A Carpenter Bee collects nectar from a milkweed plant while aphids mount an attack on the plant. The aphids are those small yellow creatures on the plant stems just to the right and below the bee.

Note: I decided to take Traveller's suggestion and switch to the uncropped version of the photo. The original entry has been moved to pending: http://www.pbase.com/cichallenge/image/163669888