How To Kill Mexican Bean Beetles

I used to ask myself, “What’s a Mexican Bean Beetle?” Now, every summer, I ask myself, “Of all the bugs in all the world, why does the Mexican Bean Beetle have to find my garden?”

Photo reproduced w/permission of Purdue University

As with any pest, it pays to know your enemy. I call this picture, “The Circle of Life” and am grateful to Purdue University Entomology Department and Dr. Christian Krupke, Principal Investigator, for letting me use it.

If you have been invaded, these are all the forms the enemy takes while ravaging your crops. Since it’s mid-July in Pennsylvania, I know the invasion of my back yard, all organic garden has begun.

Of all the pests I do battle with, the Mexican Bean Beetle is the worst of the worst when it comes to green beans.  One day there is nothing there.  The next day there are some holes in a few leaves on a couple of plants.

Flip up the leaves and if you see pudgy yellow larvae with lots of legs and one big old mouth chewing away, you’ve been invaded. Grab a bucket, sit down, methodically flip up every single leaf on every single plant and crush the yellow menace. Then get up and do it again, tomorrow and the next day or you will lose your bean crop.

Mexican Bean Beetles are members of the lady beetle family.  But they aren’t the Lady Beetle relatives you want in your garden.  Small, copper or khaki colored, these beetles are about 6 mm (1/4 inch) long and 5 mm (1/5 inch) wide.

Tiny & destructive (Photo credit: Michael Bok)

Some have 8 small black spots on each wing, resembling large lady beetles. Some are brown with barely discernible stripes. No matter what they look like, they’re really wholesale destruction machines.  And they come in force.

How do they find your garden and your bean plants so quickly?

Chances are they never left when the winter came; they simply tucked in to the ground in leaf litter and other sheltered areas in fence rows of your garden plot and waited out the freezing temperatures and the snow.

Adults begin emerging from these protected areas when beans begin sprouting and continue to emerge for up to two months. The adults feed for approximately two weeks before depositing their eggs on the underside of leaves.  And when I say feed, I mean ravage.

Mexican Bean Beetles will literally eat the life out of my bean plants, if I let them.

Yellow eggs 1 mm (1/20 inch) in length are laid in groups of 40-60 on the lower leaf surfaces.  Females may deposit an egg-mass every two to three days. Eggs hatch in 5-24 days.  Immature larvae are yellow and are covered with large spines.  Larvae feed for two to five weeks before pupation.

You have 3 chances to kill these beetles off – crush the eggs, crush the larvae and crush the mature beetles.  The first two are the easiest but you can catch and kill the beetles too.  You just have to be persistent.  I like to think of it as my summer time exercise program, bend, search, crush, start again.

If you can make it through July and early August, when the greatest amount of injury occurs and the adults begin to disappear, you might save some of your bean harvest.

So, every spring I take a chance and plant some beans.  They grow fast.  They set tons of beans.  If I plant them properly, train them right (if they’re pole beans) and aggressively crush all variations of the Mexican Bean Beetle, I can harvest and enjoy green beans all summer long.