Pesky Cucumber Beetles: a Pest of String Beans

Dealing with Cucumber Beetles on String Beans

By Rosemary McCreary, Sonoma County Master Gardener

If you’ve ever wondered if spotted green cucumber beetles attack anything other than cucumbers, you’re on the right track for finding a cause of damage to your string beans. Officially, this garden pest is named Diabrotica undecimpunctata undecimpunctata but it’s commonly called the western spotted cucumber beetle, a persistent pest that reproduces itself in several generations throughout the summer growing season.

You’ve probably seen these beetles flying about and landing or felt the sting of their bite. They look a lot like ladybugs, but are yellowish green with 12 large black spots across their backs. (The striped cucumber beetle, which does similar damage, is a yellowish orange relative with three black stripes.) If you grow flowers in your garden, you may have found them lurking inside petals or crawling out just as you’re popping roses into a vase.

In serious attacks, beetles skeletonize bean vines, though mature bush or pole beans nearly always survive even severe injury. The more critical concern is with young seedlings. They can be completely devoured before leaves ever unfurl.
The gardener’s challenge lies in protecting beans—and to some extent, melons, squash, cucumbers, eggplant, peas, and a few greens—in the seedling stage, when all of these are most vulnerable to the spotted beetles.

Sadly, there are few really good solutions to the problem. Handpicking works if you’re an early riser and can surprise bugs before they’re awake enough to move, but generally an infestation is too large and too few hands make this approach impractical. Insecticides kill cucumber beetles but toxins drift, leave residues, leach away into water supplies, kill beneficial insects, and raise other serious issues. Besides, applications must be made frequently since all of the beetles don’t sit down to dinner at the same time or on the same day or even in the same week.

Many gardeners end up replanting string beans, often two or three times, before the sprouts are pest-free enough to grow to maturity. In previous years, I’ve planted early only to have the seedlings wiped out. Then I planted again, and again. By the third round, it was late enough in the season that the spotted cucumber beetle population had either tapered off or moved on to feast on the roses and left my beans in peace. I do love harvesting early beans, but I find more success planting only a late crop.

So what to do if you want to plant early or if your struggling plants are facing an

onslaught of cucumber beetles? In small vegetable beds, try using paper cups or cones to protect seedlings until plants are of sufficient size to withstand damage. If you garden in large beds or boxes, stretch fine screening or row covers over crops, securing the sides to prevent adult beetles from flying in. Porous spun fibers such as Reemay allow water and sun to filter through but deter all types of insect pests from penetrating. Keep such protection in place until plants are large enough to withstand any subsequent damage.