There have been several reports of Mexican bean beetle attacking green beans. This insect is a bit unusual in that it is in the lady beetle family, which we usually associate with insects that feed on other insects. However, this one feeds on bean leaves, so it is important to recognize this as a pest and not as a beneficial insect. Mexican bean beetle (MBB) will feed on both garden beans and soybeans.
Mexican bean beetle is a larger 16-spotted lady beetle, about 3/8 inch in length (Figure 1). The typical symptom of MBB feeding occurs on undersides of leaves. Damage is between the veins and MBB only chews partway through leaves. Larvae (Figure 2) are only found on the undersides of leaves and look very different from larvae of beneficial lady beetles. MBB larvae are bright yellow with black-tipped yellow, branched spines. The larvae cause the same damage as the adults.
We need to keep defoliation of garden beans from insects to less than 15 to 20%. There are a number of effective insecticides listed in Home Vegetable Gardening (ID-128) and Vegetable Production Guide for Commercial Growers (ID-36), but the key to successful control will be obtaining coverage to the undersides of the leaves.
Ric Bessin, Extension Entomologist