House mouse Mus musculus

A gap the width of a pencil is all a mouse needs to get indoors, which is why they turn up in newly built houses as readily as old farm buildings — droppings along skirting, gnawed packaging and a faint scratching in the loft at night are usually the first signs before anyone’s actually seen one.

Because a single pair can produce dozens of young in a year, we treat mice the same way as rats — inspection, targeted baiting and proofing the entry points they used — rather than a one-off treatment that leaves the same gaps open for the next mouse to find.