Understanding Pesticides

Pest Notes: Introduction

Pesticides are designed to be toxic to the pests they target—whether they are insects, rodents, fungi that cause plant disease, or weeds. When used properly, pesticides can help protect plants, property, pets, or yourself from injury, damage, or discomfort. However, when the pesticide label instructions are not followed correctly, plant injury may occur, pests may not be controlled, human or plant health may be impaired, and pesticides may contribute to soil, air, or water pollution.
While pesticides can be a part of integrated pest management (IPM), carefully consider whether or not a pesticide is necessary and if a nonchemical solution might be just as effective. Correctly identify the pest before starting any control measures, especially a pesticide application.

Before you purchase and use a pesticide, learn all you can about the product, how to use it, personal protective equipment needed, and how to properly dispose of unused material as well as the empty container. This publication provides important information to help residents and licensed pest management professionals use pesticides as safely as possible.

What is a Pesticide?

A pesticide is any material (natural, organic, or synthetic) used to control, prevent, kill, suppress, or repel a pest. "Pesticide" is a broad term that includes insecticides (insect killers), herbicides (weed or plant killers), fungicides (fungus killers), rodenticides (rodent killers), plant growth regulators, miticides (used for mite control), and products that kill snails and slugs (molluscicides). Even disinfectants, some surface wipes, and pool cleaners such as chlorine are classified as pesticides. The suffix ‘cide’ comes from the Latin word ‘to kill.’

Pesticides are regulated by the U.S. Environmental Protection Agency (US EPA) and state regulatory agencies. In California, this is the California Department of Pesticide Regulation (CDPR). Products that contain a pesticide that is applied to the body such as topical treatments like mosquito repellents are regulated by US EPA. Parasite treatment products applied topically to animals (including pets) are generally regulated by US EPA if they remain on the skin to control only external parasites (e.g., collars or transdermal “spot-on” products used to control fleas and ticks) and by FDA if they are absorbed systemically into the bloodstream.

When To Use a Pesticide

Before using any pesticide, first consider what else can be done. Verify that an actual pest is really causing the damage and not something else. Consider nonchemical management methods. Understand that most pests cannot be completely eliminated—even with pesticides. Some questions to consider before using a pesticide or any other control method include:

Is a Pest Really the Cause of Your Problem?

Frequently, pesticides are applied unnecessarily because the cause of damage has been assumed or misidentified. Damage can also be the result of other factors such as incorrect irrigation, poor drainage, herbicide toxicity, or physical damage. Always be sure the cause of the problem is determined, and that it’s a pest, before applying a pesticide.

Can You Change the Conditions?

Prevention is always the best way to manage a pest problem. Will the conditions change due to the weather or other environmental factors? Is the problem due to gardening practices that can be changed?

Each pest has optimum environmental conditions for causing damage. For instance, powdery mildew in many plants is favored by shade. Providing plants with a sunny location, opening canopies to increase air circulation, and avoiding excessive fertilizing may keep the disease from becoming serious.

Many indoor and outdoor pest issues can be solved by removing available food, water, or shelter.

Other Than Pesticides, What Else Might Work?

Integrated pest management involves combining different methods to manage pests other than pesticides. These include:

• Cultural control: correct pruning, fertilizing or watering regime, or selecting pest-resistant varieties or species; reducing clutter and cleaning up food or crumbs
• Physical control: using mulches to keep weeds from growing, solarization for soilborne pathogens or weed seeds; excluding pests by installing mesh screens, caulking, or weather stripping
• Mechanical control: hand tools to remove weeds, spraying leaves with water to dislodge insects; using snap traps
• Biological control: using naturally occurring beneficial organisms such as insects and spiders that prey upon or parasitize other insects

Is Using a Pesticide Justified?

One or two caterpillars on a plant might not be a problem that warrants using a pesticide, especially if natural enemies of the caterpillars are present. However, a high caterpillar population causing severe leaf loss, damage to edible fruits or nuts, or severe defoliation of ornamental plants, may mean you would want to control the pest.

Be sure to base decisions on the number of pests present—not damage levels—and on your knowledge of the pest's life cycle. For instance, often by the time a tree is defoliated (stripped of leaves), pests are gone, and pesticides will be of no use. In the case of foliar diseases, many fungicides must be applied preventively before symptoms are noticeable.

Choosing the Right Pesticide

The first step in choosing a pesticide is to accurately identify the organism (the specific insect, weed, or plant disease) that is causing the problem. If the pest is misidentified, you will not be able to choose an effective pesticide or other management strategy. Be aware that some beneficial insects may be mistaken for pests.

If you aren't confident that you can do this using your own experience, get help from your University of California (UC) Cooperative Extension office, the UC IPM website or another reliable source. Use the plant problem-solving tables in the back of the UC Agriculture and Natural Resources publications Pests of the Garden and Small Farm and Pests of Landscape Trees and Shrubs to identify major pests on most common garden and landscape plants.

If a pesticide is needed, select one that is effective against your target pest species and also poses the least risks to human health and the environment. A good source of information for identifying effective, least-toxic methods and pesticides for use against specific pests is the UC IPM Pest Notes series and the Pesticide Active Ingredient Database.

When shopping for a pesticide product, it is important to read and follow the label to be sure the target pest and site are listed. However, don’t use the pictures on the pesticide container as your primary source for selecting the best control product. In addition to pests that are effectively controlled, pesticide labels often picture pests against which the product is only marginally effective. Getting information from university publications, UC Cooperative Extension offices, or other knowledgeable experts is a better strategy.

Before purchasing a pesticide, always check the label to be sure it is appropriate to use on your plants or treatment site. For instance:

• Be sure the plant or site you plan to treat is listed on the label.
• Never apply pesticides labeled for use only on ornamental plants to plants that will be eaten.
• Never use pesticides labeled for "outdoor use only" indoors.
• Pesticides can seriously damage some plants; read the label to be sure treated plants won't be injured.

Finally, when choosing pesticides, remember that most pesticides (even the more toxic ones) may only control certain stages of the pest. Many insecticides kill only the insect larvae such as caterpillars, not the eggs, pupae, or adults. Other insecticides are effective only on adults. Many fungicides are preventive treatments and will not eliminate infections that have already started, although they may slow their spread. Likewise, some herbicides (preemergence herbicides) kill germinating weeds but not established ones, while others (postemergence herbicides) are effective against actively growing weeds. For postemergence herbicides, it is important to note that desirable plants may also be injured or killed if the herbicides are not used correctly.

Read and Follow the Pesticide Label

Very important information can be found on a pesticide product's label. The label also serves as a legal document, required for every pesticide product registered or sold in the United States. It is illegal to use a pesticide contrary to the directions on the product's label. This includes how much to apply and where it can be used. Always keep the product in the original package. Some of the information that is contained on the label includes:

• Trade name or brand name
• Active ingredients and their percentage
• Types of plants or sites where the pesticide can be used
• Pests targeted
• How much to use
• Directions for use and when to apply
• Time required after the last application before food can be harvested (pre-harvest interval or PHI)
• Personal protective clothing and equipment (PPE)
• Signal word indicating short-term toxicity to people
• Precautionary statements defining hazards to people, domestic animals, and the environment
• Emergency and first aid measures
• Proper storage and disposal of pesticides and empty containers

Indoor Versus Outdoor Pesticides

Use only pesticides specifically labeled for indoor use inside the house. Many outdoor pesticides are designed to break down into less toxic substances when exposed to water, sunlight, and air. Without these conditions in indoor spaces, the pesticides may linger and cause unhealthy and even dangerous conditions for humans and pets.

Least Toxic Alternatives

Choose the least toxic pesticide that will solve your pest problem. The term “least toxic” refers to how the pesticide will affect human health or the environment, not the pest. Least-toxic alternatives are usually suggested in the UC IPM Pest Notes. Examples of least-toxic insecticides include oils, soaps, and microbes (such as Bacillus thuringiensis or “Bt”).

Pesticides are used because they kill or control the target pest. “Selective” pesticides kill only a few closely related organisms. Others are broader spectrum, killing a range of pests but also organisms that are not intended to be killed, such as beneficial insects.

Most pesticides have some negative impacts on the environment. For instance, some insecticides with low toxicity to people may have high toxicity to beneficial insects like parasitic wasps or other desirable organisms like honey bees, earthworms, or aquatic invertebrates. Most herbicides selectively kill some weeds but can also kill desirable plants if not used properly. Pesticide persistence—or how long it remains toxic in the environment—is also a factor in the safety of pesticides. Pesticides that break down rapidly usually have less negative impact on the environment but require more precise application. Because they don't leave toxic residues that will kill pests arriving hours or days after the application, they must be applied precisely when the vulnerable stage of the pest is present.

The signal words CAUTION, WARNING, DANGER, or DANGER-POISON on a pesticide label indicate the immediate toxicity of a single exposure of a product to humans (Table 1). These words are the consumer's primary guide to relative safety of products. However, signal words do not give an indication of potential for causing chronic problems (for example, cancer, reproductive problems or other long-term health effects). They also do not reflect potential hazards for wildlife, beneficial insects, and many other nontarget organisms. Precautionary statements on labels give additional information on harmful effects or additional safeguards that should be taken. For more information on hazards of specific pesticides, review the Safety Data Sheets (SDS) available from the pesticide manufacturer or dealer or visit the National Pesticide Information Center online at npic.orst.edu or by telephone 800–858–7378.

Table 1. Signal words and the toxicity, irritation, and sensitivity hazards associated with exposure.

Signal Word	Toxicity, Irritation, and Sensitivity Hazards
CAUTION	Slightly toxic if eaten, absorbed through the skin, inhaled; causes slight eye or skin irritation
WARNING	Moderately toxic if eaten, absorbed through the skin, inhaled; causes moderate eye or skin irritation
DANGER	Highly toxic by at least one route of exposure; may be corrosive and can cause irreversible damage to the skin or eyes
DANGER-POISON	Highly toxic when consumed, inhaled, or absorbed through the skin or eyes; fatal at very low doses

Pesticide Application Equipment

Read the pesticide product label carefully and be sure that you have the proper equipment to apply it correctly and in the safest way possible. You will need protective gear and clothing to protect yourself from exposure, even when applying least toxic pesticides. At a minimum, protective gear and clothing should include chemical resistant gloves, eye protection, a long-sleeved shirt, long pants, and closed shoes. Chemical resistant gloves are made from nitrile, neoprene, butyl, or rubber. Avoid using cotton gloves or lightweight dust masks that may absorb liquid sprays and result in prolonged contact with your skin. Read the pesticide product label carefully for any additional protective requirements. In addition to label requirements, licensed pest management professionals are required by the State of California to use specific personal protective equipment (for example, eyewear with brow and side protection) when handling or applying pesticides.

Required equipment varies according to your application site, your choice of pesticide, and your willingness to work with more complicated application devices. For many home and garden pesticide applications, the best choice is to purchase a ready-to-use product in a trigger pump sprayer. Ready-to-use products eliminate the need to dilute and mix pesticides or purchase special equipment and are excellent for spot treatments on small plants and shrubs. At the other end of the spectrum are compressed air sprayers, which require careful maintenance and operation as well as precise mixing of chemicals.

If you mix your own pesticides, keep a set of measuring spoons or cups for use only with pesticides. Label them with "PESTICIDE ONLY" to distinguish these tools from kitchen utensils and keep them stored far away from food preparation areas. A locked storage cabinet in a garden shed, garage, or well-ventilated utility area is the best place to store pesticides and the equipment used to mix or apply pesticides. If you are a professional applicator licensed by the State of California, there are additional regulations that must be followed. These can be found in DPR's webpage on Worker Health and Safety.

If you are spraying herbicides for weed control, have a sprayer specifically for that purpose and label it "WEEDS ONLY." Otherwise, herbicide residue in the sprayer may injure plants if the same sprayer is used for applying another type of pesticide or fertilizer.

After a pesticide application, remove your work clothes and take a shower as soon as possible. Wash clothing worn during pesticide application separately from other laundry items. Never smoke, drink, eat, or use the bathroom after a pesticide application without washing first.

Measuring and Diluting Pesticide Concentrates

Proper measurement of concentrated pesticide formulations is essential for their effective and safe use. The application rate for most insecticides and fungicides is given on the label in ounces per gallon of water used in the spray applicator. It is essential that you follow these procedures properly and dilute and apply materials as required.

For herbicides and some uses of insecticides and fungicides (such as applications on lawns), the label will state the amount of pesticide to use for a given area. In these cases, you'll need to measure the area you are treating to calculate how much to mix the pesticide.

Remember, if the label specifies a dilution rate, you need to follow the label directions precisely. Before mixing your pesticide, test out your sprayer with water to assure you will cover the recommended area with the recommended amount of spray. If not, you will need to adjust your application rate accordingly by applying slower or faster.

Insecticide or fungicide directions for fruit or ornamental trees often don't specify areas in square feet to be treated. They often say something such as "wet plants to dripping point, thoroughly cover both sides of leaves." For these applications or for spot treatments, it is also a good idea to test out your sprayer with water to see how much spray you need to cover a fruit or ornamental tree or other area. That way you'll know how much product to mix.

Never use more than what the directions state on the label. The pest will not be controlled faster, and you will be wasting the pesticide, your time, and money while potentially causing plant injury and contaminating the environment with excess chemicals. Mix only as much as you need immediately; don't store leftover pesticide solutions. They may be susceptible to quality changes at high or very low temperatures, be affected by minerals in the water solution, or by settling out.

Protecting the Environment

Be sure pesticides are properly applied to the target plant or site and can't move onto other plants or areas. Pesticides can easily move off target with wind. Do not spray during windy conditions (5 mph or more) when pesticides can be carried into areas where they aren't needed or wanted.

Use spot treatments where the pest is most prevalent; avoid widespread applications of pesticides throughout your garden or home. For spot treatments, mix the pesticide according to label instructions, and apply the mixture only to the affected area. Bait stations for ants, wick or shielded applicators for some herbicides, and tree trunk treatments for certain insects are other ways of limiting environmental contamination.

Pesticide applied to the ground may also move with water or on soil. Be sure the application does not run off or blow into drains, creeks, or other water bodies so you can prevent water contamination.  

Keep Pesticides Out of Our Waterways

Pesticides applied in gardens and landscapes can move off target by drifting in the air or washing off into storm drains or creeks. See the Urban Pesticides, Fertilizers, and Water Quality page for more information.

Follow these guidelines:

• Be aware of weather patterns and do not apply pesticides just prior to irrigation or rainfall (unless specified in the label instructions) or during windy conditions.
• Avoid applying pesticides to hard surfaces such as sidewalks, driveways, and foundations, because they can easily be washed off and enter storm drains.
• Check pesticide labels for warnings regarding use near bodies of water such as streams, rivers, and lakes.
• Never dispose of pesticides in storm drains, sinks, or toilets.
• Under no circumstances should pest control equipment be cleaned in a location where rinse water could flow into gutters, storm drains, or open waterways.
• Never apply more than the rate listed on a pesticide label.
• Some pesticides are more easily carried within surface water runoff than others and therefore have greater potential to move off site during irrigation or storms.

Check the Weather

To ensure pesticides and fertilizers don’t end up in waterways, and to protect yourself when applying, make sure to apply under the right weather conditions and check the forecast for the next 24 hours. Calm weather, with no rain or snow in sight and with wind less than 5 mph, is when you can safely apply. Pesticides and fertilizers can easily be carried off to waterways in storm-like weather conditions. Some products also have specific sunlight or temperature requirements for application.

Certain pesticides can react with the sun and burn plant foliage. This is known as phytotoxicity, and the product label will indicate if this is a concern for your specific product and the plant you intend to use it on. Other products may volatize and become very dangerous if applied when temperatures are too high. These environmental hazard reminders can be found on the product label under the “Precautionary Statements” section and include additional information on the products risk to sensitive species.

Disposing of Leftover Pesticides

Try to purchase only as much pesticide as you need to control the current pest issue. This will eliminate the need to store the unused products for a long time. Don't prepare (dilute) more than you can use right away. Diluted pesticide products need to be applied according to label directions (to plants or sites listed on the label and at label rates) until the spray tank is empty. Excess diluted pesticide should be disposed of at a household hazardous waste facility.

If you can't use up your pesticides, consider giving them to a friend or neighbor who can use them correctly and responsibly, but always keep these materials in their original containers. Do not use anything that could be mistaken for a food or drink container, such as a plastic beverage bottle. People have been poisoned and killed by inadvertently drinking from such containers.

Do not dump excess, unwanted, or old material down household drains or into open waterways, gutters, storm drains or sewers, or in the trash. The only legal way to dispose of pesticides is to take them to your local household hazardous waste disposal facility. In California, check the Earth911 website to find the hazardous waste disposal site closest to you.

Empty containers of home use pesticides should be triple-rinsed and disposed of in the trash. Professionals who use concentrated liquid pesticides must also rinse the container three times before disposal. The best time to rinse is when you are using up the last remaining pesticide in the container. Add the remaining pesticide to the sprayer. Add water to the empty pesticide container, put the cap on, swirl the water around the container, and transfer the liquid to the spray tank. Repeat two times. If necessary, add more water to the spray tank to reach the correct concentration. This way, you will have rinsed the bottle three times and used the rinse water to make the pesticide application.

Don't pour unused rinse liquid down any drain or sewer or in the trash. Unused rinse liquid is considered hazardous waste and must be disposed of properly at a hazardous waste facility or as suggested above.

Hiring a Pest Control Company

If you do not have the time or ability to research your pest problem and safely apply the appropriate material to control it, you may want to hire a pest control service to do the job. Licensed pesticide operators have access to some products not available online or in retail stores. Many problems, such as termites or pests on large trees, require special pesticides or equipment and technical training for most effective management. Although professional services can be expensive, the investment may be worth it to save a structure or solve a serious problem.

See the Pest Notes: Hiring a Pest Control Company for information on how to select a contractor.

References

Flint ML. 2018. Pests of the Garden and Small Farm: A Grower's Guide to Using Less Pesticide, 3rd edition. UC ANR Publication 3332. Oakland, CA.

Dreistadt SH. 2016. Pests of Landscape Trees and Shrubs, 3rd edition. UC ANR Publication 3359. Oakland, CA.

Whithaus S. 2016. Safe and Effective Use of Pesticides, 3rd edition. UC ANR Publication 3324. Oakland, CA.

Pittenger DR., ed. 2015. California Master Gardener Handbook. UC ANR Publication 3382. Oakland, CA.

Taravati S, Haver DL, Sutherland AM. 2023. Pest Notes: Hiring a Pest Control Company. UC ANR Publication 74125. Oakland, CA.

UC IPM. 2026. Pesticide Active Ingredient Database.

US EPA. 2025. Pesticide Labels.

Resources

• About Pest Notes
• Glossary
• WARNING ON THE USE OF PESTICIDES
• List of other Pest Notes

Publication Information