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A control technique at best is usually only about 95% successful, the reasons for this be due to:
1. factors of the environment, i.e.. mild winters,
2. hygiene i.e.. an abundant food supply,
3. variations within a species i.e.. shyness or likes dislikes,
4. and inappropriate building or drainage design.
5. a factor missed whilst undertaking a survey.
Although neither rodent seems to do much to control the environment (other than lining a nest), both species can be extremely adaptable to current circumstances.
We have dealt with problems in extreme dryness and wet, heat and cold and they have thrived in environments which are supposed to be quite alien to them.
Why? Because many micro-environments designed by man has a niche which these adaptive rodents can exploit and thrive to become pests. As examples of adaptively I can recall examples where house mice living in the insulated lining cold rooms have been feeding inside freezers where the temperature is minus 30 degrees Celsius, and within a mile of this location they had adapted to feeding down drainage channels and grown to an average body length of 75 mm -100 mm, the size of hamsters!
Earlier this year there have been reports of brown rats thriving on swan mussels growing on the beds of many of our rivers and streams.
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