By Cynthia B. Lauer, Master Gardener, SCMG
Bad bugs are notorious in their diversity and in the damage they do. Aphids swarm zinnias, roses, and dozens of other prized plants, and spider mites suck the life from fruit trees. Thrips destroy rhododenrons and strawberries while leafhoppers damage vegetables and flowers. Gardeners seeking a natural method to control pest insects can use a form of biocontrol, the method of using one insect species to control another.
ABOUT BENEFICIAL INSECTS
Of the one million known insect species, only 1 percent are harmful like those that eat our prized plants, that bite or sting us, and that carry diseases. The remaining 99 percent are either benign or beneficial. Thousands of species of beneficial insects live in the average garden. Their diet of other insects is a function that can, and should, be encouraged.
Entomophagous insects—the kind that eat other insects—can be divided into two groups: predators such as the lady bug that capture and consume other insects, and parasitoids that spend part of their life cycle inside of, or attached to, another insect, eventually leading to the host’s death. Despite their common name, parasitic wasps are an example of this behaviour. True parasitic insects, such as ticks and mosquitos, feed from a host but will not kill it.
WHO ARE THE BENEFICIAL INSECTS?
Some examples will illustrate the fascinating range of beneficial insects. The taxonomic family of true bugs includes assassin bugs, a generalist predator that consumes hornworms, potato beetles, aphids, leafhoppers, and many kinds of caterpillars. For both adults and nymphs, insects are its only source of nutrition. Predatory stink bugs, including the large spined soldier bug, eat Japanese beetles, Mexican bean beetles, Colorado potato beetles, and others.
There are over 890 species of syrphid flies and hoverflies in North America. Their predaceous larvae eat soft-bodied insects like aphids, thrips, leafhoppers, scales, caterpillars and more. As adults, however, they are completely dependent on pollen and nectar for survival. With their striped abdomens, they may be mistaken as bees, but they are distinguished by their single pair of wings. Tachinids are among the most common and important beneficial insect in the garden. Adults feed on pollen, nectar, and honeydew and are important pollinators. As parasitoids, they often use various caterpillars as larval hosts.
Ladybugs belong to the beetle family. With over 480 species in North America, they are diverse in colour, spot number, and pattern; some North American species have no spots at all. Predaceous as both adults and larvae, most are generalists that feed on aphids, scale, mites, mealybugs, small caterpillars, insect eggs and pupae, and whiteflies. Each hatched ladybug larva will eat about 400 aphids before entering its pupal stage. Adults will eat as many as 50 aphids per day or about 5,000 by the end of their lifecycle. They also eat tomato hornworms, mealybugs, cabbage moths, whiteflies, and scales. Gardeners may spot other beneficial beetles such as the rove beetle, fireflies, ground beetles, and leatherwings.
Lady beetle adult and larva consuming aphids, t-mizo, CC BY 2.0
Parasitic wasps are a critically important group of beneficial insects. Ranging in size from minuscule to 7.5 centimeters, nearly all of them use other insects as hosts for their young laying eggs on, or inside of, the host. After hatching, the larvae develop by consuming the organs of its host, eventually killing it. The larvae then emerge and pupate into adults. Many insect species are vulnerable to attack by parasitic wasps: aphids, beetles, flies, scales, true bugs, leafhoppers, and many species of caterpillars. Chalcid wasps includes the genus, Trichogramma whose specialization in insect eggs make these minute insects the most widely released biocontrol agents in North America. Ichneumon and braconid wasps parasitize dozens of different caterpillars like cabbage looper, fall webworm, tent caterpillars, tomato fruitworm, cabbageworm, and corn earworm.
Parasitic wasp (Aleiodes indiscretus) parasitizing a spongy moth (Lymantria dispar) caterpillar. Credit: Scott Bauer. Agricultural Research Service, United States Department of Agriculture, ID K7659-1.
USING BENEFICIAL INSECTS IN THE GARDEN
The first step to using beneficial insects in your garden is identifying the insects that are eating your plants. Once you know your pests, you can learn the proper control insects. Don’t expect absolute elimination of all pests; predators and parasitoids will not exterminate their natural food source. With experimentation and observation, you should discover the right combination that encourages the most appropriate beneficial insects for your garden.
If you want beneficials to make your garden a permanent home, it is necessary to provide for their ongoing needs of pollen, nectar, water, concealment from their enemies, and safe spots for reproducing and overwintering. Pesticides and herbicides must be strictly avoided. Research has shown that beneficials eat and parasitize more insects when they can enjoy a pesticide-free environment rich in diverse plants.
A GARDEN FOR BENEFICIAL INSECTS
To welcome beneficial insects into your garden, choose a diverse mixture of plants with differing inflorescence types, bloom times, and growth habits. Interplanting of flowers, herbs, and vegetables ensures that sources of nectar and prey will be in close proximity. Another option is to install a dedicated insectary border with six to eight hours of full sun daily and away from direct wind.
Since most of these insects do not have the specialized mouthparts needed for accessing nectar from tubular flowers, they need to feed from tiny flowers with shallow, exposed nectaries. Tailor your plant selections to suit their needs. Examples of inflorescences that support beneficial insects are the umbels of dill and angelica, the corymbs of yarrow and boneset, and the composite flowers of coreopsis and rudbeckia. Perennial North American native plants recommended for beneficial insects are common yarrow, tickseed, shrubby cinquefoil, boneset, spotted bee balm, mountain mint, black-eyed Susan, cup plant, goldenrod, meadowsweet, asters, vervain, culver’s root, golden zizia, and the annual native sunflower.
Next time you see an insect on your beloved ornamental flowers, relax. It’s very likely to be benign or beneficial. That beetle, wasp, or other tiny flying thing is just doing its job keeping unwanted insects under control. You may never see evidence of their work. But if you do the right things, they will be there.
