Last year I thought my bush bean leaves were ravished by the Japanese beetles. The beetles didn’t touch my black or white beans So I ignored the beetles to focus on bigger problems–the cabbage looper. Well, ignorance is not bliss since that “beetle” was the Mexican bean beetle which ravished all of my bean leaves this year. He or she looks just like a ladybug but don’t be fooled by its seemingly harmless demeanor. It is worse than teenage boys looking for food in your kitchen.
What is the Mexican Bean Beetle?
It is a tan colored lady bug with 16 spots on its back. Beneficial ladybugs are generally a darker copper color and have fewer dots. (See HERE for different types of ladybugs.) As the Mexican bean beetle ages, its spots fade more and turn from tan to orange brown with a bronze tinge.
FYI, the squash ladybug beetle is copper color but look alot like the Mexican bean beetle. This is another beetle you don’t want to coo over. (See HERE for a picture.)
Back to the Mexican bean beetle.
This bug loves soybeans, bush beans, cow peas, and Lima bean leaves. However, it will eat your tomatoes, peas, and squash plant leaves as well. In some cases, the beetle can devastate your crop since it eats the leaves. No leaves, no plant.
It’s appetite is voracious. Your leaves will look like skeletons if you let this bug get out of control. (Be sure to watch my video to see the type of damage this beetle can create.)
So what do the eggs look like?
Pictured above is an egg cluster. The top picture of this article is the bean larva. When you pick off the larva, they excrete some yellow liquid which will stain your fingers. Don’t worry about it.
In my case, the beetles and larvae ate all of my bean leaves plants except my cow peas since I planted them later. I was lucky that I saw them in my bean bush plants and started hand picking the beetles and the larvae and throwing them in soapy water. They ate about 30% of my leaves.
My soybeans were about 200 feet away from the bush bean plants so for some reason they seemed content eating only my bush bean plants. (The Japanese beetles was busy eating the soybean leaves.)
Could I get them? Where does the Mexican bean beetle live?
The Mexican beetle originated in Mexico and inhabits the east coast and certain places in the West including Texas, Arizona, Utah, Colorado, and Nebraska. It has been spotted as far north as southern Canada and New England and as far south as Guatemala.
It favors wet conditions like the east coast of the United States but can inhabit areas of the west where there is irrigation. Surprisingly, it favors weedless fields. (Um, my garden.)
When should I be worried?
The beetle overwinters in leaves and brush and comes out in the Spring and Summer. According to University of Florida IFAS,
“Some may, however, delay their appearance until mid-summer. In mid-May adults tend to search out snap and lima beans, but by late June they begin ovipositing in soybeans. After feeding on the tender young bean plants for one to two weeks, the females start to lay their eggs, each depositing 500 to 600 of them in batches of 40 to 75 on the underside of the foliage. The eggs are carefully attached at the end so that they all stand vertically. They hatch in a week during warm weather but may require at least two weeks under more unfavorable conditions.”
Eggs hatch in 5 to 14 days, larvae feed 2 to 4 weeks, then pupate on leaves. There are one to three generations per year. (Kill me now.)
How to Prevent the Mexican bean beetle:
Several authorities suggested the following prevention ideas. Listed below are some of my comments with regards to their suggestions.
1. Using row covers until the plants are large enough to withstand the damage. (You really have to wait to remove the row covers after all the beans are formed and just need to dry or be picked. Beans are not pollinated by bees.)
2. Plant a crop such as bush beans early so that you can capture and destroy overwintering beetles. Note, adult beetles are strong fliers and will search out bean fields. (Miss one and you are in trouble but you could definitely take a bite out of their population.)
3. Start leaf checking in the Spring to look for the eggs on the underside of the plants. I didn’t catch the beetle until July and then started daily beetle, larvae and eggs picking. The beetles are usually on the top of the plant leaf and the larvae are underneath. My best garden friend is a pail containing soapy water.
4. Spray with OMRI approved insecticidal soap or neem oil as a last resort. I always find that the soap and the neem oil burn my leaves even when I spray at night.
5. Leave flowering weeds between your plant to attract beneficial bugs. (Note, the square foot gardening method doesn’t allow much sunlight for weeds.)
6. Plant rosemary or flowers in your bean plant bed. (I am a little concerned with this suggestion since beans like to climb. They will climb any plant you plant near them-even if they are labeled bush beans.)
7. Till your beds in the spring to uncover wintering beetles.
8. My suggestion is to plant different beans far away from each other so if one bean bed is attacked by the beetle, the other bed may not. I noticed last year, when my bush beans were about 20 feet from my white and black beans, the beetles only went after the bush beans. This year, they are side by side with the bush beans.
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How do you keep the Mexican bean beetle at bay?
Photo of the beetle by Jason Reidy