RAT POPULATIONS AND CONTROL IN TWO ENGLISH VILLAGES

Abstract

Rat (Rattus norvegicus Berkenhout) populations have been studied in two villages in Devonshire. One village had a human population of 266, the other, of 364. The main activity in each village is mixed farming, with poultry and pig keeping on a small scale.From spring 1947 to spring 1950 visits were made to both villages at six‐monthly intervals. At each visit a relative census of the rat populations was taken. This was done either by test baiting, which records only the visits by rats to bait points; or by census baiting, in which a surplus of wheat is laid each day until the amount taken levels off. The level so reached is a measure of the rat population. Test baiting was found to be less reliable, as an index of changes in the rat populations, than census baiting. In some instances treatments, consisting of one, two or three strikes, i.e. poisonings after prebaiting, were carried out after the census.Complete clearance of a whole village was not achieved on any occasion. Lasting reductions of the rat populations were achieved only by comprehensive double or triple strikes. After such treatments the rat populations took more than a year to recover; probably the recovery to the maximum rat population would often have taken 2 years or more had it been allowed to take place.In each village the rats were distributed in discrete colonies, most of them in farms or chicken runs. Poor hygiene and dilapidation of buildings were important factors in promoting rat infestation. When a new source of food became available near an existing infestation, e.g. as a result of the setting up of a new chicken run, rats soon appeared.Apart from rats five species of small mammals visited the bait points: Mus musculus, Apodemus sylvaticus, Microtus agrestis, Clethrionomys glareolus and Sorex araneus. Their presence was detected by their droppings, by characteristic nibbling of the grain, and by trapping. Allowance was made for their presence in recording the results of census baiting.Mouse activity at the bait points was greatest when the rat population was low, but there is no certain evidence that mouse numbers greatly increased when the rats were reduced.It is suggested that, for effective control and the economical use of manpower, treatments in rural areas should always consist of at least two strikes. It may be expected that such treatments, if carried out with a high degree of thoroughness and efficiency, need normally be carried out only once a year; and it might often be possible to leave an area for 18 months or 2 years.