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Beneficial Bugs—Luring Predatory Insects to the Garden with Umbelliferous Plants
Plants & Gardens News Volume 16, Number 2 | Summer 2001
by Niall Dunne
The Apiaceae or Carrot Family is a grand old clan with a new name. (Until very recently, botanists called it the Umbelliferae.) It includes such common culinary favorites as coriander, dill, fennel, parsnip, anise, cumin, carrot, and parsley. Yum yum! But it also includes a lot of other plants that wouldn't taste so good in your soup, such as hemlock (Conium maculatum), the poisonous species famously used to execute the ill-fated Socrates.
The Carrot Family is comprised of around 3,000 species—reflecting roughly 300 different genera—of mainly herbaceous plants native to temperate regions around the globe. It has the distinction of being the first plant family ever to have been systematically studied. (A copy of Robert Morison's pioneering monograph of 1672, Plantarum umbelliferarum distributio nova, can be found in BBG's rare book room.)
Dill (Anethum graveolens) attracting an ichneumon wasp
The family tie between these plants is clearly visible in their flower clusters, which generally look like miniature, flat-topped parasols. Botanically, these marvels of inflorescence are termed umbels (from the Latin word umbellula, meaning—hold on to your hats—"umbrella"). In a typical umbel, individual flower stalks arise from the same point on a primary stalk and stretch at different lengths so that the small flowers on top are all roughly on a level plane.
Folks as far back as the ancient Chinese, Greeks, and Romans were aware of the shared floral characteristics of some of the plants in the Apiaceae. They were also aware of the plants' rich and varied chemistry, harvesting seeds and stalks and using them not only for food, but also perfume, medicine, and (as mentioned already) poisoning troublesome philosophers.
Historically, then, umbellifers have been of enormous biological importance as crop plants. In modern times, however, they have something else going for them as well: they're very attractive to beneficial insects—the so-called "good bugs" that act as pollinators, soil builders, or predators of pest insects in the landscape. Plants with umbels are magnets for predatory bugs in particular.
Lovage (Levisticum officinale), for instance, is beguiling to ichneumon wasps, which parasitize the larvae of herbivorous insects. Similarly, fennel (Foeniculum vulgare) attracts, among other beneficial bugs, lady beetles that prey on aphids, scale insects, thrips, mealybugs, and mites. Dill (Anethum graveolens) is very adept at luring such insects as lacewings, whose larvae are well-known aphid-devouring machines.
This kind of knowledge is very useful in an age when people are becoming increasingly concerned about pesticide use on their food, and about the health of the environment in general. The ability to attract and maintain a population of beneficial insects is very important to any large- or small-scale pest management scheme that seeks to cut down on the use of chemical sprays.
Bug Banquets
Although beneficial insects consume large numbers of pestiferous bugs, they often have to supplement their protein diets with plant pollen and nectar. Indeed, many of these insects have certain phases in their life cycles when they depend entirely on nutrients collected from plants.
In recent years, agricultural and habitat management scientists have been conducting field research to try and determine which plants offer the best nectar and pollen resources to natural enemies of insect pests. One plant family consistently comes out at, or near, the top: the Apiaceae. (Members of the Asteraceae or Aster Family and Brassicaceae or Mustard Family also have proven track records.)
In one study conducted by Oregon State University in 1997, eleven plant species were randomly arranged in small plots alongside a field of corn. Researchers then measured the feeding frequency of aphidophagous (aphid-feeding) hoverflies on each plant. It turned out that coriander (Coriandrum sativum) was the plant most visited by the hoverflies in the early growing season. Fennel became the late-season plant of choice, after coriander had stopped blooming.
What makes umbelliferous plants so appealing to beneficial insects? Dr. May Berenbaum, head of Entomology at the University of Illinois, Urbana-Champaign, and author of Bugs in the System (Addison Wesley, 1995), summed it up nicely for me: "Small flowers with accessible nectar and a nice landing platform."
Although predatory insects often have custom-fitted mouthparts for eating other bugs, they are generalists when it comes to feeding on nectar and pollen. Most of them are also on the smaller side. So, invariably, they seek out diminutive, closely spaced, easy-to-land-on flowers that are shallow but brimming with exposed grub. Other factors like bloom time, flower color (usually white or yellow in the Carrot Family), shelter, and presence of prey play an important role, too.
Botanists have recorded very little specialization in umbel-pollinator interactions. Umbelliferous flowers are morphologically so uniform and their insect visitors so varied that some scientists refer to the plants as being "promiscuous." One might normally associate "promiscuity" with waywardness or a lack of discipline, but in the case of pest management, an orgy of umbellifers can contribute, ironically, to an increase in biological control.
Triple-duty Beauties
Most of the research done on the beneficial-bug magnetism of the Apiaceae has so far focused on the culinary herbs. Gardeners looking to increase the number of predatory bugs in their vegetable patches need look no further than these plants. But what about gardeners who want to fight pest insects in the rest of the garden, too?
Well, we know that some of the culinary herbs can also make dramatic statements in purely ornamental settings. Korean angelica (Angelica gigas), for example, with its dark purple flowers and tall red stems is a great centerpiece plant for the perennial border. (It's also good at attracting lacewings, lady beetles, and parasitic wasps.) Dill, with its thread-like, blue-green foliage and lacy, aromatic, deep-yellow umbels, can look good planted almost anywhere in the yard.
In her book, Great Garden Companions (see box), Cornell Cooperative Extension specialist Sally Jean Cunningham recommends the summer-blooming, umbelliferous sea hollies (Eryngium species) as striking plants for the front of a perennial cluster. Not only do they have, according to Sally, attractive "leathery, blue-gray, spiny foliage" and "silvery blue flower heads with dome-shaped centers and silvery leaf-like bracts," but they also are good at attracting parasitic wasps.
As yet there just isn't much data in on how well the more decorative members of the Carrot Family perform as insectary plants. "The science of which plants attract or maintain which insects, at which time of year, in which climate zones is at an elementary stage," says Sally.
So this is a good opportunity for gardeners to get in on the action and study such traditionally planted ornamental genera as Astrantia (masterwort), Myrrhis (sweet Cicely), Aciphylla (speargrass), and Bupleurum (thorow-wax) for signs of beneficial insect activity. But you don't need to stop there. The ornamental palette of the Carrot Family is broadening steadily.
In the April 2001 issue of Horticulture magazine, plantsman Daniel J. Hinkley wrote about some of his favorite lesser-known umbellifers. He recommended the spring-blooming Chaerophyllum hirsutum 'Roseum' for its "low spreading mounds of handsome, deeply dissected foliage" and its "lacy heads of rose-colored flowers." He also gave special mention to Pimpinella major 'Rosea' (greater burnet), whose sturdy, 3-foot stems produce "lovely discs of light pink" from July to August.
I asked Dan if he had noticed any high insect activity on the specialty umbellifers that he grows at Heronswood Nursery. He couldn't say for sure, but his interest was definitely piqued. "You know, come to think of it, there's always a swarm of wasps flying around our angelicas."
Niall Dunne is the editor of Plants & Gardens News.