The control of re-invasion by rats of part of a sewer network

Re-infestation by rats (Rattus norvegicus Berk.) of a section of a sewer network was studied by recording bait consumption in the manhole chambers for periods up to 18 weeks after each of three attempts to rid the section of rats. The mean rate of increase of the rat population was of the order of 20 % per week both when the section was surrounded by a permanently poison-baited zone as a barrier against re-invasion from the connected, still heavily-infested sewers, and when this barrier was absent. When, however, infestation in the connected sewers was reduced by poisoning, and the poison-bait barrier was maintained, the mean weekly rate of increase in the section declined to 4·4% and could be attributed substantially to breeding. It is concluded that the type of poison-bait barrier used in the present trials is insufficient to prevent invasion of one part of a sewer network from another if the population density in the latter is high.