Quite an old thread but since I am working on to make and test a mosquito repellent circuit since few weeks, I would like to add my points here.
As there are some electronics products that claims to repel mosquitoes electronically, I tried to found circuits and build one for myself (for study purpose). After searching a few I recently build a 555 timer based circuit similar to
this one. Since the sound produced by this circuit is not audible (A very thin beep similar to those created by mosquitoes), I tried and tested for few days to find out if they actually works. But so far I haven't seen any sign that mosquitoes are scaring away from the circuit.
I researched on Google to find out more about the ultrasound concept and find this piece of information:
https://www.mosquito.org/faq#attracts
What it says is quite clear about this concept.
"At least 10 studies in the past 15 years have unanimously denounced ultrasonic devices as having no repellency value whatsoever. Yet, consumers flock in droves to hardware stores to purchase these contraptions. Why? The discovery that mosquitoes locate mates in mating swarms via wing beat frequency generated a great deal of research into ultrasound as a potential source of environmentally-friendly control. Yet, all attempts to affect mosquito behavior by ultrasound have fizzled, despite enormous amounts of money spent upon research and development. To be sure, the clever, high-tech, and imperceptible (by humans) use of ultrasound proved to be an exceedingly effective marketing tool for the repeller manufacturers. Homeowners were urged to buy ultrasonic repellers and the like to rid their houses of pests without the need to inhale "even one breath of poisonous spray". This appeal to the public's chemophobia, while extremely effective in diverting attention away from proven preventive and control measures (and toward their repeller products), has undermined an unbiased review of the subject by consumers desperate for a clean, effective, nonchemical means of mosquito control. Unfortunately, no such miracle cure exists. A pioneering study testing five different ultrasonic devices against four mosquito species convincingly demonstrated that ultrasound in the 20-70 kHz range used by these devices had no effect on reorienting flight by female mosquitoes either toward or away from human subjects. Additional tests have shown that sound generators capable of a wide range of frequencies were also ineffective in repelling mosquitoes. The fact is that these devices just do not work - marketing claims to the contrary. "