The Q&A Archives: Bean Beetles

Question: I had a highly successful first planting of bush green beans this year and once spent, pulled and planted a second round. The plants were looking more vigorous and healthier than the first round and I had dozens of green beans coming in. Then within a week, all of my plants were reduced to mere stems by swarms of a little bug about the size of a lady bug that was yellow to gray in color w/ elongated spots on its back. They were everywhere and my insecticides didn't phase them. What were they? Also, when I pulled the dead plants out of the ground, it appeared that they were attacking the roots as well and were in my soil. How can I prevent these pests from surviving our winter and attacking my garden next year?

Answer: It sounds like you were invaded by the Mexican Bean Beetle! These are a very common pest on beans. Adults overwinter on plant debris; they soon begin laying their yellow eggs on the undersides of the leaves. In about 2 weeks, the blob-like larvae begin munching on bean leaves, leaving only the skeleton veins behind.

For now, I'd be extra diligent about cleaning up garden debris, since the beetles overwinter there. But your real work will begin next spring--the key to controlling this pest is catching them early in the season. Plant your early crop as usual, but inspect these plants carefully every few days for both the adult beetles and egg masses. Be sure to check the undersides of the leaves. Destroy any of the pests you see. If you are very thorough about this, it may be possible to control future infestations. If your later crop is still affected, you can either try to battle them with hand-picking. As you've seen, they are difficult to control with insecticides. Instead, you might experiment with different planting times--this is one pest that can be foiled by carefully timing your bean plantings to avoid their peak populations. That, combined with patrolling early in the season for emerging adults/eggs, should do the trick.

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