One explanation may be that the flowers pulse their nectar production throughout the day, with different plants on different cycles. That way not everything is attractive at the same time. If you watch your swallowtails or hummingbirds they will go around the garden sipping from the same species of plants in one pass, completely ignoring nearby plants even though those plants are full of flowers. Some plants do hog the limelight and produce an abundance of nectar while others stick to a rigid schedule. I imagine your bees are hitting the other plants but they wait til you aren't looking to do it.

I grow garlic chives to attract the colorful wasps (the ones with the black bodies with bold cream and red splotches on their abdomens). I snip off the flower heads when they are done blooming to keep them from seeding in everywhere.

my mountain mint and garlic chives usually are loaded with little wasps and bees. i never really thought about it before. i don't usually have trouble with aphids except for maybe the very early spring. i never thought about them being connected. we do have many ladybugs and praying mantids, too. speaking of snipping garlic chive heads- i need to do that now. i forget each year and now have a sizeable patch!

In Asia, farmers will build cute little birdhouse like shelters for paper wasps. By encouraging them to live in their gardens they help keep caterpillars under control. Most of the larger wasps and hornets (yellow jackets are considered a hornet) really like to feed caterpillars to their babies. I imagine things as small as aphids are preyed upon by those tiny tiny wasps.

One of my favorite wasps is the Steel Blue Green Cricket Killer. You don't see them as much here on the east coast. They are a shiny black wasp with black wings and every part of them has a blue or purple irridescence (thats the "steel blue" part of their name). They only eat Green Crickets (what some people call Katydids). The cool thing is that while you are out in the garden or sitting on the porch, these wasps will be hunting and totally oblivious to you. They walk around porch railings and potted plants feeling with their antennae and really seem to be peeking into every hiding spot big enough for a cricket. They'll even climb all over you if you sit still enough.

ooh- i've seen those john! they are very pretty and not aggressive at all. good to know what they are and what they hunt. my kids hunt katydids to feed the turtles. and i had a paper wasp nest in a little ornamental/possible wren house this year. i didn't bother them, nor they me.