Why would large, yellow ladybugs be eating the leaves of pole and bush beans. Aren't they supposed to be beneficial rather than destructive to plants?

Gardens & Landscapes April 22, 2008 Print Friendly and PDF
The pole beans are probably infested with the one lady beetle that feeds on plants - specifically bush and pole beans. It's the Mexican bean beetle and is easily distinguished from its beneficial cousins because it is larger and yellow-orange in color with 14 spots. The Mexican bean beetle can be recognized as a ladybug, but beware - it is the black sheep of the family. Not only can the adults wreak havoc on the bean leaves, but the larvae are also voracious eaters and will feed for a whole month before pupating. The larvae are yellow and spiny and don't look at all like the parent, so beans should be checked to see if there are both adult and larval stages attacking the plants. The undersides of leaves also should be checked for tiny masses of yellow-orange, oval-shaped eggs. Once the adult and larval stages have been identified, control methods can be decided. If the infestation isn't serious or there aren't many plants, the insects can be handpicked off the leaves. If an insecticide must be used as a last resort, select nontoxic sprays such as those containing pyrethrins. They work on both the adults and larvae, but it is best to target the more vulnerable larval stage if possible. It is important to be rid of the pests by the time the plants are flowering and when the pods begin to set because the insects will also eat tender pods. Be sure to read the label on any insecticide and follow all the instructions on the container. It is really best to try to control the insects mechanically by picking them off or biologically by releasing a Mexican bean beetle predator such Pediobius faveolatus, a parasitic wasp. The wasp must be present when the first larvae hatch. For a good photo of the pest, follow this link: Mexican Bean Beetle

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This work is supported by the USDA National Institute of Food and Agriculture, New Technologies for Ag Extension project.