Rat traps or bait: what works inside the house

Traps or bait inside the house: which to use

Indoors, use traps first. A snap trap kills at once, the carcass is recovered instead of rotting inside a wall, there is no poison lying about for a child, a dog or a cat to find, and every catch tells you which animal you have. Bait earns its place against a large rat population in a roof space or subfloor nobody can reach, and only in a lockable station; anticoagulant bait takes several days, so nothing poison-based is immediate.

The honest trade-off is attention. Traps have to be checked every day, and a trap left sprung over a dead rat for a week is worse than no trap at all, whereas a bait station asks much less of your time but hands the choice of where the animal dies to the animal, which is usually somewhere you cannot reach. In a lived-in house, with a kitchen and bedrooms under the roof space, we would rather do the daily checking.

Mice settle the question even more firmly. A few mice in a kitchen is a trapping job every time, because a mouse seldom strays far from its nest, so a poisoned one usually dies inside the building, behind a cupboard or in the wall beside it, and the first you know of it is the smell.

Where the traps go, and why most DIY trapping misses

The most effective trap is the one sitting on the run. Rats and mice travel along walls and edges rather than across open floor, so a trap goes flat against the wall, at right angles to it with the trigger end almost touching, wherever the droppings and rub marks are. Space rat traps 3 to 6 m apart and mouse traps no more than about 3 m apart, ideally in pairs set a few centimetres apart.

In a roof space the runs are on the timber. Ship rats travel the top plates, the beams and the pipes rather than the ceiling lining, so traps go along whichever piece of timber has droppings on it, and University of California guidance suggests fixing a trap to the surface it sits on, so that the force of the snap is not wasted moving the trap. Norway rats work at floor level, so in a subfloor the traps sit along the foundation walls and beside the piles, where the soil has been disturbed.

Mice keep to small territories and seldom go more than about 9 m from their nest and their food, so their traps go close to where they live: behind the fridge, under the sink where the pipes come through, at the back of the pantry, and along the skirting of whichever room has the most droppings. A trap in the middle of the kitchen floor catches very little.

Then use more traps than feels necessary, and check them every morning. The aim, in the University of California's phrase, is a trapping period that is "short and decisive", and a dozen traps working for a week does far better than two traps left out for a month, both for the result and for how wary the survivors become.

Rat bait in New Zealand: what is sold, and what each type means

Most rat poison sold in New Zealand is an anticoagulant. The first generation actives, coumatetralyl, diphacinone and pindone, need several feeds and carry a lower risk of secondary poisoning, while the second generation actives, brodifacoum and bromadiolone, kill after a single feed and persist in the carcass. Cholecalciferol, a form of vitamin D3, works differently. None of them is instant, so the bait that works fastest is the wrong question inside a house.

Blocks, pellets and paste are formulations rather than strengths, and the choice between them is about where the bait has to sit and how the station holds it. What matters more is that the rat eats enough: one that takes a meal too small to kill it learns to avoid that flavour, the bait shyness that makes a half-finished baiting job harder for whoever comes next.

Where we use bait, every station is lockable and fixed in place so that it cannot be dragged or knocked over, every placement goes on a written map, and every station is recovered at the end. Persistent bait also travels up the food chain, and University of California guidance notes that a dead rat can carry enough poison to kill several more animals. What persistent bait means for a dog that finds a poisoned rat is set out in what rat bait means for dogs, cats and children.

A garden trap line is a different job

Backyard trapping of the kind Predator Free NZ and the Department of Conservation promote exists to protect native wildlife, with rat traps in tunnels set along a bank or a fence and checked over months and years. It is well worth doing, and Auckland has a great many community trap lines, but it is a different job from clearing rats out of a house, which is about the runs inside the building and the gap they use to get in.

The two meet on a section beside a reserve or a gully, where a trap line along the boundary cuts the supply that keeps refilling the house. It is also one more reason we keep persistent bait out of the open, because a poisoned rat dying in the garden can be eaten by a morepork the trap line is there to protect, or by a stoat or a cat. If the animal in your garden is a possum, that is a different animal and a different job, and a rat trap is not the tool for it. Predator Free NZ's guide to backyard trapping is the place to start a line of your own.