Haha, I've had this idea rattling about my brain for some time too (though mine went a bit further in scope instead - all sorts of crop pest control for small-scale farming situations, such as greenfly, caterpillars, etc, with training to avoid positive visitors such a bees)

Early on, I dismissed ultrasound - unless some kind of funky triangulation went on, I think the resolution would be far too low and risk of false positives too high - but I'd be interested to hear any thoughts you had on how that might work. That said, recognising the flight noises of certain bugs would make sense to me - only as a way to trigger a stop-and-sweep cycle though.

But as much as the idea excites me, "real" exploration of the space will only happen when I get lots of time and a fair bit of money - and maybe some land! Alas, I'm nowhere near that point yet.