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Sandy's Garden

One Bad
Lady Bug

Unlike other lady bugs, the Mexican Bean Beetle is a pest

By: Sandy Feather ©2010
Penn State Extension


QI grew green beans for the first time last summer in my vegetable garden, and lady bugs ate them.  I thought lady bugs were “good” bugs!  What gives?

  A. Most lady beetles are beneficial insects in that they feed on insect pests such as aphids, whiteflies, mealybugs, scale insects and even small caterpillars. They do not generally damage plants in any way. However, a small number of their relatives are plant pests, including Mexican bean beetles. They are one of the most common pests in Pennsylvania vegetable gardens, and likely responsible for last summer’s damage to your green beans.

ADULT BEETLES

Adult Mexican bean beetles appear like large (one-quarter inch long), copper-colored lady beetles that have 16 black spots on their wings. Immature Mexican bean beetles (larvae) are yellow and covered with black spines. Adults and larvae feed on the undersides of leaves, eating leaf tissue but leaving the veins intact. This type of feeding damage is known as skeletonizing. They can also damage stems and pods when populations are high.

LIFE CYCLE

Adults overwinter on the ground in garden debris, tall grass and weeds or other sheltered locations. They emerge from the winter hiding places when beans begin growing and remain active for roughly two months. Adults must feed for about two weeks before beginning to lay eggs. Mexican bean beetle eggs appear as small yellow dots laid in groups on the undersides of host plant leaves. It takes eggs 5 to 24 days to hatch, depending on temperature and moisture. Larvae feed for two to five weeks before pupating. We have two generations a year in our climate. The most severe damage from this pest occurs in July and August.

Control options include planting succession crops of bush beans. Bush beans do not take as long as pole beans to mature and produce a crop. Those growing and producing prior to or after peak Mexican bean beetle activity are less likely to be damaged. You can also exclude Mexican bean beetles by covering bush beans with floating row covers, something that would be more difficult with pole beans. This thin, lightweight fabric allows air, water and light to pass easily, but excludes insects from their host plants. It is light enough that the growing plants easily push it up as they get taller. Floating row covers are sold under trade names such as Reemay or Garden Blanket. Beans are largely self-fruitful, so it is not critical that floating row covers keep bees from pollinating the flowers.

HANDPICKING

You can also check plants regularly for eggs and destroy them before they hatch. Handpicking adults and larvae and dropping them into a pail of soapy water can reduce their population to a tolerable level in the home garden.

Insecticides labeled to control Mexican bean beetles include BioNeem (azadirachtin), pyrethrins and Sevin (carbaryl). Repeat applications will be necessary for season-long control. Always read and follow label directions for mixing and application intervals.

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