Extended use of rodenticides has lead to resistance for London's mice
Because of the widespread use of poisons to control London's rodent population, mice have become resistant to traditional rodenticides.
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Traditional rodenticides are typically anticoagulants, which kill mice and other rodents through excessive internal bleeding. While there are other types of poisons--not to mention other methods of controlling mice populations--anticoagulant rodenticides are some of the most commonly used.
Mice have always been prevalent in London. Given the city's longevity, mice have had countless generations to adapt to their surroundings. Mice reproduce quickly, a single female can have up to 10 litters per year with 5-6 mice per litter. As people began to fight the mouse problem in London with anticoagulant rodenticides in the 1940s and 50s, the mouse population was initially reduced to a fraction of what it was before. However, scientists observed in the 1960s that the mice that remained after the use of these rodenticides had developed an immunity to the poison--even genetic mutations to protect themselves--and the latter quickly reproduced. A A similar thing happened with a later-developed compound known as bromadiolone. This gave rise to a mouse population in London that was mostly immune to these previously commonly used anticoagulant rodenticides.
Saying that mice are resistant to rodenticides is oversimplifying the matter because there are many poisons, even anticoagulants, that can be very effective in controlling mouse populations. Just because London's mice are now resistant to some compounds like Warfarin and bromadiolone does not mean that this fully excuses or explains the city's current mouse problem.
Rejecting the premises
More on mice: https://bpca.org.uk/a-z-of-pest-advice/mouse-control-how-to-get-rid-of-and-prevent-mice-bpca-a-z-of-pests-/189003#the-dangers-of-mice Rodenticide Resistance Action Group (RRAG): bpca.org.uk/about/partners/rrag