Rats and Mice
Through the summer months, the natural population of rats and mice rises as increasing quantities of food such as fruits, nuts, and grains are available to them. Once autumn arrives, food sources dwindle and temperatures drop. Consequently, rats and mice then begin to look for shelter and alternative food supplies. All too often they find both in our homes, sheds, chicken houses, stables, and stores. Also, all too often we don't do anything about this inevitable invasion, at least not until they have moved in, set up home, and begun to raid the food available.
Rats and mice are well known to gnaw on cables causing short circuits and sometimes fire. They will gnaw pipes causing floods. They continually dribble urine and defecate regularly; mice produce up to eighty droppings per day. They will eat our and our animals' food which is a risk to health. And they cause consternation to those with a natural fear of rodents.
Now is the time to prepare for and prevent rats and mice from moving indoors to live with you or your animals. In this article you'll find simple steps to staying free of rats and mice.
Identifying Common Rodents
New Zealand is home to four introduced rodent species:
House Mouse (Mus musculus): Small, with a pointed snout and large ears. Common in homes and outbuildings.
Norway Rat (Rattus norvegicus): Larger, with a blunt snout and robust body. Often found in basements and sewers.
Black Rat (Rattus rattus): Slender, with a pointed snout and large ears. Prefers warmer areas like roofs and trees.
Kiore (Rattus exulans): Smaller, with a pointed snout and large ears. Found in coastal and forested areas.
Health Risks Associated with Rodents
Rodents can carry and transmit several diseases, including:
Leptospirosis: A bacterial infection that can cause flu-like symptoms and liver damage. Transmission occurs through contact with urine or contaminated water.
Salmonella: Bacteria that can cause gastrointestinal illness. Rodents can contaminate food and surfaces.
Hantavirus: A rare but serious virus transmitted through rodent urine, droppings, or saliva.
Rat-bite fever: A bacterial infection transmitted through bites or scratches from infected rodents.
Proofing
Survey
Stop rats and mice from finding a way into your home or other building. Take half an hour to walk around the house and outhouses looking for ways rats and mice might get in. Take a pencil with you to write down what needs to be done and use it to measure gaps. If a pencil can slip under a door a mouse can get under it. Mice and rats can flatten their rib cages to fit through gaps, if their head can fit through, their body can follow.
Seal
Gaps under doors can be sealed with a draft excluder. Gaps around pipes and cables can be filled with mortar, filler, or polyurethane foam. Although rodents can eat through the foam, and even concrete, they won't unless the sealed gap is somewhere they have travelled many times before.
Sheds and outhouses, particularly old ones, may have many possible entry points. It may not be possible to seal them all, but I would recommend sealing as many as you can. It all helps to reduce the risk of rodents finding their way in.
Look in places both high and low. Rats and mice are excellent climbers. Roof rats are better climbers than the larger Norway rats and spend a lot of time feeding in trees. They will walk along branches that overhang roofs and drop down onto the roof. From here they rarely have difficulty getting into the roof void via gaps around the eaves. Cut overhanging trees away from the roof and also trim climbing plants such as rambling roses, vines, ivy, honeysuckle, etc. to at least 30cm below the overhanging eaves.
Air vents are necessary for the good ventilation of subfloors and should not be sealed. Undamaged vents are usually designed to be mouse-proof. Repair damaged vents or cover them with fine mesh. Also, look for gaps where pipework and cables enter buildings and gaps where weatherboards do not fit flush. These can usually be filled with silicone sealers, waterproof fillers, or foam fillers.
It is also worth taking a look around the interior of the building for gaps that can be sealed. Make the building less attractive to rodents by making it harder for them to travel through. Sealing gaps around pipe work and cables are particularly helpful as the cables and pipes often run through walls making great highways for rodents.
Rats and mice are usually active at night and are not likely to gain entry through open doors and windows during the day. The exception might be when they are under real pressure to find food and shelter. Then they may take the risk of venturing out during the day. So keep doors and windows closed at all times if possible.
Remove Cover
As mentioned previously rodents do not 'like' to be in the open. So don't give them cover around your home or buildings. Don't let the grass grow too long. Don't have piles of timber, corrugated iron, etc. piled against walls. Have a perimeter of open ground that rodents would have to cross to get to your buildings.
Remove Food Sources
Remove all food sources. In your home check for food that might have been spilled behind the fridge or under other furniture. Make sure the cereals, flour, rice, and other foodstuffs in your larder are put in sealed containers.
In sheds, stables, chicken houses, and stores put all food in sealed containers. Use metal containers if possible. Rodents can gnaw through the plastic with ease but plastic bins are better that paper bags or no covering at all.
Feeding troughs can sometimes be placed so that the animals being fed can reach the food easily but rodents will find it more difficult. It is often not possible to prevent rodent access entirely, but it is worth making it as difficult for them as possible. E.g. hang chicken feeders from a cable. Make the rodents jump up or climb down to reach the food.
Baits and Traps
Nowhere can be made 100% proofed against rodents and you may already be too late to stop rodents from entering. So place baits and traps in position NOW. This will also mean that the baits and traps are ready for any rodents that gain entry by some route not found during your search. Deal with them before they become a problem.
Bait
Almost all toxic rodent baits contain anti-coagulant (AC) toxin related to the original warfarin. However, there are two main groups of anti-coagulant; first and second-generation ACs.
Second-generation ACs such as brodifacoum and bromadialone work more quickly but the toxin lasts longer in the dead or dying rodents. This means there is an increased risk of secondary poisoning of animals that might scavenge on them.Therefore these baits should be used with caution outdoors.
First-generation ACs such as coumatetralyl can be as effective, but they are safer and eco-toxic because they pose a lower risk of secondary poisoning. So safer for pets and there is a reduced persistence of the toxin in the environment. These first-generation ACs are rarely used as they've been overtaken by second-generation options.
If toxic baits are not an option there is a natural organic certified rodent bait that is effective but does not contain any toxin. It is therefore safe around pets. The bait is lethal to rodents because they are alone in not having the correct enzymes to digest the cellulose that the bait is made from. Note: this bait is only effective in dry conditions and where other food sources can be removed.
Baits come in several forms but most are based on grains. Some bait blocks are waxed to make them weatherproof and last longer outdoors or in damp conditions. There is also a bait a tracking powder available. This product is useful where other food sources cannot be removed meaning it is difficult to get the rodents to eat bait. The bait may be eaten or the rodents may take in the toxin after running through the tracking powder and licking it off their paws or fur.
Place baits in areas where rodents will travel. Place the baits undercover in bait stations, in a length of pipe, under blocks, in roof voids, etc. Rodents prefer to feed while undercover and cover to keep non-target animals such as pets and birds away from the bait.
Roof rats hoard food. So baits should be crumbled or fixed in place so that they cannot take the bait to their stash and must eat it where it is.
Regulation
Second-Generation Anticoagulants and Increased Regulation
As mentioned, second-generation anticoagulants (SGAs) such as brodifacoum and bromadiolone are highly effective in controlling rodent populations due to their potency and long-lasting effects. However, their use is increasingly being regulated due to the risk of secondary poisoning.
What is Secondary Poisoning?
Secondary poisoning occurs when a non-target animal, such as a pet, bird of prey, or scavenger, eats a rodent that has ingested poison. Because SGAs stay in the rodent's system for a long time, they can be transferred to predators or scavengers that eat the poisoned rodents. This poses a significant risk, especially to wildlife such as native birds, including kaka and kea, as well as domesticated pets like cats and dogs.
Why is Regulation Increasing?
The growing concern around secondary poisoning has led to more stringent regulations around the use of SGAs. Environmental groups, wildlife conservationists, and regulatory bodies like the Environmental Protection Authority (EPA) in New Zealand have increasingly called for restrictions on the use of these poisons. These concerns are particularly focused on:
Environmental Impact: The prolonged persistence of SGAs in the environment means that poisoned rodents can remain toxic for an extended period. This increases the chance that other animals, especially scavengers, will consume the poison.
Impact on Non-Target Species: Although SGAs are designed to target rodents, their potency can lead to poisoning in non-target species that are not typically targeted by rodenticides. For example, predators such as harrier hawks, which feed on rodents, can ingest enough poison to cause harm or death.
Regulatory Responses: As a result of these risks, New Zealand has tightened regulations on the use of second-generation anticoagulants. For instance:
Aerial Application Restrictions: Some regions have restricted or banned the aerial use of SGAs to limit their impact on the environment.
Bait Station Use: The use of SGAs is now more strictly controlled through the use of bait stations, which help limit exposure to non-target species. However, even with bait stations, there’s still the potential for secondary poisoning if the carcasses are not managed correctly.
Focus on Safer Alternatives: There’s an increasing focus on developing safer alternatives for rodent control, such as cholecalciferol (Vitamin D3), which has a lower risk of secondary poisoning compared to SGAs. Additionally, trapping is being promoted as a more humane and safer option, particularly in residential areas or places where the risk to pets and wildlife is high.
What This Means for Rodent Control
Farmers, pest control operators, and homeowners must be more cautious in their use of SGAs. They should:
Use bait stations to contain the poison and prevent exposure to non-target animals.
Monitor and remove carcasses regularly to prevent scavengers from ingesting poisoned rodents.
Consider alternatives such as cholecalciferol or mechanical control methods like trapping, especially in areas with high wildlife activity or where pets are present.
The increasing regulation of SGAs reflects the growing concern over their environmental and animal welfare impacts. While these poisons remain an effective tool in rodent control, it is crucial to use them responsibly and in accordance with updated regulations to mitigate the risks to non-target species.
Traps
Traps are rarely sufficient on their own to get rid of an infestation of rats or mice. However, they will be effective at catching 'dopey' rats and mice that have had a feed of bait, they are good for additional control in areas where toxic bait cannot be used and where you want to be sure the bodies can be removed. Recent innovations have improved trap efficiency and ease of use
Snap Traps: Quick and effective, these traps kill rodents instantly. Modern designs are more humane and easier to set.
Live Traps: Capture rodents alive for relocation. However, they require regular monitoring and are less effective in high-density infestations.
Self-Resetting Traps: Devices like Goodnature traps automatically reset after capturing a rodent, allowing for continuous control without frequent checking.
The best lures for traps are peanut butter or chocolate but these have a limited life and need to be replaced regularly. Placed correctly traps are almost as effective without bait. Place traditional snap traps at right angles to, and against walls where rats and mice would travel. Rats and mice are generally reluctant to move into open space and prefer to stay close to some vertical surface or stay in enclosed places.
It's important to note that glue traps have been banned in New Zealand since 2015 due to concerns over animal welfare and inhumane killing methods.
Repellents
Electronic, electromagnetic, and sonic devices can be useful at encouraging rats and mice to choose next door to live, but should not be seen as a way of getting rid of rodents that have already set up home. Rats and mice are wary of new things in their environment and will be quietened by such devices for a short period. But they become used to new things after about two weeks. It is better to stop them from entering in the first place. It would take a lot to make you move out of your home!
Be Pro-Active
Don't wait for a problem to arise. Put checking rodent defenses on your list of regular things to do. Checking and replenishing, traps, baits, entry point seals and housekeeping should be done at least every three months, and more often where rodents are common.
Late summer and early autumn is the time of the year when rodents are most likely to invade. But be ready for them all year round.
Follow these simple steps and you can sleep soundly in the knowledge that you are not sharing your home with rats and mice.
‘The Pest Advice’ – by David Brittain (updated May 2025)
