Catching a rat in a trap isn't a simple set-and-forget procedure. The right sort of bait has to have correct physical qualities as well as aroma.

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Peanut Butter

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Peanut butter is the most popular and effective bait for rats and mice. It's aromatic, tasty and sticks to the trigger of the trap.

Cheese

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Another aromatic option, cheese will certainly attract rats but might not catch them. Cheese tends to separate from the trigger easily, and a rat might steal it without being caught.

Bedding

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In some cases, bits of string might be more of a draw than food. Tying a bit of bacon to the trigger can be a doubly powerful attractant.

Lubricant

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Trap hinges and trigger mechanisms need occasional lubrication to work properly. Don't use machine oil; vegetable oil or a little WD-40 will loosen the action without repelling the prey.

Triggers

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Bait choice might not be the problem. A poorly functioning trap teaches animals to stay away. Test the trap and adjust the trigger for a light touch.