R.,
I haven't had to deal with this, however I think it's fortunate for you that there are TONS of "friendly" bug books and movies out there that might help. What about going to the library (by yourself) and finding some "fun" ones - cartoonish, with friendly bugs as the main characters. There is the BUG movie, The BEE Movie, even WorldWorld DVDs and morning t.v. show that has a few little friendly bugs in it (they are not the main characters but are often in the show flying around and helping). Maybe if you can work on "desensitizing her" through controlled experiences and "kid-friendly" shows it would help.
Also, keep pointing out to her ALL the times you see a bug and NOTHING happened to her. Take her on a walk. Look for birds, squirrels, butterflies, ants, etc. Make it HAPPY. Make it fun to see how many you can find (from a distance). You could even eventually giver her a little notebook to "write down" all the animals you find. I wouldn't make it ABOUT bugs, but more likely than not, you'll see more bugs than anything else. She can sit in a stroller or on a bike and just point them out, not go near them or catch them...just realize 'everyone' who lives outside.
I don't know why this phobia has grasped your daughter so strongly, perhaps ONCE someone was afraid of a bee and she has translated that into all bugs?
In addition to trying to find friendly and funny things to start exposing her to, I would try to work on her reaction. Let her know that screaming and crying is not appropriate. She is a "big girl" who can use her words and simply walk away from them. Also, start "rewarding" her when she DOES act appropriately - like a sticker.
I hope this helps! Let us know what happens.