Notes on the ecology of sewer rats in St. Louis

A study of Norway rat populations in the combined storm drains and sewers of St. Louis, Missouri is described and results are reported. Initially, packets of grain were provided to rats in sewers and drains and checked at two-week intervals during a two-year period. A rather pronounced annual cycle of sewer rat activity was documented, which was rather synchronous around the city. Prevalence of active manhole site via bait take increased from February through June, and then began a gradual but irregular decline to a low point in January. It was postulated that seasonal difference in surface runoff following summer rains may have flushed out the “surplus” rat population that was not attached to stable home ranges containing refugia. Further, winter weather may have led to a sharp decline in active sites during two successive winters. Changes in baiting strategy were initiated, which indicated either that many sewer rats were transient, or that some locations lie outside the regular travel path of rats with stable home ranges. It was also determined that rats hoarded cracked corn, and to a lesser degree, cornmeal; this is important, as bait consumption when seen in pest control operations may not be a reliable indication of rat abundance. Observations of rat behavior, coupled with an analysis of physical factors in the environment, are important in developing an effective control strategy