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11
In view of the low level of infestation obtained, it appeared to be an excellent opportunity
to ascertain, as far as possible, if this level could be maintained by direct poisoning with Zinc
Phosphide at an concentration of 10%, and it was decided to apply four such treatments and a
test baiting during the ensuring year.
Two treatments of this nature were applied in September and December, 1954, and a
similar procedure to the previous series was followed. In view of the vast rebuilding operations
now in progress throughout the City, it was deemed advisable to bait all available sewer entrances,
by reason that several entrances previously baited were no longer in existence and that disturbance
of obsolete sewers and drainage during rebuilding could have the effect of scattering any
residual rats into active lengths of sewers.
In the September treatment 402 entrances were baited, each bait being 4 ozs. comprising
damp sausage rusk evenly mixed with Zinc Phosphide at a concentration of 10% by weight. The
baits were left down for three days and then picked up and destroyed. Examination showed seven
"takes" (1×¾, 2×½ and 4×¼ or less). Of a total of 102 lbs. put down approximately 8 ozs.
only was taken. Heavy rain was experienced during the treatment, and as the result 16 baits
were washed away.
For the second control in December, 1954, 400 sewer entrances were baited, and on examination
22 "takes" (2×¾, 7×½ and 13×¼ or less) were recorded, and of 100 lbs. of bait put down,
approximately 33 ozs. was taken. Heavy rain was experienced on the first day of this treatment,
and allied to the continuous rain during the previous week, had the result of the baits becoming
very wet, and a reasonably true recording was difficult. From these two controls 25 bodies were
recovered.
The efficacy of 10% Zinc Phosphide in maintaining the low level attained by Sodium
Fluoroacetate cannot be ascertained until completion of the remaining treatments, and testbaiting
in July, 1955.
In conclusion, the graph illustrates the breakdown of the pre-bait "takes", which are
indicative of the numerical density and distribution of the rat population from December, 1943, to
May, 1948; the rise which was maintained until December, 1950, and the decline following the
institution of direct poison controls with Sodium Fluoroacetate at quarterly intervals."