Controlling Cucumber Beetles

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Question. Mike: Even though I practice crop rotation I still get cucumber beetles and the wilt they spread every year. I have read that beneficial nematodes will eat the beetle larvae underground. Is this true?

---Heidi in Ambler, PA

Last summer I noticed lots of cucumber beetles on my zucchini, pumpkin, and cucumber plants. A light horticultural oil got rid of them, but a few days later, there were tons more. Is there anything else I can do?

---Melissa in Rush, CO

How can I control those small black and yellow cucumber beetles? The wilt they spread wiped out my vine crops by August. Is there something I can spray to reduce their numbers? Thanks.

---Barry in Davidsonville, MD

As I told you in a previous email, I'm having trouble with bacterial wilt spread by cucumber beetles. To answer the questions you asked after you received that first email: The plants are growing on flat ground in an old garden established by my parents. I don't feed and use no mulch. Now, is there a way to treat the soil so the beetles are not able to winter over?

---Cheryl in Greenville, Michigan

Answer. Although there are many different species of cucumber beetles, two in particular are huge problems on American plants. And both of them transmit the bacterial wilt many of our listeners just mentioned, and nasty plant viruses as well

Striped cucumber beetles are a quarter-inch long and yellow with black stripes down their backs. They mostly feed on cukes, but occasionally make mischief on melons. The adults overwinter under plant debris, emerge in Spring, feed on your precious veggies, mate, and then the females lay tiny yellow/orange eggs at the base of the plants. Around mid-June, the eggs hatch and the larvae dig down and nibble away at the roots of those plants for about six weeks. That's when, yes, they can be controlled by beneficial nematodes. Water these microscopic good-guy predators into the soil at the base of the plants early on a July evening and the larvae should soon be devoured. Otherwise, new cuke beetle adults will emerge in early August.

Spotted cucumber beetles are about the same size but have black spots instead of stripes, are more of a greenish yellow, and lay their eggs earlier in the season. The underground larvae feed and pupate much faster, often leading to multiple generations in a typical year (and making the timing of nematode applications more critical). These beetles also pestiferize a much wider range of plants, including corn; in which case their underground pest self is known as the corn rootworm).

Control measures are the same for both kinds of beetles. Promptly remove and destroy any infested plants (including corn stalks) at the end of the season to reduce the following year's numbers. In Spring, position the plants they attack as far away as possible from the previous year's problem areas. Make sure you only put good size, healthy plants in the ground. And don't rush the season like Barry in Davidsonville! When he emailed at the end of April, he mentioned that his plants had already been out for several weeks; and while it might not kill these tropical plants outright, chilly nights and cold soil stress them greatly. That stress attracts insect pests and makes the plants more vulnerable to them.

Cheryl in Michigan also revealed that she contributed greatly to her own problems with her answers to my follow-up questions. Unfed plants growing in flat, compacted soil don't stand much of a chance against any pest, much less these terrors. Plants growing in compost-rich raised beds (see this previous Question of the Week for lots of raised bed details) are much better able to fight back with their own formidable internal defenses against pests and disease.

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You'll also want to encourage beetle-eating toads to live in your garden, as they're the only beneficial that feasts on the bitter-tasting bugs. (See this previous Question of the Week for lots of toad-attracting info.)

And have floating row covers ready at planting time, so you can immediately cover newly-installed plants with this fabulous form of pest protection. Made of spun polyester and sold in rolls, row covers allow light, air and water to reach your plants, but keep insects out. The best-known brand name is "Reemay," but they're available from many sources, including right here at good old GA! Cover the edges of the covers with soil to make a tight seal and don't even think about lifting those covers until you have multiple flowers opening up.

When the plants are full of flowers, lift the covers just long enough to hand-pollinate the plants by transferring pollen from flower to flower with a little paintbrush. Do this until the plants are nice and big and/or you're getting really bored with making horticultural whoopie. Most experts feel that simply protecting the plants while they're young and small makes them much less vulnerable to the diseases carried by the beetles.

Or grow a self-pollinating cucumber variety like "Sweet Success," and just leave the covers on all season (which would also really break the pests' life cycle in your garden). If you go this route, install metal hoops for the covers to sit overtop of so the plants have plenty of room underneath when they get nice and big.

Southern garden writer Barbara Pleasant splits the difference. She says that she leaves her covers on all season, but lifts them for about an hour early every morning to let squash bees in. Then she drops them back down before the later-awakening beetles show up.

If those dastardly beetles show up after you take your covers off for the season, vacuum them up or spray them with insecticidal soap or a light horticultural oil. Just be aware that soaps and oils work only by smothering the sprayed pest, so you can't spray the plant itself for protection. You have to soak those bad beetles directly.

But you know, if you simply put big, healthy plants out into light, loose, warm, compost-rich soil and keep them well protected while they're young, a couple of beetles later in the season won't do them any harm.

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