Controlling Garden Pest Populations

An ecosystem view of controlling garden pest populations while keeping beneficial insects healthy, including integrated pest management, pest control techniques and how to choose a pesticide.


Without deliberate manipulation by man, insects and other animal populations are naturally controlled by a variety of factors.


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William and Helga Olkowski are entomologists, founders of the Farallones Institute, authors of The City People's Book of Raising Food (Rodale, 1975), and co-directors of the Center for the Integration of Applied Science (a division of the John Muir Institute for Environmental Studies, Inc). Who, then, could be better qualified to offer us some timely advice. 

The article excerpted here first appeared in Horticulture magazine, June 1976. (Subscriptions: $18 per year from Horticulture, Subscription Department - TMEN, Boulder, Colorado.) Reprinted by permission of the authors. 


Without deliberate manipulation by man, insects and other animal populations are naturally controlled by a variety of factors. It is necessary to know something about these natural controls to understand why relying on synthetic chemical insecticides can have undesirable effects.

Weather is a commonly recognized factor. It may become too hot, too cold, too wet, or too dry for any particular pest population. Gardeners in every part of the country have their local examples of what such conditions as a freeze or a wet spring might mean in terms of problems later in the year. In California, for instance, the oak moth (Phryganidia californica ) overwinters as a small caterpillar upon the leaves of the live oak tree (Quercus agrifolia ). Normally, heavy rains and light freezes help to reduce its numbers during the winter months. But when the season is unusually dry and warm, many more caterpillars may survive to defoliate the trees when spring arrives.

Food supply is another factor affecting pest populations. On a farm, large monocultural stands of the same species of plant may allow a huge buildup of a pest. This is less likely to be a problem, however, in a small and more highly diversified home garden.

Habitat is also important as a limiting factor for animal populations. For example, when garden slugs are reproducing beneath boards that are placed directly on the ground as walkways in wet weather, by simply placing fine dry sawdust under the boards you can reduce the slug population while avoiding the problem of muddy footpaths at the same time.

Biological controls, including diseases and natural enemies, are particularly important in keeping down pest populations. While most gardeners are aware that toads, spiders, some birds, and other predators eat insects, it is not as widely recognized that an insect's major natural enemies are other insects. The natural enemies can be broken up into two groups: predators (which usually eat a lot of their prey at one sitting, and eat many different species of pests), and parasites (insects which live on, or inside, another insect, lay eggs inside, and usually cause the host to die). Examples of predators are ladybird beetles (Coccinellidae), lacewings (Chrysopidae and Hemerobiidae), and syrphid (Syrphidae), or hover, flies. The parasites are so tiny that most people are not aware of them and they have no common names. Most are members of the order Hymenoptera and are referred to as miniwasps.

The distinctions are important because parasitic insects are extremely "host specific". For example, the parasite that attacks the cabbage aphid doesn't attack the rose aphid. These are the "silver bullets" of pest management, and learning how to conserve them, import them, and release them is a very important pest management technique. They're already out there; in fact, they are the reason you've been raising anything at all, because without them all your plants would be devoured by the herbivorous insects. The cardinal principle of good pest management, therefore, is to disrupt these natural controls as little as possible, or you may end up making things worse instead of better.

Unfortunately, the injudicious use of synthetic chemical insecticides may do just that: make the problems worse or create some where none were before. These results can occur in several ways.

Resurgence is one common effect of an insecticide treatment: You see some bugs on your plants, you treat them with a pesticide, and in anywhere from a few days to a number of weeks later, the same kind of bug is back again, only this time in higher numbers than before. What happened? Well, you accidentally killed of the pest's natural enemies along with the pest. It is rarely possible to kill off all the pests, and without any predators or parasites to hold them in check, those that didn't die, or those that flew in from your neighbor's yard, were able to multiply without restriction.

The pests are plant feeders, or herbivores, and tend to move around less than their predators, the carnivores. The predators must actively seek their prey, thus covering more ground and coming into contact with more of the poison. Furthermore, there may be a lot fewer predators out there than pests. Each ladybird beetle must eat a lot of aphids to survive, so there will only be as many as its food supply can support. Also, the predators may take longer to reproduce themselves.

Another reason that the natural enemies do not "resurge" as quickly as the pests, after pesticide treatments, is that occasionally a material is selectively more toxic to the predators and parasites. Research shows that this is the case with carbaryl (marketed as Sevin), which is particularly poisonous to bees and some other Hymenoptera, the order which includes parasites of such insects as aphids, scales, mealybugs, and caterpillars. Thus, as a consequence of the use of carbaryl, the pest insects may actually resurge to greater numbers than before.

Resistance is a second major effect of exclusive reliance on synthetic chemical insecticides. Here we are, devilishly concocting the most exquisite pesticides in our laboratories, at enormous costs, and somewhere out there, in every population of pest insects, there is an insect that is resistant to it. Each time you spray an insect population, you change the balance between susceptible and resistant individuals, because all the resistant insects survive to reproduce, while most of the susceptible insects are killed. Therefore, the more you use a pesticide to control insects, the faster you produce a population of resistant pests.

Secondary pest outbreaks are a third problem caused by injudicious use of insecticides. To understand this you must remember that there are insects everywhere, on every plant, that you are not aware of. All the time that you are focused on and concerned with the highly visible pests, there are small populations of other insects that could cause problems if they were not under good control by their own natural enemies. When you treat the area with an all-purpose spray, there is a very good chance you will kill off some of the parasites and predators of these other potential pests. Then, suddenly, you'll have a whole new problem on your hands.


All of the above problems are really a result of taking too narrow an approach to managing insect pests. What is needed is an overall view that takes into account the entire ecosystem of which the plant and the insect that feeds upon it are a part.

In general, a garden is an ecosystem of living (biotic) and nonliving (abiotic) parts. Light, air, water, and minerals from the soil are converted into living matter by the green plants, and the plants are fed upon by herbivores, or plant-eaters. These, in turn, are fed upon by carnivores, which are fed upon by secondary carnivores, which may fall prey to tertiary carnivores. The predators and parasites of insects described above are all part of such a chain. All, in turn, are fed upon by the detrivores, or decomposers, the fungi and bacteria which break down biotic materials and return them to the soil where they become available to plants once again.


Integrated pest management is a decision-making process which combines all possible strategies with the aim of suppressing the population of a pest below the level where it causes economic or aesthetic damage. The strategies may be cultural, physical, biological, or chemical. An essential aspect of this approach is monitoring your garden to determine what level of pest population is a problem. This means going out and making counts. You must figure out how many insects there are per plant, or per leaf or stem, and then watch to see if their number increases. When the population is a certain size, and you determine that the plant damage is intolerable, you should then do something about the situation. Your goal is really to suppress your pest population to below that level. We're not talking about eradication, but pest suppression, which will keep the ecosystem from oscillating wildly, and will allow it to cycle at a considerably more moderate level.

Examples of some cultural controls are the selection of varieties resistant to the pest, and management of watering and fertilizing techniques. Aphids, for example, are very sensitive to nitrogen levels in the plant. By overfertilizing you can actually increase their populations. On the other hand, thrips (order, Thysanoptera), tiny juice-feeding insects that often look like straight dark lines approximately 1/16 of an inch long, may appear in response to dryness. By increasing the water to the plant, you may reduce the damage that they cause. We have used this method in managing thrips found on tomatoes.

A common physical means of control is pruning, either to remove the insects physically, or to remove the favored habitat of an insect, such as the inner canopy of sucker growth on a tree, which is favored by certain aphids.

Biological control is the manipulation of the biological components in the ecosystem. This may mean enhancing natural controls already present, or introducing missing components that would be desirable. For example, there is a naturally occurring disease of certain caterpillars called Bacillus thuringiensis , marketed under the trade names of Dipel, Thuricide, and Biotrol. If sprayed on a plant when the caterpillars are large enough to feed on both sides of the leaf, it will give good control of the pest without harming any of its natural enemies.

Many people buy carnivores such as ladybird beetles or praying mantids to release in their gardens as general predators. However, if not previously defatted, the ladybird beetles must fly to burn off stored body fat before feeding, so they will shortly leave the area in which they are released. Mantids are very unselective in what they eat and are probably better bought for purposes of observing and learning about insects than in hopes that they will satisfactorily control pest populations.

Green lacewings, excellent general predators, can also be bought commercially. Because they are shipped and released in the egg stage and hatch into voracious larvae that cannot fly, lacewings are useful biological control tools. Unfortunately, they are not yet produced commercially in many sections of the nation. The only large-scale insectary we know that offers them for sale by mail is Rincon-Vitova Insectaries, Inc., Oakview, California. Wheast, a food spray that is a by-product of the cheese industry, can be used to feed and enhance the numbers of lacewings and some other common predators found in the garden. It can be obtained from the same company.

Chemical tools are also part of an integrated pest management system, but they are used only as a last resort, when all other methods have failed and the damage is intolerable. They are used only where needed and carefully timed so as to cause the least disturbance of the natural controls. The most selective chemicals are chosen, to affect the pest but as few other organisms as possible. And they are applied very infrequently, so they will work when you need them and will not cause new problems. Used in reduced quantities, they will be less likely to pose hazards to the environment and human health.


[1] Start by overcoming some of your fear of insects. They are part of the natural world (indeed there may be more insect species than any other form of life), and few are any problem to man. Forget the concepts of "good" bug and "bad" bug used to sell you products you don't need. Even the plantfeeding insects are necessary. Those that eat the weeds help to keep them in check, and the ones that feed on cultivated plants are important too. You will never eliminate (and do not want to eliminate) the pest insects, because you have to have the pest insects around in order to keep around the natural enemies that control them. That's why there's no such thing as a "bad" bug—there should even be some of the "worst" pests around to make sure you have the predators and parasites around so that the next "bad" bug will have enough enemies there to feed upon it.

[2] When you see plant damage, ask yourself, is it tolerable? There is no way to have plants without the insects that feed on them, and there is, therefore, no way to have no insects. We have to readjust our values to tolerate some plant damage.

[3] When the damage involved is truly intolerable, then you need to determine the true cause. It is easy to blame an insect or other animal that is highly visible during the day (such as an earwig or cutworm curled up inside a hole in a tomato) while the real culprit is something else that does its damage by night. In fact, going out after dark with a flashlight may be the only way to discover what is going on. (On the tomatoes in our garden, for example, slugs were eating holes at night that provided refuge for the insects during the day.)

[4] When you have correctly identified the animal causing the problem, learn something about its habits and life cycle. You can do this through direct observation, reading, or asking your local university or county agricultural extension advisor. He or she also may be the best person to help you identify the animal that you have found.

[5] Knowing something about the pest, you can start to consider how you can modify the environment to affect it. For example, in our back yard we were severely bothered by an invading garden snail ( Helix aspersa ). By looking around, we discovered we had actually created some choice breeding areas for the snails in a large bed of succulents and under boards used for pathways, where we discovered a great many baby snails and snail eggs. So we reduced the large bed to a few specimen plants, separated so that air could circulate around them and dry them of after rains or watering, and we put dry sawdust under the boards. This helped to reduce the snail population almost immediately.

[6] Use handpicking. To continue with our snail example, we made a point of checking the garden for snails in the early evening with a flashlight every week or so. Those we found in the vegetable beds were collected by hand and fed to our chickens. (We could also have stepped on them and put them in the compost pile.)

[7] Try barriers. Isolating beds with dry sawdust can reduce snail and slug traffic between beds. Tin cans, opened at both ends and shoved down into the mulch a few inches to provide a collar around freshly transplanted seedlings, have proved effective against the same pests. Nets and chicken wire are protection against birds in early spring. Cages made of aluminum flyscreen will keep out the cabbage maggot and other root burrowing larvae of flies. Be inventive!

[8] Concoct some ingenious traps. We turned over clay flowerpots set among the plants to collect snails that wander in during the warm, sunny day and emerge to feed at night. Rolled up newspapers give earwigs places to hide. Flytraps can be simply constructed and baited with dog manure or garbage.

[9] Provide habitat and food for beneficial organisms that are the natural enemies of insect pests. Compost mulches favor the shiny black carabid beetles (Carabidae) that eat cutworms, for example. By increasing the diversity of the soil surface, you increase the habitat for beneficial insects, particularly some of the predacious ground beetles.

Plant flowers in the vegetable garden, particularly shallow-throated ones that provide easy access for the little mini-wasp parasites mentioned above. Many of the beneficial insects need some pollen and nectar during the season to provide protein for egg laying. So having some plants in flower all during the season may actually help to suppress the potential pest populations. This is an example of companion planting that works!

[10] Modify your cultural methods. Make efforts to determine whether your watering, fertilizing, and pruning or clipping methods are affecting the pest populations. One strategy for dealing with pests is overhead sprinkling, which we recommend, or direct water sprays, taking the hose and washing off a plant that has an insect on it. Not only do you have direct effects (the physical effects of the water hitting the plant, knocking off the insect, or breaking its tissues or mouth parts), but also you will increase the humidity levels of the plant or plant tissues, and this could produce fungal outbreaks in the insect populations.


When you've tried everything and nothing seems to work adequately, you need to stop and ask yourself if the plant in question is important enough to you to warrant supporting it with a chemical crutch. It may simply be the wrong variety or species for that spot. Your overall climate, or the mini-climate, may be unsuitable for that plant.

If you decide that you wish to keep the plant, then you may wish to resort to the use of a chemical tool. Here are some guidelines for choosing and using them.

[I] Pick a pesticide that is least toxic to mammals. Of course, water and water-and-soap sprays are relatively harmless from that point of view. Some detergents even have phosphorus in them, which is a plant nutrient. However, when using a commercial detergent compound, always test it on a small portion of the plant first, since you don't know what other chemicals might be in the material. Such an approach is also useful with other pesticides.

Toxicity of pesticides is measured by LD 50 , or the lethal dose required to kill 50% of the test animals, usually mice or rats. The LD 50 is expressed in milligrams per kilogram of body weight of the test animal, either dermal (when contact is through the skin) or oral (through eating). The higher the number, the less toxic the dose. Thus pyrethrin with an LD 50 of 3,000 is assumed to be less toxic than rotenone with an LD 50 of 400, or diazinon with an LD 50 of 108. In some commercial greenhouses where constant use of pesticides against whiteflies has produced populations resistant to everything else, some growers now use a material called Temik with an LD 50 of 0.93. This is so toxic that a drop accidentally spilled on a sweaty hand could kill a person.

If you wish to use a product and are curious about its toxicity, you can usually find the LD 50 of any commercially available compound by looking it up in a reference work such as "Commercial and Experimental Organic Insecticides" from the Bulletin of the Entomological Society of America, 15 (2): 85-148, often available in libraries or agricultural extension offices.

However, it is important to remember that the LD50 doesn't tell you everything. Materials are not tested in combination, so it will tell you nothing about synergistic, or combined, effects with the other contaminants in the environment to which you may be exposing yourself. Also, the tests are ended and summarized after a short period of time, and the effects may take longer to show up. This was the problem with Kepone, which was thought to be safer than it turned out to be. This is also the problem with materials such as chlordane and DDT, which are now suspected of causing cancer many years (usually 20 to 25) after exposure, but which were originally regarded as relatively nontoxic to humans.

[2] Pick the most species-specific material available. This is the advantage of Bacillus thuringiensis , mentioned earlier. It is very selective, affecting only certain caterpillar pests and not their natural enemies.

[3] Choose pesticides that break down quickly to harmless materials. All of the botanicals, as they are called—pyrethrin, rotenone, and sabadilla are examples—break down rapidly under sunlight and bacterial action, and their breakdown products are nontoxic.

[4] Spot treat. Use only as much as you really need where you need it, confined to the problem area. Use a bait, if one is available, before resorting to a broadcast treatment.

[5] Time your treatments. Don't spray just as the pest population is beginning to decline because large numbers of their natural enemies are present. Give the parasites and predators time to catch up; they always lag somewhat behind their prey. Learn to recognize the common insect predators and parasites of your area so you'll be able to see how well they are doing.

This whole approach to insect management is hardly simple. But it is worth it to keep yourself healthy, your garden thriving, and the insect populations, both plant and insect feeders, in balance with one another. This balance may ultimately be essential if we want to be able to continue growing plants at all.

EDITOR'S NOTE: In order to help you identify, and thus decide how to control, your garden's pests, we've included a guide to some of the more common vegetable-eating insects on pages 94 and 95 of this issue.