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with 2½ percent zinc phosphide on the third day. 2,855 manholes were
baited. 1,680 showed a prebait take and of these 729 showed a complete
prebait take on one or both days. For the first time the manholes were
inspected on the fourth day for poison take. 1,233 manholes showed
poison take, of which 338 were complete takes.
In the winter treatment, an attempt was made to carry out the
doubling up process recommended by the Ministry of Agriculture and
Fisheries, and amounts of bait deposited were based on results of the
rat treatment of May - June. If in June the poison bait in a manhole
was not taken, or only partially so, the amounts deposited remained at
4ozs., 4ozs., 8ozs. If in June there was a complete poison take at a
manhole, the first day prebait was doubled. If this showed only a
partial take, the amounts deposited were 8ozs., 8ozs., 16ozs., whereas
if it showed a complete take the second day prebait was doubled to
16ozs. If this second 16ozs. prebait showed only a partial take, 16ozs.
of poison bait were deposited on the third day whereas if it showed
a complete take, 24ozs. of poison bait were used. A total of 2,877 manholes
were baited in the winter. 1,063 showed poison takes, 171 of
which were complete.
During the year a new type of poison, which has the effect of
reducing the coagulability of the blood and thereby causing haemorrhages
from which the rat dies, came into use. Such drugs have been used in
medical practice for some years under the title of anti-coagulants.
The main one used at present in rodent control work is Warfarin and
during the year this method was tried at a factory where infestation
had remained in spite of other attempts at control over many months
or years. So far it appears to have worked with considerable success.
It is interesting to note that there is a tendency to criticize the
principles on which the present methods of rat control are based. Even
by the newest methods of poisoning this control remains difficult and
expensive and it has been suggested that the better solution may be found
in the study of the behaviour of rat populations. According to an
annotation in The Lancet of recent literature on this subject, if a few
rats begin to breed in a favourable area their rate of increase will be
slow at first, then more rapid ; but as the area fills up the rate of
increase will slow down until a maximum number is reached. One
implication is that reduction of such a population by about half (a
common result of attempted control) merely brings it to the level at
which it increases most rapidly. When, in Baltimore, half a rat
population was caught by trapping, the survivors rapidly gained in
weight, and within two months the pregnancy-rate had doubled.
For such reasons as these it has been suggested that poisoning and
trapping and such methods may not be profitable and, instead, action
" to increase competition" among the rats themselves has been
suggested. A worker in Baltimore has claimed that by reducing access
to food and clearing away harbourage the rat population of Baltimore
was reduced from 400,000 in 1944 to about 65,000 in 1949.