Insects that feed on bean plants inflict two types of damage. Beetles, such as the Mexican bean beetle or Japanese beetle, chew holes in the leaves. Leaf-sucking insects, such as aphids and leaf hoppers, such the fluids out of plants and are harder to detect. They cause wilted or curled leaves that resemble disease symptoms.
Mexican bean beetles resemble large brown ladybirds and usually appear around the time that bean flowers blossom. They chew large holes in the leaves and can quickly defoliate a bean plant. Look on the undersides of the leaves for the adult beetles, as well as the fuzzy yellow larva or the yellow egg masses. Hand pick beetles off the plants at all stages of development and drop them in a bucket of soapy water. Treat Japanese beetles the same way.
Aphids are small, winged insects that may be green, black or brown. They suck the juices out of the plant, causing the leaves to curl and wilt. They leave a sticky substance, known as honeydew, on the leaves or on the ground. Look for aphids on the undersides of leaves and treat them by spraying them with a steady stream of water several times per week. Coat leaves in the morning with insecticidal soap if water is ineffective.
Leafhopper are green, wedge-shaped insects that suck the fluids out of bean plants, leaving a white stippling on the leaves or causing tip-burn. Apply an insecticidal soap if you see the insects on the plant. Interplant flowering herbs, such as dill and borage, among vegetables to encourage beneficial insects, such as ladybirds and lacewings to your garden. These insects feed on leafhoppers, aphids and spider mites.
Tiny pests that suck the juices from bean plants, spider mites leave a white or yellow stipling on the surface of the leaves. The undersides of the leaves may appear grey. Treat spider mites as you would aphids or leafhoppers.