Caren White is a Master Gardener and instructor at Home Gardeners School. She has been associated with Rutgers Gardens for over a decade.
I had a problem with a rabbit making a buffet out of my vegetable garden this year. He tortured my beans. He nibbled, then let the plants grow a little before nibbling again. This was not the pest I was expecting in my beans. I thought that I would be battling the Mexican bean beetle.
What are Mexican Bean Beetles?
Mexican bean beetles (Epilachna varivestis) are native to Mexico and the Eastern United States. They cannot survive in arid areas so they are only found in heavily irrigated parts of the West. They are relatives of lady bugs. They look a lot like them too. They have the same characteristic black spots. In the case of the bean beetle, there are 16 spots, arranged in three rows of 6, 6 and 4. Their body color is usually described as orange, but can range from yellow to brown to even red. They are the only member of the lady bug family that eats plants, specifically legumes. Lady bugs usually eat other insects.
The adults overwinter in plant debris in your garden. They emerge in the late spring and immediately start feeding on the undersides of the leaves of beans, soybeans and cowpeas. They begin laying their eggs 1 or 2 weeks after on the undersides of the leaves. The eggs are easy to spot. They are bright yellow and laid in groups of 40 to 60. The larvae hatch in 1 to 2 weeks and begin feeding on the undersides of the leaves they hatched on.
All of this feeding by both the adults and the larvae results in the characteristic lacy appearance of bean beetle damaged foliage. The “lace” is actually the veins of the leaf which the beetles and larvae don’t eat. You are most likely to observe the damage in July and August.
After 2 weeks up to 5 weeks of feeding, the larvae attach themselves to the undersides of the leaves and pupate into the adults. The adults emerge in a week to 10 days and live for 4 to 6 weeks. Bacause their life cycle is so short, there can be 2 to 3 generations of Mexican bean beetles each growing season.
How to Get Rid of Mexican Bean Beetles
Floating Row Covers
The best way to prevent the beetles from getting to your beans is to place a floating row cover over your plants. Floating row covers are lightweight fabric that can be laid over your garden or supported over hoops on row crops. The cloth allows sunlight and water in but keeps the beetles out of your garden. Be sure to anchor the sides with pins, soil or rocks so that the insects can't crawl underneath. You can leave the cover on all season until harvest.
Handpick the Beetles, Larvae and Eggs
Since the adults, larvae and eggs are all found in one place, the undersides of the leaves, it’s easy to find them and destroy them. If you have a small garden and you aren’t squeamish, you can examine the undersides of your legumes and squish any beetles, larvae or eggs that you find. I’m squeamish so I knock them off of the leaves into a container of soapy water. If they don’t drown, the soap will kill them. You can use any kind of liquid soap. I use dish soap.
What’s a little cannibalism among family members? That’s right. Lady bugs eat Mexican bean beetles. If you don’t already have them in your garden, you can purchase them online to release into your garden. You can also purchase a parasitic wasp, Pediobius faveolatus, to dine on your beetles. Other insects that think the beetles are delicious are green lacewings and pirate bugs.
Insecticidal Soap or Neem Oil
When all else fails, you can spray. However, you must always be very, very careful what you spray on edible plants because whatever you spray on your vegetables, will end up on your dinner plate. Insecticidal soaps work by coating the insects and larvae and smothering them. Neem oil is always a good choice because it is non-toxic and breaks down within a few days. When spraying, be sure to coat the undersides of the leaves where the beetles, larvae and eggs are. Spraying on top of the leaves won’t harm the beetles, because they only feed on the undersides of the leaves.
Do a Thorough Fall Garden Cleanup
A great way to keep Mexican bean beetles out of your garden is to do a thorough fall garden cleanup. Remove all weeds and plant debris. The adult beetles hibernate in plant debris over the winter. If your garden is bare and there is no place for them to hibernate, they will fly away to find a more friendly environment.
© 2019 Caren White