View report page

movement and build-up of rats in lengths of sewers not treated for 18 months. The result of this
treatment was 20 "takes" only (3 x ½ and 17 x ¼ or less), the lowest recorded. In each instance
the poison baits, which were 4 ounces each, comprised sausage rusk and water in equal parts
by weight evenly mixed at a concentration of 1% Sodium Fluoroacetate. A feature of the last
treatment was that only three "takes" were obtained from the 118 additional entrances baited.
The decline in the number of poison "takes" and quantitive value was extremely satisfactory,
but the possibility of some poison prejudice having developed could not be over-ruled, and
in order to be reasonably satisfied on this point, and at the same time test the efficacy of the
measures applied, a test baiting of all available sewer entrances was carried out after a lapse
of three weeks. To overcome any possible bait prejudice also, bread rusk moistened with water
in equal parts by weight was used, each test bait being 4 ounces. A total of 417 sewer entrances
were baited and the baits left down for two days. On examination 35 "takes" (seven complete,
3 x ¾, 10 x ½ and 15 x ¼ or less) were recorded. Altogether, 104 lbs. of unpoisoned test bait
was put down, and the estimated amount taken was 72 ounces only. This was a very satisfactory
reduction on the pre-bait recording of December, 1950, when 225 "takes" (185 complete 29 x ½
and 11 x ¼) were obtained and the amount taken was 67 lbs.
The intermediate stage of the series comprised four treatments during the period September,
1952 and June, 1953. The number of entrances baited was approximately 315 in each instance
and the "takes" were consistently low. They were 13, 14, 16 and 11 respectively, and with
one exception were in the half or quarter category.
Subsequent to the fourth poison control of the second series, and after a lapse of three
weeks, 413 sewer entrances were test baited and a total of 105 lbs. was put down and left for
two days. Examination showed 23 "takes" (14 complete, 6 x ½ and 3 x ¼) and the estimated
weight was 71 ounces. This test indicated that the results obtained by the first series had been
maintained.
The third and final stage comprised four treatments during the period September 1953
June, 1954 and as in the second series, the poison "takes" were consistently low, the recordings
being 21, 4, 5, and 4 respectively. The annual test baiting was applied three weeks after
the final poison control, and the recorded result was 26 "takes" (15 complete, 5x ¾, 3x ½ and
3 x ¼) out of 405 entrances baited. In this instance 101 lbs. of test bait was put down and
approximately 82 ounces taken.
During the course of this series of controls, 262 bodies were found at the base of the sewer
shafts or in shallow flows close to the base. This number does not include those found subsequently
by sewermen in their routine work along the lengths of the sewers and in dead ends.
In one instance, 15 bodies were found in a group near to an entrance where a complete "take"
had been recorded, and no doubt many bodies were washed away in rapid flows.
The weather conditions diring such treatments can have an important bearing on the ultimate
result. If there is an appreciable amount of rain prior to and during a treatment this has the
effect of making the entrances very wet and sludgy, resulting in the baits becoming sodden, and
in the event of prolonged and heavy rain a number of baits are inevitably washed away by the
charged up flows.
Fortunately the periods selected for these "direct" controls were, in the main, fine, and of
the 90 days which the controls and tests occupied, only four were sufficiently wet to have an
adverse effect.
From an analysis of the detailed records collated, it did appear that the combination of
quarterly "direct" controls and the use of Sodium Fluoroacetate had produced the results it was
considered could be obtained, and that an extremely low level of infestation had been achieved,
thereby justifying the use of this highly toxic poison, and the labour, care and thought taken in
the dispensing and application.
During the progress of these control measures, the first to be carried out by any Local
Authority in this country, the closest co-operation and fullest exchange of information was maintained
with the Scientific Officers of the Ministry of Agriculture, Fisheries & Food, who had
carried out field trials under controlled conditions, and it was gratifying to learn some two
years later that this method of control of rats in sewers had received official blessing and was
embodied in the text book of Rodent Control issued by the Infestation Control Division.
In view of the low level of infestation now obtained, it appeared to be an excellent opportunity
to ascertain, as far as practicable, if this level could be maintained by "direct" poisoning
with Zinc Phosphide at a concentration of 10%.
Four such controls were applied during the period September, 1954 and June, 1955 and the
poison bait "takes" were 7, 22, 15 and 3 respectively. A similar test baiting to that applied
after the "1080" controls was carried out, and a total of 405 entrances were baited for three
days. The subsequent recorded result was 63 "takes" (33 complete, 5 x ¾, 19 x ½ and 6 x ¼ or
less). The test result after the final "1080" control was 26 "takes" (15 complete, 5 x¾, 3 x ½
and 3 x ¼ or less). In this series of controls 35 bodies were recovered.
15