Based on a true story:
The Knucklebiter Gnat must be defeated before attempting to go to sleep. If the character fails to destroy it, it will harass them all night and instead of getting the benefits of a long rest, they will gain one point of exhaustion.
It's a fun little thing, I think. Everytime the characters find it (investigation/perception vs. its Stealth) they get to make 1 attack against it. If it is not killed, the character may try again.
Add more Gnats until the characters give up and simply don't care.
Next day hit them with the exhaustion. From now on, you can force your party to have to fight Knucklebiter Gnats now and again because it'll be horribly annoying not to and, as people in real life, they'll get increasingly irritated and extreme in their measures to try and hunt those little monsters down.
Ideal for swamp, temperate and desert climates.
Bonus for storytelling: After a night with one of those around, narrate every failed skill check or attack with "You cannot concentrate due to the horribly itchy bites left by the Knucklebiter Gnat. Being this distracted, you fail to spot the trap."
Love the approach to asking locals on how to deal with them. Maybe there's a really simple solution like lighting a lemon scented candle or something silly like that. Very good point!
Or coating exposed skin in a dangerous unguent that may pose a moderate-term risk and definitely contributes to epigenetic issues, but seems to repel the Knucklbiter Gnats for a few hours. (Of course, in line with the game's theme, what's repellent to one may be wildly attractive to another, larger, more imposing issue...)
Fantastic! Depending on the group composition it could also be a strong aphrodisiac or repellent for one of the races involved. Just to throw some internal conflict into the mix with the external conflict
Personally, I shy away from drug-induced sexy times, real or otherwise, heh. 😅 Makes for awkward RP at best, IMHO... but a non-sentient monstrosity that just can't shake that newfound mesmerizing crush on a certain hero who's just strolled into their swamp, yet is too shy to engage with all those others around their crush all the time? Could make for an eerily persistent stalker vibe that ratchets up and crescendos to a classic He-Man ending. 🤪