Yah, the noise is definitely how I notice them. An odd little chiming noise that somehow sounds like jumping even though jumping doesn't really have a sound. Another way to notice them is to talk to villagers, since their catchphrases change and they mention being itchy.
But as for getting them to spawn? I think it's totally random. Maybe removing other places bugs could spawn on your island would help (like flowers, trees, etc.) but I'm not sure and that sounds really frustrating to do on most islands.
TBH, I found mine randomly. And I still occasionally find them randomly on my villagers- but I feel like I can definitely go ages without seeing them. Other times I keep finding them to the point where I get annoyed. Hence why I'm guessing they're random.