Mexican bean beetles feed on all types of beans
Question: I would like to grow green beans again this year but had such a problem with Mexican bean beetles last year that I am hesitant to plant them again. Is there some kind of green bean that is immune to them• Or is there something I can do to keep them off the plants?
Answer: Unfortunately, there is no green bean variety that is resistant to Mexican bean beetles. Looking much like a copper-colored ladybug on steroids, Mexican bean beetles feed on all types of beans, from soy to lima, pole to bush. They are easy to identify by the 16 black spots on their wing covers and the distinct appearance of their larvae -- yellow, spiky, slug-like creatures usually found on the undersides of bean leaves. These larvae tend to be the heaviest feeders and can quickly defoliate bean foliage and will even eat the beans and flowers. They are most active in mid to late summer in our region.
There are several different things you can do to deter them. First, the adult beetles overwinter in garden debris and emerge to begin egg-laying in June. An easy way to deter them is to not plant your beans until the end of June or July. This still allows the plants plenty of time to mature but separates them from the lifecycle of the beetle. You also can cover the young plants with a floating row cover from the time the seeds germinate until the plants come into flower (you'll need to remove the cover to allow access to pollinators). Doing so puts a protective barrier between the pest and the plant. By the time the cover is removed, the beetles have gone elsewhere to lay their eggs.
You should also interplant your garden with flowering annuals and herbs like sweet alyssum, dill, fennel, sunflowers, zinnias, and others to attract the many species of beneficial insects that prey on the Mexican bean beetles. I plant these flowers all over my vegetable garden to lure in the good bugs that help me control the bad. Several Eastern states release a species of parasitic wasp from India called the Pedio wasp (Pediobius foveolatus) every year. The gnat-sized, non-stinging wasp is an awesome control for bean beetles as it parasitizes them to feed their young. The Pedio wasp does not survive the winter so they are released annually and have brought the bean beetle numbers down significantly (and since the bean beetles are their exclusive host, there's no need to worry about other organisms). You can purchase Pedio wasps for release in your own garden from various insectaries (for a source list: ipm.ncsu.edu/urban/cropsci/c03altpr/supplier.html ).
And, as a last resort, you can turn to spinosad-based organic pesticides to control the bean beetles, though they must be used according to label instructions. Spinosad is made from a fermented bacterium that is consumed by the insect as it munches the leaves. It's a fairly new product to hit the market and is available at local garden centers under the brands Captain Jack's Dead Bug Brew and Monterey Garden Insect Spray, among others.