Pesticides include any chemical or biological substance used to kill, repel, or control unwanted organisms. They span a wide range of products, from the weed killer in your garage to the flea treatment on your dog to industrial chemicals sprayed on thousands of acres of farmland. The term covers several distinct categories based on what they target: herbicides kill plants, insecticides kill insects, fungicides control fungal diseases, and rodenticides kill rodents. Here are the most common examples across each category.
Herbicides: Weed Killers
Glyphosate is the most widely used herbicide in the world, found in products sold for agriculture, forestry, lawn care, and aquatic weed control. It works by blocking a specific enzyme that plants need to produce proteins essential for growth. Plants exposed to glyphosate show stunted growth, loss of green color, leaf wrinkling, and tissue death within 4 to 20 days. It’s a non-selective herbicide, meaning it kills virtually any plant it contacts, which is why genetically modified “Roundup Ready” crops were developed to tolerate it.
Other common herbicides include 2,4-D, one of the oldest selective herbicides still in use (it kills broadleaf weeds but leaves grasses unharmed), and atrazine, widely applied to corn and sugarcane fields. Dicamba is another selective broadleaf herbicide that has drawn attention in recent years because it can drift from fields and damage neighboring crops. For home lawns, you’ll often find products containing combinations of 2,4-D, mecoprop, and dicamba designed to kill dandelions and clover without harming grass.
Insecticides: Bug Killers
Insecticides fall into several chemical families, each working differently on an insect’s nervous system or biology.
Pyrethroids are synthetic versions of a natural compound found in chrysanthemum flowers. Permethrin, cypermethrin, and bifenthrin are common examples. You’ll find permethrin in household ant and roach sprays, clothing treatments for tick prevention, and some pet flea products. Pyrethroids paralyze insects on contact by disrupting their nerve signaling.
Neonicotinoids are the most widely used insecticides in agriculture globally. Imidacloprid, clothianidin, and thiamethoxam are key examples. They’re systemic, meaning they get absorbed into a plant’s tissues and kill insects that feed on the leaves, stems, or nectar. Imidacloprid is also the active ingredient in many flea and tick treatments for dogs. Neonicotinoids have been at the center of controversy over pollinator health, as they can be toxic to bees.
Organophosphates were once the dominant insecticide class but have been increasingly restricted due to toxicity concerns. Malathion is still used for mosquito control programs in many cities. Chlorpyrifos, once one of the most common agricultural insecticides in the U.S., was banned for food crop use by the EPA in 2021.
Fungicides: Disease Prevention
Fungicides protect plants from mold, mildew, blight, and rot. Copper sulfate is one of the oldest and most recognizable, used in both conventional and organic farming to control fungal and bacterial diseases on fruits, vegetables, and ornamental plants. Sulfur is another longstanding option, effective against powdery mildew on grapes, roses, and other susceptible plants.
In home garden products, you’ll commonly see chlorothalonil (a broad-spectrum synthetic fungicide), myclobutanil (often sold for rose and fruit tree care), and mancozeb (used on tomatoes, potatoes, and other vegetables). Neem oil pulls double duty as both a fungicide and an insecticide, making it a popular choice for organic gardeners. Newer biological fungicides use living organisms like the bacterium Bacillus subtilis or the fungus Trichoderma harzianum to outcompete plant pathogens.
Rodenticides: Rat and Mouse Poisons
Most consumer rodenticides are anticoagulants, chemicals that prevent blood from clotting. They come in two generations. First-generation anticoagulants like warfarin and chlorophacinone require multiple feedings to be lethal. Second-generation anticoagulants like brodifacoum, bromadiolone, and difethialone are far more potent, often killing after a single feeding. The potency of second-generation products creates a serious secondary poisoning risk: predators like hawks, owls, and foxes that eat poisoned rodents can accumulate lethal doses themselves.
Non-anticoagulant options include bromethalin, which causes brain swelling in rodents, zinc phosphide, which produces a toxic gas in the stomach, and cholecalciferol (a form of vitamin D that causes fatal calcium buildup). These work through different mechanisms and don’t carry the same secondary poisoning risk from anticoagulant accumulation, though they still pose hazards to pets and wildlife.
Biopesticides: Natural and Biological Options
Biopesticides are derived from living organisms or natural materials. They tend to break down faster in the environment, work in smaller quantities, and target pests more narrowly than conventional chemicals.
The most widely used biopesticide is Bacillus thuringiensis (Bt), a naturally occurring soil bacterium that produces proteins toxic to specific insect larvae. Different strains target caterpillars, mosquito larvae, or beetle grubs without affecting other insects. Bt genes have also been engineered directly into crops like corn and cotton, creating plants that produce their own insect-killing proteins. These are called plant-incorporated protectants.
Other biopesticide examples include spinosad (derived from a soil bacterium, commonly used in organic flea treatments and garden sprays), neem oil (pressed from the seeds of the neem tree), capsaicin (the compound that makes chili peppers hot, used as an animal repellent), and citronella oil (a plant-based mosquito repellent). Insect growth regulators like methoprene mimic natural hormones and prevent mosquito or flea larvae from maturing into adults, controlling populations without conventional poisons. Even corn gluten meal is registered as a biopesticide, used as a pre-emergent herbicide on lawns.
Banned Pesticides and Why They Matter
Several once-common pesticides have been banned or severely restricted worldwide. The Stockholm Convention, an international treaty, initially targeted 12 persistent organic pollutants, many of them pesticides. Understanding these banned chemicals helps explain why modern regulations exist.
DDT is the most famous example. Widely sprayed from the 1940s through the 1960s for both agricultural pest control and mosquito eradication, DDT accumulated in the food chain and caused devastating declines in bird populations by thinning eggshells. Most countries banned it in the 1970s. It remains permitted in limited circumstances for malaria mosquito control in some tropical regions.
Chlordane was used extensively against termites and on agricultural crops. It persists in soil for years, with a reported half-life of about one year, and was banned in the U.S. in 1988. Dieldrin, used against termites and crop insects, and heptachlor, used to kill soil insects and grasshoppers, were both banned for similar reasons: extreme environmental persistence and bioaccumulation in wildlife and human tissue. Endrin, sprayed on cotton and grain crops and also used as a rodenticide, rounded out the group of banned organochlorine insecticides.
How to Read a Pesticide Label
Every pesticide product sold in the U.S. carries a signal word on the label that tells you its toxicity level. The EPA assigns products to four categories. Category I products carry the word “DANGER” (and “POISON” in red letters if highly toxic through ingestion, skin absorption, or inhalation). Category II products say “WARNING,” indicating moderate toxicity. Category III products say “CAUTION,” meaning slight toxicity or mild irritation. Category IV products may optionally include “CAUTION” but represent very low toxicity.
These signal words apply to the formulated product as you’d buy it, not just the active ingredient. A highly toxic active ingredient diluted to a very low concentration might end up in a lower toxicity category. The active ingredient list on the label tells you exactly what chemical is doing the work, which is useful for comparing products, checking for allergens, or looking up environmental safety information.