When you garden ecologically, you try to keep insect pests below levels where they will cause unacceptable damage, rather than try to get rid of all of them. Preserving natural enemies may be the most important and readily available biological control practice you can undertake in your battle of the insects. As you rely on natural enemies to help you, you need to foster them by providing their needs. One method consists of increasing the diversity of plants in or near the garden to attract more beneficial insects to the area.
Natural enemies, or "good bugs", can provide a safe, environmentally friendly means of suppressing pests. Most insects that live in or near our gardens are not harmful; in fact, many are beneficial! Insects' natural enemies can be predators or parasitoids. Predators such as lady beetles and lacewings are mainly free living species that consume many prey during their lifetime. Another important group of predators are the spiders; scientists are just beginning to recognize the importance of spiders as natural enemies and in some countries they are being brought in to help control insect pests.
Parasitoids, which include many wasps and flies, are more specialized than predators; the immature stage actually develops within the body of a single insect, ultimately killing it. The adults are free living and often visit flowers for nectar and pollen.
Select plants for your garden that are known to lure insects' natural enemies to help you attract and conserve these garden helpers. Two large groups, or families of plants, are excellent "lures" - the parsley family (Umbelliferae) and the sunflower or daisy family (Compositae).
You can spot members of the Umbelliferae family by their umbrella-shaped clusters of small 5-petaled flowers. The overall appearance is often a large flat head of white or yellow flowers; Queen Anne's lace is a good example. The flower head provides a place to land for many insects, especially beneficial wasps. Using a variety of these plants that bloom at different times can make your garden look attractive, too. A number of culinary herbs in this plant family including parsley, dill, caraway, cilantro or coriander, and fennel. Some of these herbs are very attractive to syrphid and tachinid flies, assassin bugs, lacewings and parasitic wasps. One caution - these plants will spread quickly if left to go to seed, so remove flower heads after they stop producing nectar, but before seeds mature. Also, some are biennials, so you won't see flowers appear for a year.
The Compositae family is characterized by flower heads that are actually made up of many small flowers growing together. Many flowers are composed of rays around a disk-like center. Many well-known ornamental flowers including marigolds, dahlias, daisies, asters, cosmos, calendula, coreopsis, tansy, yarrow, zinnia, and sunflowers are in this family. Flowering often lasts over a long period of time and there is usually more than one flower per plant. This provides a slow flow of nectar over a long period for the insects. Ladybugs, lacewings, parasitic wasps and some predaceous wasps are attracted to plants in this family. Soldier beetles, flower beetles, and some lady beetles will feed on pollen in addition to feeding on insects. Dandelions offer early spring pollen to some of these insect predators.
Legumes such as clovers and vetch also attract beneficials. They add nitrogen to the soil, provide good shelter and moisture for insects, and may even serve as a source of alternative prey for natural enemies. Beneficial insects such as ground beetles, rove beetles and robber fly larvae are often found in the soil.
Cover crops offer protection to natural enemies when our annual garden plants are not actively growing. Often, beneficial insects move over from the cover crops as these crops begin to die back, feeding on "bad" insects that are in turn consuming the desirable garden plants. Buckwheat is a good one because it not only provides shelter, but has flowers which attract flies, ladybugs and pollinating bees. One caution, however, is that it does self-seed readily! A small permanent planting of buckwheat near the garden allows immature natural enemies to complete development without seeding up your garden.
If attracting - and keeping - the "good guys" to your garden is what you would like to do, try planting a few of the "lure" plants from the parsley and sunflower families this year!
Biological Control: A Guide to Natural Enemies in North America.
Natural Enemies of Vegetable Insect Pests. Michael P. Hoffmann and A. C. Frodsham. Cornell Cooperative Extension Publication. 48pp.
Fatal Attraction by Roger Bossley in National Gardening, May 1989. pp 34-37.
Beneficial Borders by Joanna Poncavage in Organic Gardening May/June 1991. PP 42-45
Flower Power by Robert Kourik in Garbage, May/June 1992. pp 26-31.
Carolyn Klass and Michael. P. Hoffmann, Department of Entomology, 11/95