Belinda Castles shares the Consumer NZ findings on fly sprays.
The following 6 products resulted in 100 percent mortality within 24 hours in the Consumer NZ study:
- Mortein Fast Knockdown Multi Insect Killer
- Black Flag Rapid Kill Blowfly Strength Fly & Insect Killer
- Raid One Shot Multipurpose Insect Killer Odourless
- Expra Fly & Insect Spray Low Odour
- Zap Multi Insect Killer Odourless Low Irritant
Mortein Fast Knockdown Multi Insect Killer (13 cents per spray) and Black Flag Rapid Kill Blowfly Strength Fly & Insect Killer (15 cents per spray) killed 90 percent of flies in less than a minute.
This may be partly because these sprays are around twice as dense as the others (i.e. spraying more with each squirt) so have about double the active ingredients, Belinda says.
"That means you're going to go through a can a lot faster, but they worked a lot faster because the fly is getting a bigger dose."
Zap Multi Insect Killer Odourless Low Irritant is a cheap but slow-acting option, she says.
"It's only 5 cents a spray, but you have to wait 5 minutes.
"It kind of depends what your priorities are – get rid of the flies quickly or save a bit of money."
Aerosol fly sprays kill with a combination of two ingredients.
The main active ingredient is a 'knock down agent' (natural pyrethrin or synthetic pyrethroid) which affects the nervous system of the fly and paralyses it.
Because sometimes the fly's digestive system can break this down, fly sprays also contain a synergist (such as piperonyl butoxide or N-octyl biocycloheptene dicarboximide) which slows down the fly's digestive system while the 'knock down' gets to work.
While aerosol fly sprays are classified as a hazardous substance, there is no evidence they cause harm to humans when used at usual household levels, Belinda says.
But if you have fish at home, cover the fish tank while you're spraying as pyrethins and pyrethroids are toxic to fish.
The only spray in the test containing natural pyrethrins was Raid Earth Options Fly & Mosquito Killer, but after 24 hours it had only killed 19 percent of flies.
Belinda says fly paper "does the trick" for her family in Gisbourne.
She recommends making your house as unwelcome to flies as you can.
"Cover your rubbish, keep your compost bins as far away from the house as you can, don't leave stuff lying out on the bench… mesh fly screens are good. Also, flies don't like wind so have some fans blowing around."