of common rats in the City sewers continued
and direct poisoning with "1080", Sodium Fluoroacetate, was again the method of control.
Four treatments were applied at quarterly intervals in March, June, September and December.
The annual full scale non-poison test-baiting was carried out 3 weeks after the June control
Altogether 12 controls by this method have been applied since May, 1951, and 2 treatments
plus a full-scale test-baiting in 1954 will complete the series of 14 controls spread over three
years. This work was undertaken in conjunction with the Technical Officers, Infestation
Division, Ministry of Agriculture & Fisheries.
For the March control 327 sewer entrances were poison baited, and 16 "takes" (4 x ½ and
12 x ¼ or less) were recorded, each bait being 4 ozs. The June treatment was the yearly full
scale control, and 407 sewer entrances were baited. Examination showed 11 "takes" (1 full,
1 x ½ and 9 x ¼ or less). Following a lapse of three weeks, the annual complete test baiting
was applied, and 413 sewer entrances were baited with unpoisoned bread rusk, each bait being
4 ozs., a total of 105 lbs. The test baits were left down for 2 days, and on examination only
23 "takes" (14 full, 6x½ and 3x¼) were recorded, the weight taken being approximately 60 ozs.
The result of the test baiting appears to be very satisfactory when compared with the pre-bait
"takes" of the December, 1950 treatment, prior to the present system of control. In that
instance, after a twoday pre-bait, 225 "takes" (185 full, 29 x ½ and 11 x ¼) were recorded a total
weight of over 60 lbs. being consumed.
In the September control 315 sewer entrances were baited and 21 "takes" (1 x 3/4, 2 x½ and
18 x ¼ or less) were recorded. Of the 18 "takes" classified as ¼ or less, only 4 really came within
the category of 1 oz. taken, the remainder being recorded solely by reason of a body being found
near the bait. It was estimated that only 13 ozs. of the 80 lbs. of poison bait put down was
consumed. The estimated amount "taken" in the first control of the series May, 1951 was
187 ozs., and during the last control by the old system, December, 1950 approximately 430 ozs.
of Zinc Phosphide bait. In previous controls Sausage rusk was used as the bait base for the
poison and Bread rusk for test baiting. For this treatment wheat was used, being available for
the first time owing to derationing of animal feeding stuffs. The wheat was soaked in a water
solution of "1080", at a concentration of 1%, for 24 hours.
For the December control 311 sewer entrances were baited and 4x ¼ "takes" were recorded.
The overall result, so far as it can be ascertained, shows that a low level of rat infestation
in the sewers has been achieved and maintained. The sewermen who assist in these treatments
state that it is rare for them to see a rat during the course of their routine duties.
The graph illustrates the breakdown of the pre-bait "takes", which are indicative of the
density and distribution of the rat population, from December, 1943 to May, 1948; the rise
which was maintained until December, 1950; and the rapid decline following the introduction
of direct poisoning with "1080" Sodium Fluoroacetate at quarterly intervals.