Fly spray aerosol cans

Not all fly sprays get the job done

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% of flies still alive.

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“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% of the flies within 24 hours, but some products worked much faster than others.

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

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.”

More information is available in the full report.

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