Answer: The dust-sized white insect sounds like a whitefly, a tiny insect that is usually found on the underside of leaves, where its immature scale form sucks the juices out of the leaves. Leaves might show tiny pin pricks of discoloration and then become more discolored and even wilt, if the insects take over. Soapy water sprays starting with one teaspoon and increasing gradually to 2 tablespoons of dish detergent to one gallon of water is the recommended control. Use regular, not concentrated soap. Don't use soaps with lemon, as the citric acid can burn plants. Start with the lower amount and work up as needed. There is no insecticide that kills them completely. It's always a good idea to test any spray on a few leaves before spraying the whole plant. I'm surprised your beans died. Did you spray during a sunny day? That can burn the plants. Try to spray in the early morning hours. Whiteflies are also attracted to the color yellow. You can purchase or make yellow "sticky" traps from yellow cardboard smeared with petroleum jelly. They fly to it and get stuck.
You say your plant leaves were "devoured." Determining what insects are eating plants usually starts with determining what kind of damage is done, i.e., ragged leaf edges indicates chewing, tiny points of discoloration means sucking sap, etc. I assume when you say devoured that something large was actually chewing the leaves, such as a cabbage looper, a small green caterpillar that is often found on vegetable crops. However, you should be able to find these larger culprits. In addition to chewed leaves, look for small black pellet-shaped droppings. You might want to check your plants at night with a flashlight. If you find what is actually doing the damage, we can recommend a control for you. I don't think it's a problem in the soil, or your other plants wouldn't do well either. Good luck!
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