A wasps venom is especially geared toward OTHER bees and wasps, and if a bee gets a hit, they die. I've seen spiders take multiple hits from wasps and they lived! So it's going to hurt A LOT!!! Possibly even cause temporary paralysis. I'm not too sure if it'd kill a medium sized fish though. I mean I've seen Firemouth cichlids take them under and tear them apart before, and archer fish eat them all the time and no negative effects are seen. But a bettas size, compared with the relatively high toxicity of wasp venom (neurotoxin, kills nerve endings, the reason why wasp and bee stings hurt even after said bug was removed) would most likely paralyze the betta, then asphyxiate it by stopping all movement and creating the inability to breathe. Oh and wasps CAN sting underwater, because their stingers are mini-needles. They work like the dorsal spines of scorpion fish, apply pressure and they inject.