Question. Mike: We have an organic vegetable garden and like to grow old favorites like Kentucky Wonder pole beans as well as newer varieties. But we have a problem with bean beetles every year, even though we trash any plants that were infested at the end of the season. Do you have any suggestions?
- ---Louise in Northwestern Massachusetts
Starting in mid June, just when the first beans appear, Mexican bean beetles attack. Control is especially tough because this is a community garden, with lots of plots and gardeners with differing attention levels. So even if you are diligent in picking the darn beetles and their nymphs off your precious green beans, the person with the plot next to you or three plots over might not care—or even know that they're a problem! I heard one gardener say that they didn't understand why these "lovely yellow ladybugs" didn't seem to be helping with pest control! Is there anything we can do to keep the bean beetles at bay?
- ---Christine from the Chester Ave. Community Garden in Philadelphia
Are there any remedies for the Mexican bean beetle other than toads? My beans were devastated last year.
- ---Michael in Media, PA
Answer. Michael is referring to a previous Question of the Week on attracting amphibians in which we mentioned that bean beetles are a favorite food of toads, and that creating a little toad-friendly habitat in or around your garden often makes them (and lots of other tough insect pests) go away. As in ---"burp!"---go away. You get to enjoy non-defoliated string beans (that's my preferred term; you may call them green beans or snap beans instead) and a little payback at the same time. And toadies are cute. Here's a link to that previous Question so you can see how easy it is to attract and keep these helpful amphibians around.
After toads, the best control is supplied by two beneficial insects, says our good buddy Bill Quarles of the Bio Integral Resource Center in Berkeley, California, publisher of the fine journals Common Sense Pest Control Quarterly and The IPM Practitioner. I consider the BIRC to be the specialists in chemical-free and 'least-toxic' pest control tactics.
The first is the spined soldier bug, a superb predator that looks like a stinkbug but is a confirmed carnivore. Rather than bother your plants, this bug uses its long proboscis to spear prey like potato and bean beetles and suck them dry. (More beany revenge!) It's remarkably effective; just a few soldiers—a mere two to five per square yard—can take out a large number of enemy fighters. So look carefully before you crush what you think might be a stinkbug; if the shoulders are sharply pointed, it could be a beetle-eating beneficial!
Spined soldier bug eggs are available for purchase at prices that varied wildly among the beneficial insect websites I checked, from a low of sixty bucks to a high of a hundred and sixty for 250 eggs (you place the eggs around your bean plants as directed and the good bugs will hatch out and take it from there). A highly effective lure was developed in the 1990s that brought native soldier bugs into your garden, but the manufacturer stopped making them a few years back. Some web sites still seem to have old ones for sale, so if you find a "Rescue Soldier Bug Attractor", snap it up! (The manufacturer tells me they should still work fine if they were stored correctly.)
You can also attract spined soldier bugs the old fashioned way, with pollen providing plants, especially perennials, which also provide winter shelter. Goldenrod, hydrangeas and milkweed are said to be especially good. And with milkweed you get monarchs! (The butterflies, that is, not European royalty…)
The other beetle beating beneficial is Pediobius foveolatus, one of the many 'so-tiny-they're-almost-invisible' parasitic wasps. These wasps are also available commercially; and you can attract them (and similar mini-good-guys) by planting tansy, dill, fennel, caraway and other small-flowered pollen-providers in and around your garden. (See this previous Question of the Week for lots more tips on attracting beneficial insects.)
Releasing and/or attracting beneficial insects and toads is a great bean beetle solution in a home or community garden; commercial plantings too.
Other control tactics: Be sure to destroy infested plants at the end of the season, and the mulch and debris around them, as these become overwintering sites for the pests. Try and position your bean plants as far away from each other as possible; don't group them all together. Grow successive runs (which you should anyway to insure a steady supply). Use row covers to protect the plants when they're young and most vulnerable.
Tape a mirror to the bottom of a hoe and use it to spot the beetle's eggs, which are orange-yellow and laid in VERY large clusters on the undersides of leaves. (Don't harm clusters of about a dozen metallic bronze eggs; that's a squadron of spined soldier bugs getting ready to report for duty!)
The larval stage of bean beetles (what our community garden listener called 'nymphs') is orange, about a third of an inch long and covered with long branched spines. Spray any adults or larvae you see with insecticidal soap or a light horticultural oil.
And finally, Mexican bean beetles are a type of ladybug; one of only two rogues in that huge family of otherwise pest-eaters. The best way to tell the good from the bad is by the number of spots on their back; bad bean beetles always have 16, whereby the number of spots on good ladybugs varies widely.