Beneficial insects, or natural enemies.
In Four-Season Harvest, Eliot Coleman describes a visit to a French greenhouse grower, Monsieur Audier. In the middle of a 2-acre glass greenhouse, M. Audier keeps an insectary, where both beneficial and pest insects are kept side by side. M. Audier’s idea is to keep a ready supply of beneficial insects by balancing them with the pest insects they prey upon, and giving all insects the host plants they require. The epitome of biodiversity in a greenhouse.
Coleman calls this the reactive approach to insect control, which organic farmers and gardeners use in place of pesticides. Pesticides indiscriminately kill pests and beneficials alike, leaving no opportunity to use methods other than more pesticides as pest problems reoccur, which they will.
Coleman also describes a preventive approach to pest insects, also an organic approach, which is to supply plants with the best environment possible–the best, most fertile soil, the right amount of water, required sun and warmth, correct timing of planting, etc. Healthy plants, grown in a healthy environment, are stress-free and do not attract pest insects.
Alternatively, stressed plants, grown in poor conditions, attract pest insects to the garden and are then unable to withstand the resulting onslaught.
I was delighted to read Coleman’s method for discovering the causes of pest problems in his vegetable garden. He says you should imagine yourself as the roots of a plant so you can figure out what’s wrong with the plant’s environment. Think like a plant. Or a plant’s roots, more precisely. Is the soil compacted? Is it too wet? Too dry? Is it fertile? Think like a plant’s roots. When plants grow in a healthy, stress-free environment, pest insects are not attracted.
Douglas W. Tallamy, in Bringing Nature Home: How You Can Sustain Wildlife with Native Plants, calls beneficial insects by another name. He calls them natural enemies. Natural enemies of pest insects. His reasoning is similar to that of M. Audier above. Keep a diversity of insects in the environment, and they will achieve a natural balance in a predator-prey relationship.
Of course, native species are crucial to this balance. As Tallamy points out, an environment of 15 plant species, in which 13 are alien plants that feed no or few insects, leaves 2 plants to support all the insects in the environment. That’s not biodiversity, which you need to keep a supply of beneficial insects, or natural enemies of garden pests.
It’s also true that pest insects are often alien insects that arrive on imported plants and have no natural enemies. For example, the Japanese beetle (Popillia japonica) arrived on imported Japanese iris in 1916. Japanese beetle larvae thrive on grass roots. As Tallamy points out, 40 million acres of lawns in our country provide an environment in which the Japanese beetle larvae thrive, and natural enemies are at a minimum or nonexistent. Out of balance, for sure. There are many other examples of alien, invasive insects–the hemlock woolly adelgid is one, which is now threatening eastern hemlocks, with no natural enemy available.
But back to beneficial insects, which live on plant species native to them and keep native pests in balance. If you are worrying about the appearance of native plants that are being chewed on by multitudes of insects, Tallamy reports that from 10 feet away, his white oak, on which he counted 410 caterpillars, and which is known to host 534 lepidoptera species, looks fine. Further, Tallamy quotes a study finding that 10% of a plant can by eaten by insects before most people notice any damage.
I have a story about beneficial insects, or natural enemies. I only wish that I had the photos to illustrate my amazing personal experience. Several summers ago, 2 young river birch (Betula nigra) were attacked by aphids. Actually, I didn’t know what the problem was, only that something was causing clumps of river birch leaves to curl up and turn color. I expected the worst. Having planted the little trees the summer before, I still remembered vividly how much they cost and the labor involved in planting them.
By paying more attention to insect life than I usually did, I soon discovered masses of yellow eggs on the birch leaves, and then wierd-looking longish larva with tiny yellow spots, and then oblong red creatures with black spots. All of these manifestations could be seen at the same time on different areas of the river birch. I’m relying on memory here since I didn’t take notes or photos. That’s how I remember it. Oh, no, I thought. Things are going from bad to worse.
Those of you with more insect awareness than I had at the time already know that lady beetles–I still think of them as ladybugs–had come to the rescue of the river birch and laid their yellow eggs. The larvae had hatched and were busy eating birch aphids and turning into mature lady beetles, just as they are supposed to do.
It’s the most common beneficial insect story ever told, yet I had to be concerned enough and look closely enough to become aware of the saga of the aphid and the lady beetle being played out one more time. I was elated. I had seen for myself. I became a believer.
If I had grabbed the pesticide spray at the first sign of curled river birch leaves, I would have killed the lady beetle eggs and larvae along with the birch aphids. Even if I had followed good organic methods and sprayed the trees with a strong jet of water from a hose, or used insecticidal soap or neem, I might well have killed the beneficials, the natural enemies, with the pests.
So, know as much as possible about the environment of the garden. Know when to take action and when to let the balance of nature take its course. This works best, of course, if a balance of nature actually exists in the garden. Where did those lady beetles come from, so quickly flying to the rescue. I don’t know. No other plant that I know of was being attacked by aphids. There is a fair amount of yarrow in the wildflower patch as well as growing among the grass and weeds we mow and call a lawn. Lady beetles like yarrow. Tallamy’s plan for diversity of native plant species makes good sense in this scenario. It’s hard to provide specific host plants for specific beneficials. A good mix of diverse native plants is a better solution.
Thinking like the roots of the little river birch trees, I decided they were probably too dry. The trees had been planted the summer before. Their roots may not have developed adequately to supply the water the trees needed. I used a barely-dripping hose at the trunk of each tree to water them for the rest of the summer. They haven’t been attacked by aphids again, for whatever reason. They are now well enough established not to need extra watering.
River birches are native to this region. Tallamy says that 413 lepidoptera species are supported by birch trees. These river birch trees are a variety called Little King, a multi-stemed, dwarf species of river birch. I shudder to think that I may come to find out that dwarf varieties of river birch are not native. But so far . . . .
This summer I hope to discover more of the insects that call river birches home. I suppose that’s one way to determine native species. Lots of insects, or not.
I also hope to provide the healthiest environment possible in my garden, particularly the raised beds of the vegetable garden, and to learn by my mistakes in doing so.