I have vents in my cycling helmet that are open so that air can help cool you when cycling.
However I am also getting wasps and bees entering the vents while cycling, that in itself is a problem, however I get a VERY bad reaction to the sting and I am trying to find a screen that will fit over the helmet.

Thoughts / Ideas?

Any help would be appreciated,

Sympathizing. Same problem here. Not bees so much though. I think mostly I get biting flies. Not fun. I've often wondered why more helmets aren't built with some netting already in place.

found these on other forums

"Black (women's) hairnets, bought from a chemist or tesco, stretched over the helmet will
do the trick. 2 in a packet for 99 pence. That's what I use"

"Kompact Pro, Shield, Diablo and Neon all have insect nets fitted."

"The OGK Kabuto Mostro and the new Redimos both have insect screens and are excellent helmets."

"My Giro Ionos has a removable insect net which works very well. My Spiuk Nexion has a built in one too"

"bought a new road helmet this year with a focus on stopping bees and went with the Lazer Genesis. The anti-insect netting is a separate accessory for about $25. Other options: Rudy Project Sterling. Uvex Boss. Spiuk Nexion and Zirion, but Spiuk can be hard to get in the US right now. Krask is a newer helmet brand that also has 1-2 models that have anti-insect netting pads as separate accessories. One of the other Italian helmet brands like Lamar may have it as well"

Cheap hairnet seems like the best solution, assuming it can actually stretch all the way around a helmet, which is a LOT bigger than a head.

My son caught a bee in his helmet vent and got stung a couple weeks ago, he was freaking out that he couldn't get rid of it, when I got to him the bee was struggling to get away from his guts that were embedded in the scalp. The sting wasn't that bad itself (fortunately my son's not allergic), but the uncertainty of why that bee won't go away, and does he have friends, etc etc...

A hairnet will certainly do the trick. Some helmets come with an "insect liner" that replaces the normal internal pads with a big part-net-part-padding apparatus kinda shaped like a stingray. My wife's Kali Phenom came with one. Have never used it.

I have had too many bees to count go into my helmet (they still grow oranges around here) and about half of those guys manage to get the stinger into my head, which is super fun-- 2015 led the charts with half a dozen stings (including one on my thigh.) So earlier this year I purchased myself a Smith Overtake, and will never have a bee sting me on top of the head again... at least while on a bicycle.

So at the opposite ends of the spending spectrum, hairnets or an Overtake. I went the helmet route, because I'm not gonna stretch a hairnet over my helmet every time I leave the driveway.

How about a cycling cap under the helmet? It'd take a strong stinger to get through that canvas!

I am follicularly challenged, so I always wear a cap or liner under my helmet. Stinger goes right through, I assure you. If you have a full head of hair any cap would work... right?

I tried a cap once and relegated it for cooler temperature because I overheat in it (it's a Halo Skull cap). Although I've never been stung once since I started commuted daily last August, I'm rethinking that cap idea...