Recently British TV has featured ads for a product called the JML Pest Shield, a plug-in device which purportedly "creates a digital forcefield that helps drive away rats, mice and cockroaches from your home". The advert is here at tellyads.com.
There was another version of the ad - gone now, see below - at JML's own sales page for Pest Shield, which expanded the claim, saying thay it "transforms the wiring in your home into a giant digital pest repellent. This digital pulsing forcefield drives away mice, rats and crawling insects from your home by interfering with their nervous system". Interestingly, its online press release (PDF) has an entirely different explanation for the mode of operation, saying "It irritates them by emitting a sound that fluctuates from high to low frequency. This change in sound irritates insects and ruffles rodents". So which is it? You'd think at least they could agree on a consistent story. Also interestingly, "because the Pest Shield attacks the rodent-based nervous system, it's important you don't use it around hamsters, gerbils, guinea pigs, tarantulas or chinchillas".
Such devices are widespread, but it doesn't take much research to find that there is little or no evidence for their effectiveness. Electronic Rodent Repellent Devices: A Review of Efficacy Test Protocols and Regulatory Actions looks at the history of such claims, particularly a fad in the 1970s for devices "advertized as capable of generating their own magnetic fields or distorting the earth's magnetic fields in such a manner that animal pest species (but not beneficial species) stopped eating, drinking, and reproducing". Thes devices were marketed without efficacy data, because there wasn't any. For instance, Commensal rodents from the Utah State University wildlife management series concludes: "many devices which produce electromagnetic fields have been marketed as an effective rodent repellent. Again, however, scientific evidence clearly shows that these devices are not useful in repelling rats or mice".
A recent article, People And Rodent Pests in PCT Online, a rodent control trade magazine, likewise concludes "Ultrasonic and electromagnetic rodent repellent devices are available on the consumer market but good data supporting their effectiveness is lacking. Controlled studies with commensal rodents have been generally negative". In the USA, at least one manufacturer has been charged by the Federal Trade Commission - see FTC Swats Lentek's Claims - as making false and unsubstantiated claims for electromagnetic pest repellent devices, And in Australia, the Australian Competition and Consumer Commission obtained a punitive undertaking, including customers' right to refund, after judging that a direct marketing firm, Danoz, had made misleading claims of efficacy for devices of this type. See the undertaking register for Danoz Direct Pty Ltd - now that is consumer legislation with the teeth the ASA here ought to have.
In short, even if you hadn't guessed from the pseudoscientific description that the makers aren't even consistent about (and thinking a tarantula is a rodent isn't a good sign either) in my opinion Pest Shield is almost certainly a total crock. I've shopped it to the ASA, but unfortunately a quick Google on electromagnetic pest repeller shows it's far from alone.
Addendum, Nov 6th: I just had a letter from the ASA saying they're already investigating this issue, and have added my comments to the portfolio.
Addendum, Nov 29th 2007: the ASA has just published its adjudication on the JML TV ad for Pest Shield - see JML Direct Ltd t/a Shop Now TV, 28th Nov - upholding all complaints on grounds of Evidence (efficacy being unproven) and being misleading, as well as falsely denigrating other products. Although this ruling has no jurisdiction over video ads published on their own website, I notice that Pest Shield has been removed from the JML Direct shopping site.
Addendum: I'm sorry; I don't normally redact comments, but I'm nervous about the legality of hosting anecdotes that might be read as allegations of criminal negligence. So no stories of e.g. how a Pest Shield exploded and took out half of a city block. If something like this did happen to you, best move is to take it up with your Local Trading Standards people, who handle product safety issues.