Lady Beetle cousins leave trail of ruin
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This closeup view shows yellow Mexican bean beetle larvae (yellow) and parasitic wasp larvae (brown) on a damaged bush bean plant leaf. 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.
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Q. I grew green beans for the first time last summer, and lady beetles ate them. I thought lady beetles 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 local vegetable gardens and likely responsible for last summer's damage to your green beans.
Adult beetles appear like large (1/4-inch long) copper-colored lady beetles that have 16 black spots on their wings. Immature 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.
Adults overwinter on the ground in garden debris, tall grass and weeds or other sheltered locations. They emerge from 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 five 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 these 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 it 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, which are sold under trade names such as Reemay or Garden Blanket, also exclude bees and other pollinators. But this is not a problem because beans are largely self-fruitful.
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.
First Published May 8, 2010 12:00 am
