I started using these several years ago with moderate success, using the cheapest olive oil I could find, plus soy sauce. That's the most common recipe for this on the internet.
Well, there is no such thing any more as cheap olive oil. I would also just as soon keep my soy sauce for myself. I have tried cheapo vegetable oils (still with soy sauce), but the earwigs don't seem as drawn to it.
It turns out that what does work just fine is used bacon grease, plus a bit of cheap vegetable oil. It seems the bacon bits provide the umami the earwigs are looking for. I may try skipping the liquid oil when it gets warmer, but for now the nights are cool enough for bacon grease alone to solidify, so I'm still adding enough oil to be sure they will drown. Now I can deploy a lot more traps since I won't have to factor a quart of olive oil into my gardening costs.
ALSO, it's not necessary to bury the container, which is also usually recommended. Earwigs are climbers and they will absolutely climb a shallow can. I still bury them partway if I feel like it, but as summer goes on I know I will get tired of doing this.
I'm hoping for better luck with my corn this year if I start trapping early. I would otherwise be much more able to tolerate the buggers, but they eat the corn silks, preventing pollination, and they nestle in the husks, which vastly diminishes the pleasure of ostentatiously picking corn and immediately cooking it for guests.
(Frugal triple play: Using bacon grease to fry homegrown potatoes, then using it to trap earwigs!)