Mexican Bean Beetles can strip all the green tissue off bean plants surprisingly quickly. If your organic garden is attacked by these pests pretty much every year, odds are high that you will see them again this summer.
How can you recognize these pests? The adults look a lot like ladybugs, only more orange or yellow. Their babies (aka “larvae”) are bright yellow, and look a bit like stubby, spiky caterpillars. (See the picture collage lower in this post.) The damage to the leaves, which become very brown as the beetles and their babies eat the green plant tissue, is the clue most gardeners notice first.
Having some strategies in place, before the bean beetles arrive, can help organic gardeners to harvest plenty of beans in spite of the coming pests.
Strategies for controlling (or minimizing) an attack by Mexican bean beetles have some overlap with those for some other pests, such as aphids or the caterpillars of cabbage moths and cabbage butterflies.
Using Barriers to Keep Mexican Bean Beetles Off the Plants
For caterpillars like cabbage worms and loopers (babies of the cabbage moths and butterflies), we can use netting over our plants to keep the adult flyers away. If the adults can’t reach the plants to lay eggs on them, then we won’t have caterpillars on the plants.
For Mexican bean beetles, we can adapt this strategy by using a floating row cover instead of netting over the bean plants. The non-woven construction of the fabric means there are no holes for insects to sneak though. The row cover is lightweight and can be draped right over the plants, but it seems kinder to the beans to secure it over a support structure. The bean patch needs to be weed-free before the area is covered. Beans, in general, are self-pollinating; most varieties will make beans even when bees and other pollinators can’t visit the flowers.
Waiting for Parasites and Predators
There is a parasitic wasp that lays eggs inside the larvae of the bean beetles, just like there is a parasitic wasp that lays its eggs inside aphids. Sadly, the bean-wasp doesn’t seem to hang out in my yard, even though my garden gets attacked by Mexican bean beetles nearly every summer. Arbico Organics sells “mummies” (Mexican bean beetle larvae that contain wasp eggs/babies) as a bean-beetle-control for use on farms, but this is a very expensive option. It also is not really practical for home gardeners. Many of us are not growing even the minimum 70 square feet of beans per mummy recommended to make this work. It is nice, though, to know that there are insects working on behalf of farmers and gardeners to control these pests.
Spined soldier bugs and assassin bugs eat Mexican bean beetles, too. Some of these could already be present in your organic garden, but probably not enough. Certainly, there are not enough of these helpful predators in my own garden.
Insect-eating birds also can help reduce the population of bean beetles in the organic garden. In my bean patch, wrens move through the patch, hidden from my view below the canopy of leaves. All I can see is movement of the tops of the plants, until the wrens spring into flight after eating. It is great to know that these garden helpers are in action.
However, the birds seem to leave a lot of beetles uneaten. Maybe Mexican bean beetles are not very tasty.
Using Pest Control Products
In general, organic spray-type options for insect pests are not very effective, and options for beetles are worse. Spraying the undersides of the bean leaves with insecticidal soap (not an easy feat!) may kill the youngest larvae.
If you are desperate, try a product that includes Neem. Neither version of insecticidal soaps will have much effect on the adults, though.
You may have better luck than me at applying a spray to the undersides of leaves. If so, then insecticidal soap could slow down an infestation in your garden. Personally, I do not use a spray option.
For me, hand-smashing the larvae and eggs seems to slow down an escalating infestation about as effectively as using insecticidal soaps. So does knocking adults into a tub of soapy water, where they will drown. In other words, none of these options is super-effective.
Adjusting Your Planting Dates
This is the strategy that has given me the most abundant harvests. This strategy requires that gardeners plant their bush beans early — right around the estimated last-frost-date for their yard. This strategy allows gardeners to begin harvesting beans as soon as possible.
Mexican bean beetles don’t typically become abundant until sometime in the first half of July in my garden, and my estimated last frost is around mid-April. Planting near this date, when it can still be very cool, lets me bring beans to the kitchen beginning at the end of May, through all of June, and into early July, before bean beetles destroy the crop.
When the Mexican bean beetles have shifted from just feeding on leaves to also feeding on the beans, it is time (or past time!) to remove the crop completely from the garden. Be sure to clear it all away. Plant something else in this spot.
In 2-3 weeks, in another part of the garden, you can then plant a second crop of bush beans. In my garden, this second crop is untroubled by Mexican bean beetles. It does, eventually, get bean leaf rollers, but those do a lot less damage to the bean plants and they never eat the beans.