Pesticides in the Home Garden

Summer of 2003

By Lee Oliphant, Ed.D., Master Gardener


No Master Gardeners worth their salt are going to recommend that you indiscriminately spray insecticides as a means of pest control.  There are numerous proven strategies available for the home gardener that will create a natural balance in the environment and manage pests, while minimizing damage to plants and wildlife - all without the use of chemical substances.

But let's face it; there are times when the home gardener feels the need to rely on technical support through the use of biochemical warfare.  Before doing so, thoughtfully evaluate your reasons for this action.  Are pests causing unacceptable losses in your garden beyond your tolerance?  Do you completely understand what is causing the damage?  Are you treating the offender(s) at the proper time in their life cycles?  Do you know how to read the pesticide label and use the appropriate dilution formula?

Know Your Enemy


The
University of California has developed strategies known as integrated pest management (IPM) encouraging gardeners to take a holistic approach to managing pests.  Important components of the IPM program are pest identification; monitoring and predicting pest outbreaks; knowledge of the biology of the pest including how and when it feeds; and knowledge of the pest's natural enemies. 

By identifying the culprit that is causing the damage in your garden, you will have completed the first step in selecting the appropriate pesticide and applying it effectively.  There are some excellent resources available that will help you identify the insect or mite, for example, which may be causing damage in your garden.  Pests of Landscape trees and Shrubs, and Pests of the Garden and Small Farm are published and available from the
University of California.  Pest notes on individual insects, and appropriate control measures are available at no cost online at www.ipm.ucdavis.edu or may be requested by mail by calling the Master Gardeners desk noted below. 

While the mere presence of an insect on a plant does not necessarily mean that it is the offending critter, finding and capturing the insect is the best way to get help with its identification.  Some pests are elusive and this may not always be possible.  To help you identify the damaging insect, note whether or not the leaf has been chewed, or is discolored or withered from the sucking mouthparts of insects such as mites, scales or aphids.  Identify the kind of plant that is affected.  Some plants are prone to particular insect infestations.  Know the lifecycle of local insects.  Some go through complete metamorphosis (beetles and butterflies) while others, like aphids, change little as they mature. 

Identifying and understanding the offending insect, its growth and its habits, will help you select the appropriate control measure and apply it at the proper time in the life cycle of the insect.  Subsequent articles on types of pesticide and formulations, and reading pesticide labels will follow, as well as information on the use of pesticides for maximum results.

Pesticides are often grouped by the type of pests they control.  Notice that all of the pesticides end in –cide (a suffix meaning killer or killing).  There are insecticides to control insects, miticides to control mites, molluscicides to control slugs and snails, nematicides to control nematodes, and rodenticides to control rodents.  There are fungicides to control fungal diseases and bactericides to control bacterial diseases.  Understanding what you need and what is involved in using pesticides is essential before purchasing. 

Pesticide Formulations

Pesticides are mixtures of active ingredients and other materials, know as inert ingredients, combined into what is called the pesticide formulation.  The formulations are symbolized by a letter or letters that are printed on the pesticide label.  It is helpful to become familiar with these symbols when choosing the pesticide you need.  Emulsifiable Concentrate (EC or E) is a common formulation.  Its active ingredient is mixed with an oil base (usually a petroleum derivative) that is diluted with water.  Agitation is required to keep the oil and water mixed.  Solutions (S) are pesticide formulations that, once dissolved in water, require little agitation.  Flowables (F or L) are also liquids mixed with water but require constant agitation or will settle in the spray tank.  Aerosols (A) are applied as a fine spray from a container (convenient but expensive) and wettable powders (WP or W) are a combination of the active ingredients with fine powder and mixed with water.  Soluble powders (SP) dissolve with water and constant agitation is not needed for suspension.  Pesticides also come in bait formulation (B), granules (G) and dusts (D), and are applied directly from the package. 

The Label

The label and any brochures accompanying the product are required reading for the responsible home gardener.  The label is easy to read and follows a specific format.  It will include the brand name as well as the chemical name.  You will find the formulation along with active ingredients in percentages by weight.  The inert ingredients may not be named if the label shows the percentage contained.  The U.S. EPA labels pesticides as unclassified or restricted-use depending on their effect on people and the environment.  The home gardener should avoid restricted-use pesticides. 

The pesticide product label will contain a “signal word” from “Caution” to “Danger-Poison” depending on its toxicity, and a “Keep Out of Reach of Children” warning.  A Precautionary Statement tells the gardener how to avoid exposure and lists protective equipment required

The Directions for Use section lists target pests plus plant species where pesticide may be used.  It tells how to mix and apply the product, how often, and where to use it.  It is extremely important for the consumer to read the Precautionary Statement as well as the Directions For Use. 

The rest of this article provides home gardeners with information on choosing the appropriate pesticide for controlling garden pests.  As noted in previous articles, before you consider using pesticides in your garden, be sure you have correctly identified the pest to be controlled and have explored other cultural and mechanical controls. 

Some insecticides can control insects only at particular stages in their development.  For instance, for a moth whose larvae eat leaves, the insecticide must be administered when the insect is in the larval stage and is actually eating.  Stomach poisons for the larvae will not affect the insect in the egg, pupae, or moth stage.  The location of the pest is important.  Some insects live inside the fruit and are not affected by foliar spraying after the fruit is formed.  Certain pesticides must be applied at particular times.  Dormant sprays help control overwintering insects and mites with minimal impact on natural enemies.  Information on the pest’s life cycle, location on the plant and timing of applications for many common garden pests are presented in
University of California “Pest Notes”, available from the Master Gardeners’ desk or online.

Understanding how a pesticide works, or its mode of action, helps determine its effectiveness because pesticides are only effective when used at the correct time, in the correct manner.  Is the active ingredient carried in the tissues of treated plants (systemic) or does it need to contact the pest?  Does it selectively kill only certain kinds of insects?  Does it kill by contact or does the poison need to be eaten? 

The following types of pesticides, which break down rapidly in the environment, are best suited for the home garden.  Horticultural oil has low or no toxicity to humans or wildlife and is effective in controlling mites and insects by suffocating them.  It is also effective against eggs, immature forms and soft-bodied adults, scale, mealy-bugs, aphids and whiteflies.  Horticultural oil, applied to deciduous fruit trees during the dormant season, is effective in controlling eggs of western tussock moth, green fruitworm, aphid and leafroller eggs, scale, mealy bugs and mites.  Insecticidal soaps help to control mites and aphids and can also reduce populations of whiteflies, immature scale insects, thrips and leafhoppers.  Pyrethrums are derived from the flowers of a Chrysanthemum species.  It causes paralysis in mosquitoes, flies, aphids, beetles, moth larvae, thrips and mealy bugs.  Bacillus thuringiensis is a microbial insecticide and causes a disease in specific groups of insects (larvae of some butterflies, moths, beetles and mosquitoes), yet is harmless to warm-blooded organisms. 

Always begin with the least toxic pesticide.  Choose one based on its ability to break down quickly and selectively control pests.  According to the
University of California’s publication Pests of the Garden and Small Farm #3332, synthetic pesticides are toxic but can be used in the home garden if lower risk control measures have failed and application on the label is followed.  Malathion, for example, is effective on some garden insect pests and less toxic to beneficials and bees than carbaryl.  Read the pesticide label before purchasing to consider environmental precautions and weigh the advantages of using it.  Lastly, and most importantly, carefully follow the directions on the label.  That information is meant to protect beneficial insects, wildlife, pets, and you. 

To simplify information, trade names of products may have been used.  No endorsement of named products is intended, nor is criticism implied of similar products which are not mentioned. 

University of California Cooperative Extension Master Gardener Volunteers can provide additional gardening information upon request .Call the San Luis Obispo office at 781-5939 on Mondays and Thursdays from 1 to 5 PM.  You may also call the Paso Robles office at 237-3100 on Wednesdays from 9 AM to 12 PM.  The San Luis Obispo Master Gardeners website is at http://groups.ucanr.org/slomg/.  Questions can be e-mailed to: mgsanluisobispo@ucdavis.edu.