Q. My bush bean leaves are riddled with holes, yet I don’t see any insects on the beans themselves. What is causing the damage, and how can I stop it?
A. Your beans are being attacked by an insect called the Mexican bean beetle, one of the few harmful members of the lady beetle family. The adults resemble large copper-colored lady beetles with eight small black spots on each wing. The larvae are yellow and are covered with large spines. Yellow eggs are laid in groups of 40 to 60 on the lower leaf surfaces.
The Mexican bean beetle is believed to be a native of the plateau region of southern Mexico. It likely was also a pest of beans grown by the Native Americans of the southwestern U.S. several hundred years ago. The insect was accidentally introduced to northern Alabama about 1918, in shipments of alfalfa hay from Colorado and New Mexico. By 1922, the Mexican bean beetle had invaded most of the southeastern U.S. It reached Ohio and Pennsylvania in 1925. The beetles, once gaining access to Eastern states, spread rapidly to the Northeast and West. It is now found all over the continental U.S.
Mexican bean beetles overwinter as adults under litter and other rubbish in hedgerows, ditch banks and woods. The adults emerge in early spring. After a feeding period of one to two weeks, the females begin to lay groups of 40 to 60 eggs on the underside of leaves. Each female may lay an egg mass every two or three days, producing an average of 460 eggs. There are at least three generations per year produced in the Midwest.
Weather does play a role in how severe infestations are in our area. The pest certainly prefers wet weather, which we experienced this spring. Drier springs discourage the beetle.
Controlling Mexican bean beetle can be difficult. Row covers placed over the beans can help. Use a material called ReMay (available online and at some garden centers).
Soldier and Assassin bugs love to feast on bean beetle larvae, so promote these in your garden by planting flowers – nasturtium, marigolds and petunias are reported to be good companions – with your vegetables.
Inspect the leaves and remove any egg clusters you find.
Planting beans early also can help reduce damage.
Rotating crops yearly also is important.
I have had some success in reducing damage from Mexican bean beetles by dusting Diatomaceous Earth on the foliage and around the plants. This organic material abrades and dries out the insect. Neem, pyrethrum and Pyrethroid applications also can help.
Our friends at the Rodale Institute also mention Mycotrol as a natural control for leaf-feeding beetles. Mycotrol contains a naturally occurring fungus called Beauveria bassiana. The fungus infects and kills the host insect and then releases more spores as the insect dies.
Unfortunately, Mycotrol is quite pricey and may not be the most economical control for this common pest of beans in home gardens.
