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Ultrasonic repellents are one type of questionable control device. They seem like an attractive choice for flea control, because they're simple, non-chemical, environmentally friendly, have an invisible effect, and require no maintenance besides batteries. Manufacturers have exploited the public's desire for such devices, with claims of repelling domestic insect pests, as well as a variety of other pests like mosquitoes. However, there’s no evidence that they're effective
Ultrasound is high frequency sound inaudible to humans (above 20 kHz). Some insects produce and receive acoustic signals for a variety of functions including alarm, courtship, and avoidance. Certain moths, for example, will avoid ultrasound in the 20-40 kHz range, because it's used by many insectivorous bats in prey detection.
Device manufacturers cite the literature on how moths and crickets avoid ultrasound. They then falsely claim that the same concept is valid for controlling household pests. Although moths have evolved organs to detect ultrasonic frequencies generated by bats, household insects haven't been subjected to the same evolutionary pressures. There’s no reason to suggest, and little evidence to show, that ultrasound repels domestic pests like cockroaches, silverfish, mosquitoes, fleas, or ticks.
It's unlikely that domestic pests have ultrasonic receptors. However, it's been suggested that fleas may detect sound frequencies between 100 to 1000 kHz. They may use ultrasound to communicate, for example advertising a suitable host's location.Still, even if it's true, this is far above the 20 to 60 kHz output of commercial ultrasonic pest repellers.
All fleas possess a complex sensory apparatus, called a sensillum, on the posterior end of their abdomen. This structure consists of an array of numerous, long, thin trichobothria and microtrichia (‘hairs’). The sensillum’s function isn’t known, but it may be a mechanoreceptor to aid in mating alignment. Or it may detect air movement (e.g. host exhalation). The sensillum may also detect ultrasound. When feeding, fleas are face-down in host tissue. The abdomen extends upwards, with the sensillum being the uppermost surface, a good position for receiving sound. Spiracles (respiratory openings) and microtrichia on abdomen may create ultrasound. And the sensillum may serve as an ultrasonic receptor.
Ultrasonic devices, such as collars, are sometimes marketed for flea control. They are reputed to prevent fleas, repel fleas off animals, or kill fleas. While repellency is most claimed, numerous other effects have also been studied. Studies have examined ultrasound's ability to repel and kill fleas, as well as inhibit feeding, mating, egg laying, and maturation of eggs and larvae.
At the end of one trial, fleas were observed within 1 cm of the transponder. Similar studies on cockroaches and ticks have found the pests near, or nesting inside, the ultrasonic devices.
Are fleas killed by ultrasound?
Ultrasound has no effect on the mortality of adult fleas.
100 actively reproducing female fleas were exposed to ultrasound. Blood consumption didn't differ from the control group, showing that feeding is not inhibited. Likewise, egg production isn’t affected by ultrasonic frequencies. The number of eggs produced on a cat wearing an ultrasonic collar did not differ from a cat not wearing a collar. This also shows that fleas were not repelled or killed, and were able to successfully mate.6 A similar study saw fleas continuing to feed and reproduce normally while exposed to ultrasound.
Ultrasonic repellers don’t effect how quickly flea eggs and larvae mature. Eggs reach the pupal stage just as quickly as control groups. In one study, an ultrasound-treated room produced a very large number of fleas by the end of the trial, showing that they continued to mate, lay eggs, develop as larvae, and pupate.
The frequency and amplitude of output of ultrasonic devices may not be accurate as labeled. The output is too low on some, with the label being exaggerated. And on others, the devices exceed established guidelines for continuous human exposure to sound radiation. Still, some are accurately labeled by the manufacturer.
Although manufacturers claim that ultrasound can penetrate voids to control pests, the presence of furniture and objects in a room significantly decreases the intensity. The physics of ultrasound limits its potential efficacy in normal homes.
The frequencies produced by ultrasonic devices are within the hearing range of dogs and cats, and their behavior can be altered when exposed to the sound. Despite the claims of the manufacturers, the sounds generated should be clearly audible to domestic cats. Dogs also may hear the signals, but not so clearly.
However, the sounds are highly directional, and objects larger than 9 mm produce considerable shadows. Therefore, although cats and some dogs can hear the sound generated by ultrasonic collars, when worn around the neck, their heads and ears may prevent it from being audible.
Numerous testimonials claim that electronic flea repellents do control fleas. It’s difficult to resolve these experiences of satisfied customers, since the scientific evidence shows that ultrasound has no effect on pests. Pet owners may have used another successful method of flea control along with the ultrasonic flea collar. Then the control that was attributed to the ultrasound. For example, if the homeowner sprayed an insect growth regulator, there's often a delay until control is established.
Some people also get placebo-like psychological relief from having an ultrasonic device at home or on their pet. This can be frustrating to pest professionals, since users are deriving benefit from a useless device.