Not to try to scare you, but something did happen to me once that I wish someone had warned me about. My horse was second over a nest of ground bees that the first rider (my husband) stirred up. I didn't realize it at first, all I knew is that she was suddenly crow hopping and trying to run back down the other side. It was only my 3rd trail ride, my first one on this horse, and I panicked. I kept her from running away but I lost control and she tried to jump a bush on the side of the trail, which she ended up literally straddling with all 4 feet off the ground, and I lost both reins and a stirrup. My husband kept his head (easy for him, he'd gotten away and wasn't sitting on a horse on top of a bush with no reins and only 1 stirrup with bees swarming him LOL) and yelled at me to dismount and lead her out. I kind of rolled out of the saddle and she kind of rolled off the bush and we ran right back through the bees. Amazingly I was only stung once but poor Molly was stung at least 15 times, I was pulling bees off of her left and right. She is an incredible horse to have done nothing more than a few crow hops and one attempt to run away, which she stopped when I yelled whoa. She only tried to jump the bush because I was panicking and giving her no leadership whatsoever.
My point to this long-winded post is that I later learned that the only thing to do in that situation is to make your horse move forward. Keep control of the horse's head and make it move forward as fast as possible right through the bees. I guess maybe the horse doesn't care which direction its running in, as long as its running from the bees, so don't let it run back the way it came if it's going to separate you from the riders who've already made it through. It sounds hard but it's what my husband did, and later, we ran into some other riders who'd encountered the same nest and it's what they did - one of them was on a 4-yr. old Arab stallion!! Moving fast, the bees will only follow you for a few seconds, and leave you alone as soon as they've driven you off their territory.
And don't panic - easier said than done, I know, and something I only learned because I panicked. I failed my horse in that situation and I'm grateful nothing worse happened because of it.
Anyway, I really, really hope you don't encounter any ground bees!