The Dangers of photographing outside.

as far as sun, it's different from doll to doll, so you can't really say. I've taken all my dolls out for several hours of photo shoots as well as meets where we've been out doors for 4+ hours, some have becoming extremely discolored, while others show no signs of yellowing at all.
Your doll is going to yellow regardless of what you do with it, so I'd say just enjoy it, do what you want with it.
If you're really worried, get a doll bag to take it outside and then get your photos and stuff it right back in.

bugs, dirt, seeds, etc should be the least of your worries. Worry more about yourself coming into contact with that then your doll.
you can brush your doll off, shake out wigs and clothing, and disassemble your doll to scrub it down if need be. I've had bugs, seeds, grass, dirt, food, and more on my dolls at least once before, and they've all be fine :P

Setting your doll in dampness is also fine. Several people have submerged dolls for photoshoots (though they've taken their dolls apart and dried them) That being said. I would not take a doll with black pants and sit them on something wet, because you're going to stain their behind most likely.
So just use your best judgement.

 

My suggestions would be to wait until cool dry days like Jan-March, when outside insects aren't around. And, instead of full sun, use reflected light from a mirror, shiney surface like aluminum foil covering cardboard, also use a flash to keep from getting hard dark shadows on your bjd.
To prevent moisture, dirt, grim, etc., from your bjd, I use a piece of canvas cloth, old towel, & cover this fabric with finely chipped mulch, ferns, pea gravel, with natural vegetation in background.
One of my favorites is an old tree trunk that has an opening at base, with exposed tree roots, & fine small grass &/or tiny moss growing around it. This setting I use only when it is dry & cool, not after it has rained, & when insects aren't around as those sites are popular with spiders.

Hope this helps you and good luck with all your photo sessions.

 

You don't need to be that concerned -- a little sun while you're taking pictures isn't going to hurt your doll. I've done the same with mine, and nobody has had unusually fast or bad discoloration -- I'm a little more careful with my darkskin guy -- he has a cloak with a hood he can pull up (he's a fantasy character so it's fitting, lol) to keep excess sun off until I've either made it inside to the meetup or am ready to take his pic. My other NS guys I don't worry about at all. I just make sure they aren't displayed in a sunny place where they'd be sitting hour after hour day after day.

Insects aren't going to do anything to resin -- it's not food to them, and if your doll gets damp, just dry him off. The only time water is really an issue is if it gets inside him and into the elastic -- then you'd have to take him apart to let the elastic dry thoroughly to prevent rotting, however, some damp grass is highly unlikely to cause any trouble. Dirt, seeds, stray bugs etc can be wiped off. The most troublesome part of shooting outside in my experience is having the wind pick up while I'm trying to pose someone. BJDs really aren't that fragile, so don't be afraid to take him out and have fun with your camera. The last doll pics I took were in the middle of a snow storm -- Charlotte was none the worse for wear and the pics were neat.

 

Whippet said:

This has made a huge difference for me, even just a piece of white foamcore can make the details on the shadow side of your doll look much better, and then you can set the doll down on a clean dry surface if you need to adjust something in the setting or mess with your camera. Having a shoulder bag or basket with a lid to set the doll is great too--the only time I ever was worried about the doll itself was when a neighbor's loose dog came over to see what I was doing! Luckily she's a nice dog, so I didn't worry about my ankles, but I did get my doll up off the ground in record time!