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Not all fly sprays get the job done, says Consumer NZ

3 November 2017

Not all fly sprays get the job done, says Consumer NZ

If you’re after a “natural” fly spray, you’ll be disappointed. The only fly spray containing natural pyrethrins in a Consumer NZ test left more than 80 percent of flies still alive.

“We’ve had complaints from our members some fly sprays don’t do the job. Or, if they do knock the critters down, hours later they’re back up,” Consumer NZ chief executive Sue Chetwin says.

Consumer NZ tested 10 aerosol fly sprays from five brands – Black Flag, Expra, Mortein, Raid and Zap. The test timed how quickly a spray knocked down flies and the percentage of flies killed within 24 hours of being sprayed. Six sprays killed 100 percent of the flies within 24 hours, but some products worked much faster than others.

Ms Chetwin says the fastest-acting products knocked down 50 percent of the flies in less than 20 seconds, and 90 percent in less than a minute. The product with natural pyrethrins took 26 minutes to fell 50 percent of the flies, and didn’t manage to knock down 90 percent.

Synthetic pyrethroids or natural pyrethrin, which is made from a species of the chrysanthemum plant, are the main active ingredient in fly sprays. These “knock down” agents interfere with a fly’s nervous system, causing paralysis. However, a fly’s digestive system can break down these ingredients, which is why some recover. For this reason, some products also contain synergists. Synergists don’t kill flies but delay the fly breaking down the active ingredients, giving them longer to work.

Ms Chetwin says how you use a fly spray is also important. “For the best results each fly must be hit directly by a spray droplet, rather than randomly spraying a room.”

The report is available in the November issue of Consumer magazine. More information is available at


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