GARDENING : Supply-Side Gastronomics of Pest-Eaters

In the thriving trade in killer insects, the catalogue descriptions always sound so good:

“One pound of ladybugs will wipe out aphids forever!” “The Green Berets of the insect world--let praying mantises be your ally!”

Earthworms are gentler creatures, but no less useful. “A no-more-till garden with hard-working earthworms!”

The benefits of hosting assorted and abundant life forms in the garden have never been questioned. Indeed, diversity of plant and animal life is the essence of environmentally correct gardening. The idea is that such variety creates a balanced, self-sustaining back yard that is easy to take care of and always beautiful.

To achieve this, however, do we really need to import bugs and other garden guests to do what they would naturally do if they already lived there in sufficient numbers?

I think not. If beneficial insects and earthworms are not abundant in your garden, or are not there at all, importing them will not fix your ecological imbalance.

The gardening world is not immune to the flimflam of society at large. A common, if specious, argument is that if one ladybug will devour 600 aphids in a week, then a dozen will consume 12 times that number and, therefore, you should have a dozen of them in your garden.

After watching a praying mantis slowly savor a fat cabbage-moth caterpillar, which in turn has been enjoying your broccoli more than you have, the immediate urge is to try to multiply that effect by fostering the mantis population.

And what about those hard-working earthworms? They too are valuable friends in the garden. One study at the National Soil Tilth Lab in Ames, Iowa, demonstrated that in farm fields rich in worm populations, irrigation runoff was virtually nonexistent: Two inches of water was absorbed in 12 minutes, contrasted with 12 hours for the same two inches in soils with low earthworm populations. No runoff, better water absorption, less erosion--all of these benefits are laid at the altar of earthworms.

So if you want a better garden, shouldn’t you invest in a major way in earthworms? Or would that be a waste of money better spent on plants?

The real rub, I think, is that although the value of earthworms and beneficial insects has never been challenged, what is rarely demonstrated is the link between bringing them in and reducing a garden’s pest populations.

The issue of the soldier beetle is a case in point. A couple of years ago, the U.S. Department of Agriculture announced that an entomologist had designed a lure that would allow home gardeners to entice vast populations of these insect-eating beetles to their own back yards. The beetles are particularly tireless predatory insects that consume a veritable smorgasbord of garden pests.

As it turned out, the project was being driven by a belief--untested by field studies--that hordes of soldier beetles would bring about great pest carnage.

The logic is that if a few do a good job, a lot must do a great job. But in certain cases the evidence seems to lean the other way. Ladybugs, for example, will just pack up and leave town if there are no bad bugs to eat. You can import them all you want, but they won’t hang around unless you’ve got aphids. (Conversely, they can smell a good meal from some distance, and will descend on your garden if aphids become a problem.)

Praying mantises eat each other if they’re too numerous in one place and have no other prey.

Earthworms curl up and die if their soil lacks the organic nutrients to support them, or if the soil contains a lot of synthetic material such as fertilizers or pesticides.

The key to attracting--and keeping--beneficial insects is to create the proper environment for them. Earthworms will luxuriate in organically rich soil that is free of chemical additives. Rather than buying boxes of diligent night crawlers from catalogues, add leaves, compost and organic fertilizers to your garden soil: The worms will come.

To keep praying mantises, soldier beetles and ladybugs on the prowl, avoid using pesticides of any kind--don’t kill the bad guys and you won’t kill the good guys that kill the bad guys.

To attract some of these predators and keep them around during the slow times between pest infestations, plant lots of flowers, especially those with flat, open faces that are easy for beetles to sup from.

And if you decide to invest in predatory insects, ask for documentation that shows they actually reduce plant damage when imported in large numbers. If you can’t get the data, don’t get the bugs.