FEATURE ARTICLE

Guarding the Garden:
Habitat Manipulation to Favor Natural Enemies

The effectiveness of natural enemies is largely a function of the pests, crops, and cultural practices involved. Cultural activities such as tillage, weeding, intercropping, and harvesting can have serious effects on the insects of the garden or farm. To the pest, the crop is a dense and pure concentration of its basic food resource, so of course, it is delighted to have the garden or field planted, weeded, and watered each season. For the natural enemies, such overly simplified cropping systems are less hospitable because natural enemies require more than prey or hosts to complete their life-cycles. Many parasitoid adults, for instance, require pollen and nectar to sustain themselves while they search for hosts. The parasitoids look for this resource in nearby flowering "weeds." And considering that most natural enemies, especially ground beetles, do not disperse far distances from where they overwinter, having permanent habitat near or within the garden or field will enhance their ability to get an early start on burgeoning pest populations. Attending to all aspects of natural enemy biology, therefore, is an excellent way of minimizing the disruptive impact of modern crop production.

Natural enemy conservation is especially important to the gardener. Since gardens operate on a much smaller scale than the typical farm, the garden containing several different crops is implicitly intercropped (and thus, avoids having a large, "pure" concentration of pest resources). It also goes without saying that gardens are smaller targets for pests and natural enemies as they disperse across the landscape. Since most pests are generally better at dispersal than natural enemies, one can see how a garden might get colonized by pests long before natural enemies. For this reason, it is all the more important that flowering borders, hedges, and other perennial habitat be provided for the natural enemies. A more diverse plant canopy will provide not only alternate food sources for natural enemies but also shelter and critical overwintering sites. Research has shown that the presence of such non-crop habitat within or immediately adjacent to the cropped area can increase the abundance and diversity of natural enemies present. In some instances, this has led to increased yields or at least reduced pest control costs; of course, the converse has been known to happen as well. Non-crop habitat may be as simple as a straw mulch, which provides humid, sheltered hiding places for nocturnal predators like spiders and ground beetles. Straw mulch may also make it harder for flying insects such as aphids and leafhoppers to "see" the crop, by reducing the visual contrast between the foliage and the bare soil surface. Mulched garden plots have been shown to have significantly less insect damage through this combination of effects.

Certain flowering plants can greatly increase the longevity and fecundity of many natural enemies. For example, a study in Canadian apple orchards showed that parasitism of orchard pests was 4 to 18 times higher in orchards with many wild flowers than in orchards with few flowers. Our knowledge of which plants are most useful in this regard is far from complete. A number of plant species have been shown to encourage natural enemies (see table), but this list is certainly far from complete. We offer the following guidelines to use in considering additional flowers as habitat for natural enemies. Flowers provide pollen and nectar as a sort of bribe to induce insects to transfer pollen from one flower to the next and cause pollination. However, the size and shape of a given flower limit the kinds of insects that can access its pollen and nectar. Many of the natural enemies that can benefit most from floral resources are small parasitic wasps, often smaller than a mosquito. Consequently, flowers that are good for natural enemies are usually small, not overly tubular, and relatively open. Equally important as flower size and shape is the timing of pollen and nectar production by the flowers. Many natural enemy species are active as adults only for discrete periods during the growing season. Pollen and nectar must be available when the adults are active. This is most easily achieved by planting a mixture of plants that have relatively long blooming periods that overlap in time. Perennial plants often have shorter blooming periods than annuals, so particular attention should be given to plant diversity and blooming times in perennial borders designed as habitat for natural enemies.

Good Flowers For Bugs
Umbelliferae (Carrot Family)
   caraway Carum carvi
   coriander (cilantro) Coriandrum sativum
   dill Anethum graveolens
   fennel Foeniculum vulgare
   flowering ammi or Bishop's flower Ammi majus
   Queen Anne's Lace (wild carrot) Daucus carota
   toothpick ammi Ammi visnaga
   wild parsnip Pastinaca sativa
Compositae (Aster Family)
   blanketflower Gaillardia spp.
   coneflower Echinacea spp.
   coreopsis Coreopsis spp.
   cosmos Cosmos spp.
   goldenrod Solidago spp.
   sunflower Helianthus spp.
   tansy Tanacetum vulgare
   yarrow Achillea spp.
Legumes
   alfalfa Medicago sativa
   big flower vetch Vicia spp.
   fava bean Vicia fava
   hairy vetch Vicia villosa
   sweet clover Melilotus spp.
Brassicaceae (Mustard Family)
   Basket-of-Gold alyssum Aurinium saxatilis
   hoary alyssum Berteroa incana
   mustards Brassica spp.
   sweet alyssum Lobularia maritima
   yellow rocket Barbarea vulgaris
   wild mustard Brassica kaber
Other plant families
   buckwheat Fagopyrum sagittatum
   cinquefoil Potentilla spp.
   milkweeds Asclepias spp.
   phacelia Phacelia spp.

Several plant families contain species that are especially noted for their attractiveness to beneficial insects. The Umbelliferae, or carrot family, contains many such species. In an organic market garden in Massachusetts, flowering fennel attracted 48 species of ichneumonid wasps and 8 species of predatory wasps. Queen Anne's lace and wild parsnip have been shown to be attractive and effective in both field and lab studies. In the home garden, sequential plantings of dill, coriander, and caraway can be made to provide a continuous source of their valuable flowers.

The Compositae, or aster family, also contains many valuable species. Tansy is a long blooming perennial that is especially attractive to lady beetles. This is partly because of its flowers and partly because it is often infested with tansy aphids, an alternate prey which encourages the lady beetles to remain in the garden and produce more offspring. Several types of sunflowers have extrafloral nectaries (nectar-producing glands on stems or foliage) that attract and feed many types of natural enemies. Cosmos, coneflowers, coreopsis, and blanketflower are other common garden flowers that are attractive to natural enemies. Some organic market gardeners let a few lettuce or chicory plants go to flower as an easy way of providing flowers within their fields.

Many cover crops in the legume family provide ample habitat and alternate prey for a wide variety of natural enemies. If space is not a limitation, the dual benefits of soil improvement and natural enemy habitat can be achieved by planting a portion of the garden to one or more of the following: white or yellow sweet clover, other clovers, hairy vetch, big flower vetch, alfalfa, and fava beans. Mixing these with a small grain like oats, barley, or winter rye provides a more complex cover crop and may enhance both benefits associated with cover crops. Fava beans, like sunflowers, have extrafloral nectaries that attract many natural enemies.

The last plant family that has a large number of beneficial species is the Brassicaceae, or mustard family. As with lettuce and chicory, mustard family crops such as radish, mustard, arugula, broccoli, and bok choi can be left to flower after harvest. Sweet alyssum was used as a habitat plant in commercial lettuce fields in a California study due to its attractiveness to natural enemies, its quick time to bloom, and it's ease of establishment. Candy-tufts is another garden flower that may provide useful flowers.

It should be apparent, however, that a recipe for success using habitat manipulation does not exist. Instead, a thorough knowledge of the pests and natural enemies involved is necessary. Use resources such as books, journal articles, and extension publications to help determine exactly what eats the crop, when it does, what it looks like, what natural enemies eat the pest, and what alternate resources the natural enemies need. Consider altering gardening or farming practices such as planting times, cultivars, soil health, and the mix of crops planted together to thwart pests and enhance natural enemy survival.

The imported cabbageworm is a widespread and perennial pest of cabbage (and other crucifers). We will use it as an example of how to approach a pest control strategy using flowering herbs and/or shrubs. The imported cabbageworm overwinters as a pupa and adults emerge in May to feed either on wild mustards or on early cole crops. Adult cabbageworms are extremely mobile, fast-flying butterflies that can disperse over vast distances in search of plants on which to lay eggs. The larvae feed on leaves and may cause direct damage to the cabbage head. Many parasitic wasps and flies attack the cabbageworm in the Midwest. These wasps are very small and are frequently difficult to find in the field, and the flies are inconspicuous, looking rather like a house fly. Between these parasitoids, the cabbageworm is vulnerable to attack at every immature life-stage: Trichogramma evanescens wasps attack the egg; the wasps Cotesia glomerata and C. rubecula and the flies Lespesia and Phryxe attack larvae; and the wasp Pteromalus puparum goes for the pupae. Other natural enemies, such as hover flies and green lacewing larvae, contribute to egg and early-instar cabbageworm control.

The adults of all these natural enemies feed at flowers to supplement their energy and nutritional needs. Given the importance of parasitoids and predators in controlling pest populations, attending to the needs of these natural enemies should be a primary concern. A recent study in Michigan showed that yellow rocket, hoary alyssum, wild mustard, and wild carrot can significantly increase the lifespan and/or fecundity of a parasitic wasp that attacks the diamondback moth, another pest of cabbage. Other wildflowers, such as statice, daisies, coneflowers, parsnips, phacelias, and wild mustard species, are preferred nectar and pollen sources for parasitoids and predators alike. Planting a mix of these herbs along the border of a cabbage garden will keep the natural enemies active, long-lived, and laying eggs. Perennial borders are popular, but having narrow strips of such flowers coursing through the garden would be even better. Unfortunately, the cabbageworm butterflies feed at flowers, too. Past work has shown that cabbageworm populations as well as parasitoids can benefit from diverse plantings of flowers. Since cabbageworm pupae overwinter on wild mustards, it may be wise to cultivate these plants under at the end of the season. In this instance, the flowers serve a dual purpose: an alternate resource for natural enemies in the summer and a trap crop for cabbageworms in the late fall. Experimenting with various plant combinations will provide the gardener with an idea of what plants thrive in the local soil-type, how the plants co-exist, whether natural enemy populations are higher with these plants available and most importantly, whether the cabbageworm population is reduced.

Many plants other than those mentioned have been shown to have positive effects on natural enemies. However, these effects may vary from year to year, with location, or by which crop plants are involved, and this variability makes absolute recommendations difficult. This article is intended to provide some ideas and a few specific suggestions on providing habitat for natural enemies in your garden. We hope that by experimenting with these ideas and observing the results, you will begin to develop a system of habitat management that works for your own garden.

- Shawn Steffan & Paul Whitaker, University of Wisconsin - Madison

There are numerous references discussing in much more detail the topics outlined in this article, including:

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