Bugged by Bugs

Wasps can be beneficial in a garden since they often feast on leaf-eating caterpillars. (Photo courtesy of ArtToday.com.)

It's a bug-eat-bug world out there. Though gardeners all over the country are stubbornly shaking their heads right now in disagreement. Gardeners know that bugs eat plants.

In fact, only 10 percent of the insects found around our homes--gardeners' homes included--are harmful to plants. Just 10 percent. And only a fraction of those inflict enough damage to warrant intervention.

Most insects eat other insects. Or pollen, or nectar, or carrion--even dung. Many draw sustenance from decaying organic matter. A few suck blood from animals.

Without bugs to recycle the Earth's detritus, we'd all be knee-deep in dead plants, fallen leaves, insect remains and animal droppings. Fortunately for gardeners, insects also recycle nutrients. Healthy plants depend on a healthy insect population to return nutrients to the soil where they can be absorbed by plant roots.

So why do we despise bugs so? Most of our disgust is simply a matter of perception. We imagine the danger to ourselves or to our plants, escalate it out of proportion, and take action to "correct" it.

With a hand lens trained on the gardener's world of bugs, entomologist (and avid gardener himself) Eric Grissell exposes our foibles in his fascinating new book Insects and Gardens (2001, Timber Press), while poking a little fun at our perception of insects as "evil."

In a section on beneficial insects, he points out how the sudden discovery of a football-size paper wasps' nest sends us scurrying for a can of bug spray, even though the nest has been there the whole summer without anyone realizing it.

We think nothing of them, as long as we don't see the nest. But as soon as we do, some killing instinct in our psyche arises and we douse the whole colony with gasoline or noxious chemicals, he observes.

Exactly true. And when the wasps' nest is gone, the leaf-eating caterpillars they'd been feasting on all summer continue the systematic demolition of our garden plants, unchecked now by the "deadly" wasps.

He dismantles a few other popular myths about insects as well: Praying mantis aren't nearly as beneficial as we've been led to believe, for instance. They're indiscriminate feeders, snatching as many good bugs as bad out of the air. Wasps, on the other hand, are much more helpful to gardeners than ever given credit for; they go about ravaging the pests that are ravaging our gardens. Even earwigs are OK--they prefer mites and aphids to our flowers.

Have you thanked a wasp today?

Grissell's philosophical approach to insect control in the garden coincides with many of the basic tenets of integrated pest management (IPM)--only more so. He urges us as gardeners to raise our tolerance threshold (the point at which damage to our plants is unacceptable and we decide to take action) and recommends "doing nothing" in most cases.

"The hallmark of a great gardener may well be knowing how and when to let nature correct the gardener's own mistakes," he concludes.

While we may not all be convinced to abandon current practices and adopt Grissell's live-and-let-live attitude towards bugs, statistical evidence is mounting that IPM may be stronger--and more cost effective--than the chemicals we use to combat those insects.

A recent University of Maryland study demonstrates just how dramatically we upset nature's balance when we spray indiscriminately: In surveys of actual landscapes, researchers discovered that scale insects were much more prevalent in yards treated with cover sprays than those that were not.

Cover sprays--routine pesticide applications regardless of actual insect sightings--epitomize the traditional just-in-case approach towards pest control. In Maryland, just the opposite effect from the one desired was observed; repeated spraying resulted in an increase rather than a decrease of target pest populations.

Try as we might, deploying 1.1 billion pounds of pesticides annually, we won't outsmart them. Entomologists confirm that insects are evolving at the same rate we introduce new chemicals to kill them.

And we'll never outlast them. Springtails have crawled the Earth for 400 million years; cockroaches for 30 million. Some of the pest species that plague modern gardeners have survived, just as they are today, for 20,000 years. Our feeble attempts at control just make them stronger.

Here on our seven-acre farm in southwestern Pennsylvania, my husband and I know that as many as 1,500 different types of insects may pass through during a single growing season. Most of them are good.

We spray very little, letting the natural--though sometimes tenuous--balance prevail. For the most part, we're successful. There are just a few I can't abide: When blister beetles appear, they get knocked back to manageable numbers with Pyrethrum or Orthene, as do black vine weevils. We occasionally treat for spider mites on a valued weeping hemlock, and trunk injections saved three huge oak trees when the gypsy moth came through. Honeysuckle vines get sprayed for aphids.

Many pests we just pick by hand: Japanese beetles, cabbage loopers, and slugs. When the hemlock woolly adelgid gets here, we'll have our large hemlocks treated with horticultural oil and hope the predatory ladybugs they're introducing in the East catch up soon.

(Lindsay Bond Totten, a horticulturist, writes about gardening for Scripps Howard News Service.)