On this spring afternoon one corner of our vegetable garden is dominated by a clump of volunteer coriander, from seed I 'missed' last year. The opposite corner is marked by a bright spray of canary yellow broccoli flowers, from a few side shoots we left in place when we harvested the crop. A few feet from the broccoli, in the perennial herb bed, there is a long, low patch of fragrant white sweet alyssum ("Snow Crystals"). All three plantings, seen from my comfortable seat on the back porch, are surrounded by swarms of small, dark, dancing motes. Were I to get up and go for a closer look (something my spring fever does not encourage, to be quite honest) I would see a host of tiny insects - many of them wasps and flies - darting in to feed on the nectar and pollen of small pink, yellow, or white blossoms, then darting away to do whatever it is such insects do when not feeding. Among those activities, I sincerely hope, will be mating and reproduction, and if I am very lucky, their offspring will call our garden home.
These wasps and flies make up a large part of that group of insects gardeners like to call beneficial insects. The larvae of these tiny, quick-winged creatures can be voracious predators on and parasites of many insects gardeners view with alarm and distaste, the insects we consider pests. And so the coriander was encouraged, the broccoli not completely harvested, and the alyssum planted with the idea of encouraging these little beasties to call a part of our backyard home. As the season progresses a series of plants will be cultivated with an eye toward maintaining the useful presence of these and other beneficial insects, who will then become an integral part in the management of the garden pests we're sure to see over the course of the year.
A beneficial insect is usually defined by the gardener as one that preys on insects that can damage plants of value to that very same gardener. For pest control purposes this is a good enough definition, and although I would include any insect that pollinates crops plants (some of those flies and wasps will do both, so attracting them has a double benefit) that is the definition I'll stick with here. It isn't strictly an either/or definition, of course. Paper wasps are beneficial in the sense that they carry off caterpillars (to be fed to wasp larvae at the nest), but I would certainly be cautious in encouraging these often aggressive insects to breed in my yard. The praying mantis could (and should) be considered beneficial, but bear in mind (when you mail-order those egg cases) that they are as likely to snatch and devour other beneficial insects - especially pollinators - as the pests you hope they will catch. But in the balance more good is done by such insects than harm, and so you would do well to attract and shelter them in your garden. (And yes, I class spiders as beneficials, though they are NOT insects. Can't help it, I'm a purist in these matters. Spiders have their place in the garden, but they are a story unto themselves.)
When you learn a little entomology, you quickly become aware that there are a lot of insects out there that can be considered beneficial by the above garden-oriented definition. There are a great many parasitic wasps, at least as many flies, the better known lacewings and ladybugs, and the minute pirate bugs. This is by no means an exhaustive list; these are merely the groups most commonly encountered, and the most easily attracted simply by planting flowers you won't mind growing anyway.
When I extol the virtues of wasps to other gardeners, the idea is always greeted with alarm. The word wasp almost immediately conjures up an image of hornets swarming from papery football-shaped nests, or the fierce stings of the common paper wasp. The wasps I'm talking about, however, are really small. They can't hurt you, and would have no reason to try. Some are so small that the average gardener is unlikely to ever even see them, unless of course you spend lots of time watching alyssum blossoms. If you do take to bug watching (and why not?) you will know the wasps when you see them; they look much as you would expect wasps to look. Variations on the theme of wasp. These are not insects that form colonies, as do their larger and more aggressive cousins. They lead solitary lives, seeking the larvae or eggs of other insects in which to lay their own eggs. The wasp larvae that hatch out devour their prey from the inside out, killing the egg or caterpillar in the process. (Yes, it's a gruesome thing to think about. Nature is always beautiful, but sometimes it just isn't pretty.) Some of these wasps are small enough to attack aphids or whiteflies, while others parasitize larger prey such as cabbage loopers, cutworms, and tomato hornworms. (You've got to love any insect that helps you grow good tomatoes!) Watch your garden closely and see if you don't find, from time to time, a hornworm covered with little white bumps. Those bumps are wasp eggs; leave that hornworm in the garden instead of destroying it, as you normally would, and soon a swarm of new wasps will hatch out of it, ready to further reduce the hornworm population of your garden.
The flies are more easily noticed, and some in fact are mistaken for bees. There are two fairly large groups of beneficial flies: tachinid flies and syrphid flies. The tachinid flies look the way you would expect a fly to look, and are in fact close kin to the common housefly. They come in a variety of shades of black, brown, or gray, and often have patterned wings. For the most part tachinid flies prey on caterpillars, including cutworms, gypsy moths, tent caterpillars (whose sticky web tents do not protect them completely from these flies), and the ever present cabbage looper. Unlike their less popular housefly kin, these flies are not attracted to garbage or dead things; they feed on nectar and pollen as adults, and so are not carriers of disease as far as humans are concerned.
The syrphid flies include those strange little winged wonders we call hoverflies. Many of them mimic the appearance of bees, perhaps as a way of putting off birds who might otherwise eat them. Appearances are deceiving; they have no stingers, and are completely harmless. (Well, to us they are harmless.) Watching these insects can pose a challenge, since they can seem to vanish while you are looking, as if teleporting from place to place instead of simply flying. These little guys are fast! Blink, and you've missed them. The adults feed on pollen and nectar, and many are good pollinators, especially of plants with flowers too small to get the attention of honey bees. The larvae of syrphid flies are hunters, preying on mealybugs, aphids, and any other small insect unlucky enough to get in the way.
The larvae of ladybugs and green lacewings are also hunters of small, soft-bodied insect pests. Ladybugs have become a sort of symbol for organic pest control practices, but unless they stay around and reproduce, they aren't much help in the long run. I have found the green lacewing to be much more dependable. The adults are easily recognized, being green with delicate, gauzy, iridescent wings, and eyes like tiny gems; you often see them attracted to porch lights on summer evenings. I find the green lacewing to be the most attractive of the beneficial insects. They lay their small white eggs on thin stalks that are attached to a leaf or stem; altogether it looks like one half of a tiny cotton swab. Lacewings of some form (there are also species that are small and amber brown and unfortunately easily overlooked) live just about anywhere people garden, and are easily attracted. Both lacewings and ladybugs eat small insects as adults as well as during their larval stages, but readily supplement this diet with pollen and nectar.
Less easily noticed, but still important, are the minute pirate bugs. I myself have only noticed them in my gardens twice, but I suspect they are often present whether I see them or not. (After all, I make sure what they need in terms of plants are growing, but we'll get to that in a moment.) These insects are very small, usually less than a quarter of an inch long, and are usually black and white. Very small, very plain, and voracious! They feed primarily on spider mites, small - as in newly hatched - caterpillars, and thrips. (Yes, thrips; now that got the attention of rose fanciers, I'll bet!)
All of the above, as adults, eat nectar and pollen, and prefer to obtain these foods from very small flowers. It may be a matter of scale that explains this preference, or it may have something to do with avoiding competition with larger insects. Whatever the explanation, the inclusion of very small flowers in your garden plan is the best way to attract and encourage these valuable insects. The list of plants that can be used for the purpose of attracting beneficial insects is almost ridiculously long, and if you are concerned about how the garden will look, you will be able to find species that are as pleasing to your eyes as they are to the tastes of lacewings and tiny wasps. The following should give you some idea of the range you have available: coriander, fennel, dill, Queen Anne's lace, rue, thyme, mint, goldenrod, buckwheat, sweet alyssum, sunflower, Buddleyia, bee balm, horehound, parsley, yarrow, and ceanothus. And, oh yes, broccoli. I could go on, but by now you get the point.
Certain families of plants dominate the list, such as the carrot family and the mints. These two require a bit of extra planning to use as attractors of beneficial insects. For the carrot clan it is simply a matter of timing or careful selection. Many of these plants are biennial, which means it takes two years for them to complete the cycle from seed to flower. Keep this in mind as you choose them, since they won't be attracting many beneficial insects this year. Or use plants that flower in the first year, such as coriander. Plants such as dill, coriander, and Queen Anne's lace - when allowed to flower - are among the best beneficial insect attractors you can grow. Of course, if you want your own dill seed or coriander, you'll want them to flower, so you have a double benefit from growing these plants.
The mints, especially the peppermints, are notorious for their ability to spread. To call the common peppermint invasive would be an understatement. The problem can be solved by growing the peppermints and spearmints in hanging baskets near the garden. (Although it's a good idea to plant your tiny flowers in and among the vegetables, it is not strictly necessary. Insects have wings, and they know how to use them!) However, since most of us grow peppermint for an entirely different purpose, and harvest the stuff regularly enough to prevent flowering, consider a few of the other members of the mint family available for the garden. One of my favorites is horehound, a robust plant that is nearly indestructible and produces clusters of small white flowers that draw tiny flies like magnets.
One flower that might catch your attention on the above list, given that I'm talking about matching flower and insect sizes, is the sunflower. After all, that's a really big flower, right? It is - and it isn't. Sunflowers belong to a plant family often called Compositae, the Composites, so named because the flower we see is actually a mass of tiny flowers crowded together on a disk-shaped structure. In the case of the sunflower there are a great many tiny flowers stuffed side by side onto that disk, with only those on the outer rim having a single, strap-like petal. (Those outer flowers are called ray flowers - for that petal - while the inner flowers are called disk flowers.) Each tiny flower will give rise to a single fruit, which we salt and eat as a sunflower seed. Members of the composite family are powerful attractors of beneficial insects, as well as bees and butterflies. Chamomile and cosmos are two other composites you might consider when planning your garden for the sake of beneficial insects. Both would be dual-use plants, the former being a useful herb, the latter being just beautiful.
Planting flowers that attract beneficial insects is an important first step, the key ingredient, so to speak. There are others things you can do to make the small predators feel that your garden is home. A shallow water source can be very important, especially in warmer weather, or during a dry spell. If you have an ornamental pool or pond nearby, you're all set. If not or if, like me, you live in a dry climate, a flat, shallow dish of water will be greatly appreciated. Set it on the ground, or set it into the soil almost to the rim, whichever situation makes it least likely for it to be tipped over by accident. A location that receives bright morning sun seems to be best. Fill this dish to within a quarter of an inch of the rim with coarse gravel, and then top it off with water until the tips of the pebbles just stick out. This creates a situation that prevents insects being trapped by the surface tension of the water when they come for a drink. If they get caught (and you've no doubt seen insects meet this fate in a pond or swimming pool) they can pull themselves free on the nearest pebble.
You might wonder that I would recommend a water source that seems tailor-made to encourage snails and slugs. If you live in a climate damp enough that such beasties are a chronic and devastating problem, you probably don't need an extra water source for beneficial insects, unless there is a drought. Elsewhere the beneficial insects attracted to the water will control pests that you will find harder to manage by hand than slugs and snails. (Frankly, the moluscan weakness for beer has always made them an easy pest to control in my garden.) The trade-off is worthwhile, since flies such as aphid midges have been found to double their egg production when supplemental water is on hand.
With the plants you provide shelter and food; with a shallow pan of water you provide extra moisture. The garden itself provides the food for their larvae. What else? There is one more very important consideration: pesticides. If you want to attract, encourage, and make use of the smallest of the insect predators, pesticide use (whether modern chemical or more benign "natural" materials) must be kept to the lowest lever possible, or eliminated altogether. Any general use pesticide will kill insects indiscriminately, pests and beneficials alike. Obviously this defeats the purpose of planting to encourage beneficial insects. The problem is far more insidious, however, since the beneficials (under even the best circumstances) reproduce more slowly than the insects they depend on. This makes perfect sense, when you think of it; basic ecology in action. If the predator breeds faster than the prey, eventually the predators run out of food and starve. So when you blast a garden with a general pesticide, you kill everything in the path of the sprayer, friend and foe alike. Unfortunately the pests will breed a lot faster than their predators, and so when the spray wears out, they have a population explosion. Eventually the predators and parasites rebound and take advantage of all that food waiting for them, but by then the pests may have done considerable damage to your garden. Unless of course you simply spray again. And again.
A word at this point on purchasing eggs or insects from catalogs: this is a good back-up plan if you are starting to garden in a suburban yard that has been subjected to repeated pesticide applications. It is unlikely that even the most zealous anti-bug campaign has eliminated every insect in the area, but such a regime almost certainly has reduced local populations. Suppose you have inherited a situation that has seen such abuse, and want a garden. Are there beneficial insects still around? The only way to find out is to plant a garden and then watch what happens. Unfortunately, given the awesome reproductive rates of mites, thrips, and aphids, what you are likely to see first are lots and lots of pest problems. If in doubt, (and if you don't want to go the chemical route) try ordering such beneficial insects as are available commercially, and then plant the garden in a way that provides the plant diversity needed to attract and support the beneficial insects native to your area. As time passes the population of beneficial insects will recover (Congratulation, you now have a really small wildlife refuge!) and it should not be necessary to buy anything except the right kind of seed for attractive flowers.
Even if you are not overly concerned with alleged damage to the environment, all that spraying starts to cost money in a hurry. You can never eliminate the pests permanently; you can only control them with chemicals, something the beneficial insects can (if given the chance) help you do far more cheaply. They become a backup system for the time-honored method of going out every chance you have, checking for pests, and zapping them with Soap-Shield, pepper spray, or the pinch of thumb and forefinger. Chemical controls also, for my money, leave you with a much less interesting garden than you might have had otherwise. Insects are fantastic creatures, worth going out in the garden and watching. They can be as comical as the syrphid flies that seem to vanish before your eyes, only to reappear a moment later. They can be as graceful as the swallowtail butterflies drifting by on a summer day. Or they can - as in the praying mantis - be as strange as any science fictional beastie ever imagined. If you lay off the chemicals and encourage beneficial insects you will always have a few pests to take care of come the weekend, but you will rarely (unless you neglect your garden duties) face any real disasters. And your garden will always be an interesting place, even when the roses are not blooming and tomatoes are not ripe.
And if you plant flowers that attract the tiniest of the beneficial insects to your yard, it will also be a very beautiful garden, well worth an afternoon of contemplation on a warm spring day.
About the Author Thomas Watson is graduate of the University of Arizona College of Agriculture, where he earned a BS in Agriculture, majoring in plant sciences. When he is not writing he is usually working in the garden, re-landscaping his Tucson home, studying botany and plant identification, or "out there" somewhere, studying the natural history of the Sonoran Desert. This is all, of course, based on the assumption that it isn't baseball season, in which case he'll get all of the above done before the next Diamondbacks game.