What are Mexican Bean Beetles?

First of all, what the heck are Mexican Bean Beetles? Most people mistakenly identify them as ladybugs. This clever critter may look like a ladybug, but it’s actually a ladybug species called Epilachna varivestis that happens to be a vegetarian. The beneficial red and black ladybug gardeners love so much is a meat-eater (well, at least insect eater) which is why we love it. Mexican Bean Beetle are particularly partial to beans, but they can also much on peas, beet, squash, and tomatoes.

How to Identify a Mexican Bean Beetle

Mexican bean beetles look like yellow ladybugs with a few tell-tell features. Here are some distinctive identifying features:

• The adult beetles are about ¼ inch long and have sixteen black spots displayed on their tan/yellow bodies. The tan/yellow will change from bright yellow to orange-yellow as they get older.
• The eggs are yellow and laid in clusters of 30 to 40 and are on the undersides of plant leaves
• The larvae are also yellow and spiny and can grow up to ½ inch

Identifying Mexican Bean Beetle Damage

There is no mistaking the damage of a bean beetle. Mexican bean beetles will eat away at tissue from the undersides of the leaves, leaving them a skeletal shell. Typically the damage is to the leaves only, but sometimes they will eat at the stems. Occasionally, larvae will feed on young bean pods.

How to Control Mexcian Bean Beetles

The best way to control the beetle damage is to prevent the eggs from hatching. When the eggs hatch, the larvae have no mercy. It will chew away leaf tissue and bean pods, leaving nothing but a ghost of a plant. As your plants are maturing or even if you haven’t planted anything yet, here are a few ways to prevent Mexican bean beetles from damaging your bean crop:

• Plant earlier. Planing the beans early in the season (as soon your last frost date) will greatly decrease your chance for an infestation. Most of bean beetle damage occurs mid-summer.
• Clean up the mess. Mexican bean beetles hibernate in accumulated dry leaves, etc. Rake up the debris between crops and destroy it prior to replanting an unrelated crop on the same bed.
• Rotate your crops. Since adult beetles will overwinter in the soil, give your beans a fighting chance by planting them in a new bed.
• Check the undersides of leaves for the tell-tell yellow clusters, especially from early spring until June, to look for and remove eggs and pupal stage larvae. I use a stick or my nail to gently scrape off the eggs off the leaves.
• Hand pick off and destroy adult beetles and larvae. Yep, squish them.
• Attract a diversity of beneficial insects to the garden, as this tends to limit population explosions for individual species.

Lessons Learned From My Experience

In my second planting of bush beans, I saw a few Mexican bean beetles and noticed the population slowly increasing, but I finished harvesting and removed the first crop before there was any real damage.

My first mistake was that I had planted the second crop while the first one was still in place. This allowed the mobile adults to fly on over to the new planting at will, especially once the first crop was removed.  While the larvae were destroyed on the first crop, there were already eggs laid on the second crop before the first was even out of the ground.  A better course would have been to wait a few weeks after having removed the first crop before planting the second one.

My second mistake was that I planted the second crop too close to the first one.  My garden is laid out in a series of 4×12 foot “beds”, and while I am good about rotating crops according to a plan, that plan had these crops bordering each other.  A better situation would have been to have the two bean crop locations separated by at least one bed.

Happy gardening!