Do Companion Plants Support Natural Enemies?

Dr. Norm Leppla, University of Florida, IFAS

Comprehensive lists of plants that ostensibly attract specific natural enemies are commonly published by organic farming magazines and county extension agents. Examples include Farmer Fred in Southern California; University of Maryland Extension; and Barbara J. Bromley, The Mercer County Horticulturist at Rutgers Extension in Trenton, New Jersey. Companion plants apparently are very useful for both attracting and retaining beneficial arthropods. They provide abundant sources of pollen and nectar for adult parasitoids and predators at critical times during the vegetable and ornamental plant growing seasons, and can help to increase natural enemies in the landscape. So called "pollen and nectar plants" include plants in the carrot family (Umbelliferae), dill (Anethum graveolens), fennel (Foeniculum vulgare), and parsley (Petroselinium spp.); mint family (Labiatae), peppermint (Mentha piperata), lemon balm (Melissa officinalis), catnip (Nepeta cataria), spearmint (Mentha spicata), and thyme (Thymus spp.); and Daisy family (Compositae), coneflowers (Echinacea spp. and Rudbeckia spp.), yarrow (Achilles spp.), daisies (many), and goldenrod (Solidago spp). Documentation of the value of these and other pollen and nectar sources to adult parasitoids and predators generally has been provided by Rodale Press publications and articles in magazines (Rodale's Successful Organic GardeningTM Companion Planting, Rodale Press, 1994). Unfortunately, however, popular books and magazines rarely cite scientific literature based on quantified studies of tritrophic systems containing different kinds of host plants, herbivores and natural enemies.

The efficacy of companion plants (polyculture) has been difficult to demonstrate for reducing pest populations and associated damage by attracting and supporting more natural enemies than found in monocultures. However, in a review of research on the effect of vegetational diversity on the relative abundance of herbivores and their parasitoids and predators, it was generally determined that natural enemies cause higher levels of herbivore mortality in polycultures than in monocultures (E. P. Russell 1989, Environmental Entomology 18:590-595). It has been shown that certain parasitoids and predators are attracted to specific annual and perennial plants, and that these plants support greater natural enemy reproduction and longevity. Examples include parasitoid wasps and hoverflies that are attracted to alyssum, cilantro, buckwheat, mustard, phacelia, fennel and yarrow (J. Luna et al., Organic Farming Research 6:7-9). In most situations, there is an inverse relationship between the number of natural enemies and herbivores, causing less plant damage due to feeding as herbivore populations decline. Thus, companion plants attract and maintain natural enemies that help to manage pests on adjacent crops; however, the level of benefit depends on the cropping system and geographic location. Research is needed to clearly link and quantify the effects of specific crop cultural practices, companion plant composition and phenology, herbivore composition, and natural enemy species and population levels relative to crop damage.