If all you have is what's built-in to AE, I'd suggest the "CC Particle World" effect. You'll need two layers -- one solid that fills the entire screen, and one CUT OUT image of a bee (cut it out in photoshop, or use masks in AE and PRECOMP it). Apply CC Particle World to the solid, then under Particle, set the Particle Type to "Textured Disc". Again, under Particle, set the Texture to your Bee image. You should see a bunch of bees shooting out of the particle emitter. From here's it's just a question of tweaking your settings until you're happy.