Natural Enemies in Your Garden: A Homeowner's Guide (E2719)


October 27, 2015 - Author: Douglas Landis


The home landscape is a complex habitat possibly consisting of vegetables, flowers, turf, woody ornamentals and other desired, and in some cases, undesired plants. For most of us, our garden is a relaxing place where we tailor the environment to our aesthetic and physical needs. Yet the garden is also home to creatures we consider pests when we find them in our broccoli or apples or on our prized rose bushes. As a result, the home landscape has become the repository of nearly 11 percent of the conventional pesticides used in this country. Indeed, acre for acre, your cousin Vinny's tomato patch has more pesticides than farmer Joe's soybean field! Fortunately, the garden is also home to our friends, the natural enemies of pests.

Most gardeners learn a great deal about their plants' growth needs, but they often know little about the insects in their gardens. Most of the insects in a garden are not harmful pests. The vast majority of insect species in North America are either beneficial or harmless to humans and garden plants. To take advantage of the work that natural enemies do (kill pests), we must first know which ones we have and help them flourish. Using natural enemies to control pests reduces your need to use pesticides and lets you take a bite from cousin Vinny's tomatoes, right off the vine!

Who are Our Friends in the Garden?

Most of us are familiar with spiders, ladybugs and praying mantids and know they eat a lot of bad bugs. Luckily, many other natural enemies are also taking care of pests. There are three major groups of natural enemies: predators, parasitoids and pathogens.

Predators, such as ladybugs and spiders, eat many prey in a lifetime. Often they are larger and stronger than their prey and the most visible natural enemies in our garden. Some are quick running hunters, while others sit and wait for a victim to pounce on.

Parasitoids are specialized insects that develop as young in one host, eventually killing it. Unlike predators, they usually kill only one prey during their immature stage. Many flies and wasps are parasitoids, but they are usually small and therefore go unnoticed.

Pathogens — nematodes, viruses, bacteria, fungi and protozoans — cause diseases. Many of these naturally occur in our gardens; others need to be introduced. Commercial companies have begun to develop many of these pathogens as bait or spray formulations, making them easier for us to use.

Making Biological Control Work for Us

Biological control uses natural enemies to keep unwanted pests at low levels. To practice biological control in the yard, you should know the three basic approaches.

Classical biological control is used when pests are exotic in origin and exotic natural enemies are imported and released to bring about control. This is conducted by federal and state agencies. Although we homeowners will not be importing natural enemies into our backyards, some of the natural enemies described in this book were brought from other countries and established here. The importation of parasitic wasps to control alfalfa weevil in the Midwest has been a widely successful classical biological control program.

Conservation biological control encourages existing natural enemy populations to flourish in the area and suppress pests. This involves reducing practices that harm natural enemies as well as implementing practices that improve natural enemy longevity, reproductive rate and effectiveness.

Augmentation biological control is the release of natural enemies into the environment in high numbers. This is done when natural enemies do not thrive well in the environment or are not active during the time of pest activity. Conservation and augmentation of natural enemies are the home gardener's tools to subdue pests, so let's get started!

Conserving Natural Enemies

Conservation Step 1: Don't Reach for the Pesticide Spray

To conserve natural enemies in the home landscape, first and foremost, we need to reduce insecticide use. The chemicals we spray to get caterpillars off broccoli also kill or reduce the livelihood of natural enemies. Natural enemies take longer to re-establish themselves than pests do. Using pesticides may also create new pests because it kills natural enemies that are suppressing minor pests without our knowledge. When the natural enemies are killed by a pesticide, these minor pests can become major problems.

Kinder Options

Home gardeners have many insect pest management options other than insecticides. Adopting these options with attention to the life cycles of pest and beneficial insects is a key component of integrated pest management (IPM). Some IPM practices include preplant cultural operations, such as selecting insect-resistant varieties, crop rotations and companion plantings. After the garden has been planted, harmful insects can be managed in a variety of ways.

• If the garden is relatively small and insect pests are few, hand picking remains one of the most effective means of insect control for a gardener.

• Traps or barriers can be useful for some pests, and biological control agents that are commercially available can be very effective against specific insect pests. A word of caution: not all traps are effective (see section on electric traps).

• When all other measures have failed, very selective and well timed spot treatments of individual plant parts with a low-impact insecticide (such as insecticidal soaps or horticultural oils, which are relatively safe compounds for beneficials) may be considered.

• Tolerating a modest level of insect feeding on your garden vegetables will reduce the need for chemical inputs. If cabbageworms eat part of your cabbage head, you can always cut off the nibbled part and use the rest.

Conservation Step 2: Making a Home for Natural Enemies (Habitat Manipulation)

Natural enemies require more than just food (pests to eat) to complete their life cycles. Predators and parasitoids may need an overwintering site, protection from heat and desiccation, plant food sources and early season prey to sustain them if pests are not present. Managing the garden habitat to meet the needs of predators and parasitoids is an excellent way to conserve these garden friends and minimize the harmful effects of crop production on them.

Overwintering sites — Most pests are generally better at dispersal than their natural enemies, so a garden may get colonized by pests long before natural enemies arrive. For this reason, it is all the more important that overwintering sites, such as flowering borders, hedges and other perennial habitats be provided for natural enemies. These vegetative sites insulate natural enemies from the winter chill. While some natural enemies may overwinter in the bare ground, we later prepare the ground for planting and disrupt their homes, sometimes killing them. When parts of the garden include undisturbed perennial plantings, natural enemies are more likely to survive the winter.

Mulches — Using mulches can reduce weed growth while providing humid, sheltered hiding places for nocturnal predators such as spiders and ground beetles. Also, the mulch may 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 soil surface.

Flowers — Having certain flowering plants available can greatly increase the longevity and fertility of many natural enemies. A study in Canadian apple orchards showed that parasitism of orchard pests was four to 18 times higher in orchards with many wildflowers than in orchards with few flowers. A number of plant species have been shown to encourage natural enemies (see Table 2). Ladybug and lacewing adults often feed on pollen. Many natural enemies that benefit from floral nectar are small parasitic wasps, often smaller than a mosquito. Consequently, flowers that are good for them are usually small, not overly tubular and relatively open. In addition, flowers ought to synchronize with natural enemy activity. Planting a mixture of plants that bloom for long periods and overlap in time will ensure that food sources are available when natural enemies are active.

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 for natural enemies. Sequential plantings of dill, coriander and caraway can be made to provide a continuous source of valuable flowers.

Ground covers — Leguminous cover crops improve soil fertility and provide shelter, floral food sources and alternate prey for a wide variety of natural enemies. In cover crops, non-pest prey may be present and sustain natural enemies if their favorite pest has not invaded the area.

It should be apparent that a single recipe for success using habitat manipulation does not exist. Consider altering gardening practices such as planting times, selecting cultivars and mixing crops together to thwart pests and enhance natural enemy survival.

Augmenting Natural Enemies

If naturally occurring predators and parasites are not sufficient to control garden pests, augmenting them with releases of commercially available natural enemies may be effective. Insect pathogens are often produced as formulations called microbial insecticides for use in the yard. Not all natural enemies are available because rearing them may be difficult or costly.

Natural Enemies for Hire

What is available? Green lacewings are commonly sold and are a good option for aphid control. These predators can be purchased and released early in the growing season for earlier and more effective control. The eggs are shipped in small containers mixed with bran (or another filler, such as vermiculite) that protects the eggs during shipping. All you have to do is sprinkle the contents of the container on the plants. Other very popular natural enemies are Trichogramma wasps to control cabbageworms, cabbage loopers, tomato hornworms and other caterpillars, and beneficial nematodes to control a wide variety of garden pests. The most commonly used microbial agent is a bacterium, Bacillus thuringiensis, or Bt. Microbial insecticides are sold by many chemical companies and common in stores. If you are interested in purchasing predators or parasitoids, this is a good free source of information:

— Suppliers of Beneficial Organisms in North America by C. D. Hunter, from: California Environmental Protection Agency Department of Pesticide Regulation Environmental Monitoring and Pest Management P.O. Box 942871 Sacramento, CA 94271-0001 bensuppl.htm

What to Consider When Ordering Natural Enemies

Ask the supplier for specific information and recommendations for your particular situation. Ordering and releasing natural enemies can be successful if you:

• Know the specific pests you need to control.

• Know the best natural enemies, either singly or in combination, for the target pest or pests. Make sure companies provide the exact name of the natural enemy that they are selling.

• Know the proper time to release the natural enemy. Timing should be based upon the life cycles of the pest and the natural enemies.

• Know the proper release rate for each natural enemy.

• Calculate the number of natural enemies needed on the basis of release rate, area to be covered and severity of pest infestation.

• Know the recommended frequency of release if multiple releases are necessary.

• Provide a safe delivery address, one where the shipment will be cared for as soon as it arrives and where it will not be exposed to temperature extremes.

• Understand proper release practices so that you will be prepared to make releases when the shipment arrives.

• Understand proper storage requirements if releases are not to be made immediately after arrival.

Making Augmentation Successful

Releasing loads of natural enemies in the garden will take money from your pocket, but will it guarantee success? Sometimes predators and parasitoids will leave the area or will not survive long enough to have an impact on pests. Maintaining a suitable habitat, as discussed in conservation biological control, is critical to improving the natural enemy's efficacy. Be very judicious with insecticide use when releasing natural enemies. Commercially reared insects will likely find it just as hard to survive insecticide exposure as your normal yard insects. Also, a diversity of plantings (flowering plants, ground covers) will help maintain an adequate food supply and shelter so released natural enemies will remain in your garden doing what you paid for them to do.

Discovering Natural Enemies in Your Backyard — How to Sample

Your garden contains a wealth of small critters. We tend to notice the bad or pretty ones, but there are plenty more out there — we just need to keep an eye open for them. Here is how you can discover the wonderful diversity of insects and other critters.

• Spend an hour or two outside sitting quietly and observing. When flowers are in bloom, there is a lot of activity — many flower feeders and pollinators come by. If you are lucky, you might see a predator that was hiding in the flower leap out and attack its unsuspecting prey. If the plant has aphids, you may notice a ladybug or other predator feeding on them. Sometimes you may see ants taking care of the aphids. The aphids provide ants with honeydew, so the ants will guard them against any predator.

• Dig up some soil and spread it out on a white surface. You'll probably see earthworms, a good sign. Perhaps there are little things jumping about. These springtails are important decomposers and relatively harmless to crops. You may also see less mobile pupae and grubs as well as adult ground beetles that run frantically when disturbed.

• Put bright yellow cards covered in petroleum jelly in various areas — in tree foliage, above grasses and in your vegetable planting. Why bright yellow? This color is attractive to many pest and beneficial insects. The traps will kill them, so it is advisable not to keep them out for long. Adult parasitoids, small wasps, often fly into these traps, and you never knew they were there before in your garden.

• Fill yellow plastic pans with soapy water and place them on the ground. Many aphids and flies will find their way into the pans.

• Make pitfall traps by inserting plastic cups into the soil with the rim just below the soil surface so that any strolling insects will fall in. Leave them out overnight and you may be surprised by the number of ground dwellers active at night. Spiders and ground beetles, both good predators, will dominate the traps. If there are insect remains, they may have been feasting on one another through the night.

• Place a white sheet under a tree or bush. Beat the branches with a stick or shake them with your hands. You will find that spiders, true bugs, caterpillars and sawflies come tumbling down.

Natural Enemies of Common Pests

With this table, you can quickly identify the particular natural enemies that can help control common pests in your garden and home landscape. Look in the left column for the pest you want to control. The right column will guide you to the pages for more information.


Predators kill more than one prey in their lifetime. Birds, mice, frogs, insects and other arthropods all eat insect pests. For pest management, the six-legged and eight-legged creatures, insects and arachnids, are the most important control agents.

Predators may be stronger and larger than their victims. Some ground-dwelling predators have long running legs and use speed to catch prey. Others are camouflaged and strike when prey are in close range. Some really smart predators build traps to bring them dinner. Have you seen cone shaped pits in sandy ground? At the bottom of the pit is a young ant lion waiting for an ant to stroll by. When an ant walks near, the young ant lion throws up sand to get the ant to tumble down into the pit. Using strength, speed or trickery, predators can catch and eat a lot of prey, and that is good news for us.


Parasitoids develop as larvae within the body of the host. Generally, the adults are free living and their main job is to mate, find hosts and then lay eggs in or near hosts. Adult parasitoids are often short-lived; they may feed on plant sources. Some also feed on the hosts, which means they kill more pests in a lifetime.

The most important parasitoids are mainly flies and wasps. These wasps are not like the yellow jackets that may come to mind. They are often very small, some no bigger than the period at the end of this sentence. The females may have long, slender tubes at their hind end. These are ovipositors, special tubes for laying eggs.

You will notice that the physical descriptions of the parasitoids are not very detailed. Most entomologists have a hard time telling them apart! Nevertheless, anyone can spot their activities. Parasitized aphids often turn black or brown and develop a hard casing. Parasitized eggs may also become much darker. You may find a caterpillar with cocoons on its back. They are the unwitting factories for making more helpful parasitoids.


You may have seen a dark and mushy caterpillar or a fly stuck to a window with white threads. These insects have succumbed to pathogens. Many microorganisms — bacteria, fungi, nematodes, protozoans and viruses — can naturally bring down pest populations in the field if certain environmental conditions are right. Pathogens are hard to manipulate because we cannot control the climate. Thus, their use in biological control is mainly augmentative, and some are sold commercially as microbial insecticides.

All pathogens except nematodes are regulated much as chemical insecticides are. This can be a plus for consumers because more information is available about them than other natural enemies. These microbes are more sensitive than chemicals — they have a shorter shelf life and special storage requirements and application procedures.

Microbial insecticides are generally harmless to animals and humans and can specifically target certain pests. They do not persist long outdoors, and this reduces the risk of insects becoming resistant to them. When used as instructed, these microbes can be valuable biocontrol agents themselves and help to conserve predators and parasitoids from more harmful chemical pesticides.



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