Well consider that the correct thinking is in the realm of energy "to kill", to be more helpful. The unit ofcourse is 'joules'. It takes, off the top of my head, 15-50 joules to do the hapless soul of a person into the unkown. Ofcourse this is mitigated by the time that energy is delivered to that 'hapless soul'. (however in the case of electrical zaps these numbers are roughly correct) Considerations of the current to the driving voltage define this "energy". So at 10mA with a driving voltage of 2000V, you will deliver 20W. Each spark generally commutates at around 100usec. A watt is a joule a sec. So, you will have delivered in the neighbor-hood of .002 joules. This is 133 millionth the energy required by a human at the low endof the spectrum. A misquito, by quick look on the internet denotes a weight of 2.25 millionths of a pound or about 400 millionths the weight of a human. This gives your spark roughly the 3 times the potency by comparison to the human DOA instigator.
In thought, here, but not necessarly to a comprehensive one, one would think that they would need something to partition the voltage "sparks". If you consider that the mosquito is completing a circuit in some sense. i would make sure that the circuit has some method of regulation, less one finds his "sparker" sparking till the FBT says it's had enough. Hope this helps.....
-this reply was written in the interest of scintillating and substinative thought....and hopefully discourse.
....also if this if this lends some length of doubt. One can safely relegate himself to the numbers and perhaps operation of an engine ignition system. Not enough to be but of superficial harm to a person, but certainly enough of perturbance to the fluttering little heart of a mosquito. Consider numbers such as 30-35kV with 100-300millijoules with a commutation time of around 1-2 milliseconds.