Few things dampen enthusiasm for growing your own vegetables quite like finding hordes of bugs chewing, sucking and otherwise destroying the leaves of your plants. Aphids, thrips, spider mites, some caterpillars, Mexican bean beetles and bean leaf beetles all find bean leaves delicious. Thankfully, most of these are fairly easy to wash away from the plant.
Problems with Broad-Spectrum Sprays
Before your run to get a spray to kill anything that moves on your green beans, keep in mind that those huge bunches of, say, aphids are drawing every ladybug, praying mantis, lacewing bugs and other predatory insects ready to decimate the "bad bug" population. Broad spectrum sprays, or sprays that target pretty much every kind of insect, will kill the good along with the bad, and are rarely recommended for vegetables due to their toxicity. Once all the good bugs are gone, you're on your own when a new, more insistent pest comes along.
Targeting Just the Bad Bugs
A homemade spray of dish soap and water will dry out soft-bodied insects such as thrips, aphids, mites and the juvenile stages of some beetles. Most every extension center has a recipe for this simple blend, so check with the one nearest you. Soap sprays rarely harm the beneficial, predatory bugs, and are very effective so long as you drench all the stems and leaves (the bottom, too) of your bean plant, and pour a bit around the base of the plants in case some are hiding in the first inch or two of soil.
Other Bug Sprays
Bacillus thuringensis (known as BT) is incorporated not only into sprays for your bean leaves, but also powders. You can apply this to your bean leaves for caterpillar problems and it won't harm a single other organism in your garden -- including you! However, if it rains or if your irrigation system wets the leaves of your beans, you'll have to reapply once the leaves are dry.
The Clemson Cooperative Extension recommends hand-picking beetle pests and their eggs from your bean plants. Beetles don't have soft bodies (except when they're young), so soap sprays won't do much good. They aren't likely to kill your beans, but they may reduce your crop. You can also time your bean planting and select an early-fruiting variety so that you're done with your crop before the majority of pests come along in late spring and summer.