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From Cecil's Mailbag by the Straight Dope Science Advisory Board

Do ultrasonic bug repellers keep away cockroaches and crickets?

24-Jan-2000

Dear Straight Dope:

My mom has been having some trouble lately with pests: cockroaches and crickets. Do the ultrasonic bug repellers work? --Loveleen

SDSTAFF Doug replies:

In 1977 Cecil answered a question about whether ultrasonic bug repellers work on mosquitoes. Answer: no.  Nothing has changed since.  Absolutely no insects of any type are repelled in any way by ultrasonic devices, nor are any insects of any type repelled by those devices that are supposed to mimic the wingbeats of dragonflies.

The best one can hope for is that if a device sends ultrasonic pulses, then moths that have ears (many do) will flop to the ground temporarily whenever they fly within a foot or so of the device, since that's what happens when a bat swoops towards them and shrieks. I don't think that counts for much as far as being a "repellent."

You might ask:  if these things don't work, how can people still sell them? Isn't that fraud?

Yup. In fact, the Federal Trade Commission has warned consumers about ultrasonic bug repellers on its Website. See http://www.ftc.gov/opa/predawn/F86/sagaintl.htm and  http://www.ftc.gov/opa/predawn/F86/saga2.htm for an example of a lawsuit where people got refunds. The FTC knows these devices don't work, and if you see ads, you should probably report them.

--SDSTAFF Doug
Straight Dope Science Advisory Board