Organic Mulch Helps
A layer of organic mulch such as chopped leaves, hay, or straw do lots of good things for the soil but when 2 to 4 inches thick around the bean plants, it harbors the ants, spiders, predator beetles, and other creatures that just love nothing else than to feast on beetle eggs and larvae.
Control by Hand Picking
In normal backyard gardens, the size of the bean crop usually allows for control of the bean beetles by handpicking. You can choose between two approaches:
You want no damage - With this more nervous approach you try to attend to the very first adults that appear to set back their egg laying activities. You must check leaves frequently and pick off the yellow egg clusters first; obiously an approach for folks with a bit of time on their hands. Handpicking here means actually plucking the beetles from the vegetables and killing them by squishing between your fingers or by dropping them into a can half full of kerosene or soapy water.
Serious Damage Is Now Possible - Here you have allowed some defoliation but you’ve reached the point where action is needed. Beetles can be dislodged and caused to drop to the ground by straddling the row and slapping the plants with an open hand. Before you apply this spanking technique you have layed an old sheet or some other fabric on the ground along the row and up close to the plants. The beetles will fall on to the cloth and then you stomp them or eliminate them in some other fashion. One person can clear 90% of the resident beetles on a 25 foot row of young mean plants in about 15 minutes and you should finished for the season in the North. In the South it may take another shot later in the season.
Try A Barrier Over Plants
If you are willing to go to the trouble, you can cover your bean plants as soon as they sprout with fleece, a very light spunbound material that keeps the beetle away from the plants will allowing light, water, and air free access. Cover transplants or newly seeded areas with the fleece material immediately to prevent both insect damage and insect egg-laying which can lead to later damage. Lay the barrier material directly over the plants, sealing all the edges to the ground with dirt. Provide lots of extra material so when the plants grow larger they don't strain against the covering.