Fleas
Do flea bombs work?
What a flea bomb actually reaches, why it misses the cocoon stage and everything under the furniture, how to set one off safely, and what to do instead.
The short answer
Partly. A flea bomb kills the adult fleas its mist lands on, and most sold in New Zealand also carry an insect growth regulator that stops eggs and larvae developing where it settles. What it cannot do is get under beds and sofas, deep into carpet pile or into cracks, and it does not touch fleas still in their cocoons, which hatch over the following weeks. On its own it rarely ends an infestation, which is the gap professional flea treatment is planned around.
At a glance
What a flea bomb reaches
| Where the fleas are | Does the fog get there? | Why |
|---|---|---|
| Open carpet in the middle of a room | Yes, the surface | The mist rises, then settles onto open floor |
| Deep in carpet pile | Poorly | Settling droplets stay near the tips of the fibres |
| Under beds, sofas and cabinets | Little or none | The furniture shields the floor beneath it |
| Pet bedding and sofa cushions | Only the exposed face | Folds, seams and undersides stay untreated |
| Cocoons (pupae) | Not effectively | The cocoon shields the flea; adults emerge later |
| Fleas on the pet | No | That is your vet's job, not a fogger's |
What a flea bomb actually is
A flea bomb, also sold as a fogger or bug bomb, is a total release aerosol: you press the tab, put the can down and leave, and it empties itself in one go as a fine mist that rises, hangs in the room and then settles. The flea bombs on New Zealand shelves typically pair a pyrethroid insecticide, permethrin, with an insect growth regulator, pyriproxyfen, pushed out by a hydrocarbon propellant, which is why the can carries a warning that it is extremely flammable.
Formulas differ between products and change over time, so the label on the can you buy is the one to follow, both for what is in it and for how to use it. It is also the legal instruction for that product.
Do flea bombs really work?
They work on what they reach, and the table above sets out how little of a house that is. The mist is released upwards and comes down on open, horizontal surfaces, so the top of the carpet at the centre of a room gets a dose while the places fleas actually concentrate, under furniture, along skirting, deep down in carpet and inside pet bedding tucked into a corner, get little or none.
University of California pest management guidance makes the same point from the other direction: room foggers do not give the coverage or lasting effect of a direct spray unless they contain methoprene or pyriproxyfen. So the growth regulator in current New Zealand bombs is a real improvement on older products, because it keeps working on eggs and larvae where it lands. It still only lands where the fog falls.
Why fleas come back a week or two after a bomb
Because of the cocoon. Flea pupae sit inside a silk cocoon coated in dust and carpet fibre, and that cocoon protects them from insecticide sprays and fogs alike. Inside it, fully formed adults can wait for weeks or months until vibration, warmth or the carbon dioxide in your breath tells them a host is near, then they emerge together. A bomb is often followed, about a week later, by a fresh wave of biting fleas.
The growth regulator helps with the next generation, since eggs laid onto a treated surface fail to develop. It does nothing to adults already formed inside their cocoons. If you cannot tell whether what you are seeing is that late hatch or a population that is still breeding, there is a guide to how to tell whether fleas are still breeding, from flea dirt to larvae.
Is it better to spray or bomb for fleas?
A targeted surface spray that contains a growth regulator usually beats a bomb, because you put it where the fleas are: under and behind furniture, along skirting boards, on the carpet where the pet rests and around the base of the sofa. The one real advantage of a bomb is that it takes no effort. If you use one, treat it as a single step in a plan rather than the plan itself.
That plan starts with the pet and the vacuum cleaner, not the can, and we have set out the order that actually clears a house, with the reason for each step.
How to use a flea bomb safely
Read the label first, because it is the legal instruction for that product and the figures below are typical rather than universal. Then work through the essentials in order, starting with anything that could ignite the propellant.
Turn off gas appliances and put out pilot lights and any naked flame. Keep the can well away from anything that sparks or switches itself on, such as a fridge or a heat pump; the US Environmental Protection Agency advises at least six feet, roughly two metres, from every ignition source.
Use one can per room and never more than the label allows. Do not set one off in a small closed space such as a cupboard or wardrobe; New Zealand labels typically rule out any space under 5 m³. Too much product in too small a space is how the flammable vapour builds up to the point where fires and explosions happen.
Get everyone out, including pets and birds, and cover fish tanks and switch off their air pumps. Stay out for the time on the label, typically two to three hours for New Zealand products, then go back in only to open the doors and windows, and leave again for about thirty minutes while the house airs. Wash down kitchen benches and food preparation surfaces afterwards.
If someone swallows the product, breathes in a lot of it or gets it in their eyes, ring the National Poisons Centre, free at any hour, on 0800 764 766. Call 111 for anyone having trouble breathing.
What to do after a flea bomb
Air the house as the label says, wipe kitchen benches and anything food touches, hot wash your bedding and all pet bedding, then start vacuuming every day for two to three weeks so the cocoons hatch onto the treated carpet. You can sleep in the bed once the room has been aired. You do not need to wash every surface in the house, but you do need the pet treated through your vet that same week, or the bomb is wasted.
Vacuuming matters more after a bomb than before it. The vibration and pressure of the vacuum are among the cues that bring fleas out of their cocoons, and a beater-bar vacuum also lifts out a share of the eggs and larvae. Tip the bag or canister into a bag you can tie off, and take it straight out to the bin each time.
When a bomb is not worth buying
There are situations where a flea bomb is money spent for a week of relief. A whole house with carpet throughout is one, because every room needs its own can and the fog still misses under the furniture. A cat or other animal living under the house is another, since the fleas are breeding where no bomb will ever reach. A rental changeover that needs a written treatment record is a third, and so is a second bomb after the first one seemed not to work, because the reason it failed has not changed.
In those cases a treatment that puts a residual insecticide and a growth regulator where the fleas actually live is usually cheaper than a run of cans. We publish what a house flea treatment costs, so you can compare it with the price of a shelf of bombs.
Questions
Flea bomb questions
How long do you have to stay out after a flea bomb?
For the time on the label, which for New Zealand flea bombs is typically two to three hours. Then open the doors and windows and leave again for about thirty minutes while the house airs. If anyone in the household has asthma or another breathing condition, or is pregnant, ask a pharmacist or GP before using one, and consider staying out longer.
How many flea bombs do I need for a house?
One per room, and no more than the label allows. A can states the floor area it covers, and some New Zealand products claim up to about 100 m², but walls and closed doors break a house into separate spaces, so rooms are the unit rather than total floor area. Extra cans in one room are how flammable vapour builds up.
Do flea bombs kill flea eggs?
The insecticide in them does not reliably kill eggs. The growth regulator that most current flea bombs contain stops eggs and larvae developing on the surfaces where it settles, which is useful. Neither part does anything to fleas sealed inside their cocoons, and neither reaches eggs under furniture or deep in the carpet.
Are flea bombs safe for pets?
Not while they are in the house. Dogs, cats, birds and other animals must be out of the building, and fish tanks covered with their air pumps switched off, until the house has been aired as the label directs. A bomb also does nothing for fleas on the animal itself; that needs a product your vet recommends.
Can a flea bomb start a fire?
Yes. The propellant is flammable, and fires and explosions have been caused by foggers set off near a pilot light, a gas flame or an appliance that switches on by itself, or by using too many cans in a small space. Turn off gas appliances, put out pilot lights and keep the can about two metres from anything that could spark.
What is the best flea bomb in NZ?
We do not recommend brands. If you buy one, choose a product that lists a growth regulator, such as pyriproxyfen or S-methoprene, alongside the insecticide, use one can per room, and treat it as one step in a wider plan that includes the pet, daily vacuuming and hot washing rather than as the whole answer.
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