I have a constant problem with cats using my garden as their lavatory and the roof of my car as a sunning spot. I have seen various sonic cat repellers advertised at prices from £20 to £60. Are they any good?

PB, by e-mail

Sonic and ultrasonic pest repellers scare an animal with a sudden, loud noise that only the pest can hear. As well as devices to scare off cats, you can also get products to deter dogs, rodents, moles, cockroaches, even ants.

Ultrasonic technology promises an appealing but by no means infallible solution to your problem. The flaw with ultrasonics as a repellent technique is, the higher the frequency, the shorter the distance it will travel, and the more readily it will be absorbed by anything in its path – so its efficiency is limited. Also, much more energy is required to broadcast ultrasound than frequencies in human hearing range, so the cost of running a device can mount up. If you want to go ahead, then look for references that cite scientific trials.

Other methods to deter wandering cats from your garden include a device that squirts a jet of water at the offending animal. Grovelands (www.grovelands.com) has three examples for sale, priced from £45. Or you can try lion dung: Grovelands has pellets at £9.99 per pack.

Richard Strand runs Pest Information Consultancy; rstrand@pestinfo.co.uk

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