Different Types of Pesticides: A Working Guide to the Major Classes
“Pesticide” isn’t a chemical. It’s a catch-all category that covers anything designed to kill, repel, or suppress a living thing that humans have decided is a pest. That’s a wide net. The biggest groups inside it are herbicides (for weeds), insecticides (for bugs), fungicides (for mold and rot), and rodenticides (for rats and mice). There are smaller categories too, aimed at slugs, soil worms, and birds, but the four above account for most of what gets sprayed in the US. The EPA registers roughly a thousand different active ingredients (the pesticide chemicals doing the killing) across these categories, and those active ingredients end up in more than 17,000 products on store shelves and farm supply invoices.
Sorting the pesticides into classes is the reference layer underneath the broader pesticides overview. Knowing which class a chemical belongs to is the difference between guessing at risk and reading the label with some idea of what you’re looking at.
The class matters. It shapes everything else: how long the chemical sticks around after you spray it, what it does inside a human body, what kind of symptoms show up if someone gets overexposed, and which group of people is most at risk. None of that information appears on the front of a bottle.
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Herbicides
Herbicides kill plants by interfering with something the plant needs in order to grow. The mechanisms get pretty specific. Glyphosate (and a few related chemicals like the sulfonylureas) blocks an enzyme that plants use to build their own amino acids. Cut off the amino acids, the plant can’t build proteins, the plant dies. Atrazine and simazine work on a totally different system: they block photosynthesis itself, which is how plants make energy out of sunlight. The growth-regulator herbicides like 2,4-D and dicamba take a different approach: they mimic the plant’s own growth hormones, forcing the plant to grow uncontrollably until it can’t sustain itself. And then there are the membrane disruptors like paraquat and diquat, which just break the plant’s cell walls open. Each class works on a different piece of plant biology. The classes worth reading carefully are the ones that act on biology humans share.
Glyphosate is the largest-volume herbicide in the world. Its cancer status is contested (EPA says no, IARC says probable), Bayer has paid $11 billion in US settlements, and residues turn up in oats and wheat from pre-harvest desiccation.
Atrazine is the second-most-used herbicide in the United States and the pesticide that turns up most often in US drinking water testing. It’s a corn-belt staple, applied on roughly two-thirds of US corn acres at an estimated 70 to 80 million pounds annually. Atrazine interferes with hormones at very low doses. Researchers have documented it scrambling the sex development of frogs at parts-per-billion levels, and there’s a growing body of research linking it to reproductive problems in people. The European Union banned atrazine in 2004 over groundwater contamination concerns. The US still allows it.
2,4-D is the broadleaf herbicide that turns lawns into grass without dandelions. It was one of the two compounds in Agent Orange. People often raise that point; it’s true but incomplete, since the dioxin contamination of the wartime formulation, not the 2,4-D itself, drove the worst Agent Orange health effects. Modern 2,4-D is still classified as a possible human carcinogen by IARC. It’s also one of the most widely used residential lawn herbicides in the US.
Paraquat is acutely toxic to humans at small doses, and there’s no antidote. A single mouthful is typically fatal. Paraquat is banned in more than 70 countries including the EU, China, and Brazil. It remains legal in the US for restricted use by certified applicators. The growing body of evidence linking chronic low-level paraquat handling to Parkinson’s disease has driven thousands of US lawsuits against Syngenta, the primary manufacturer. The risk picture for farmworkers and applicators is sharper than for the general public because handler exposure is so much higher.
Diquat is paraquat’s lower-profile cousin and increasingly the substitute when paraquat is restricted. It is also acutely toxic. EPA reclassified diquat in 2023 and tightened use restrictions but hasn’t banned it.
Dicamba is the herbicide Monsanto and BASF pivoted toward as weeds developed resistance to glyphosate. Dicamba evaporates off treated fields and drifts onto neighboring crops, gardens, and yards at levels high enough to cause visible damage and a wave of lawsuits. The “low-volatility” formulations the industry developed haven’t solved the drift problem. Multiple federal court rulings have thrown out the EPA’s approvals of dicamba formulations, the most recent one in 2024.
Insecticides
Insecticides kill insects by disrupting the nervous system, growth regulation, or energy metabolism. The four major classes in commercial use today are organophosphates, carbamates, pyrethroids, and neonicotinoids. There’s also a fifth class, the organochlorines (DDT, chlordane, dieldrin, lindane), that’s largely banned in the US but still present in soil, sediment, and human body tissue from decades of prior use.
Organophosphates are the oldest type of insecticide still in wide use, and they remain some of the most important. They kill insects by blocking an enzyme that turns nerve signals off. Without the enzyme, the insect’s nerves keep firing and the insect dies. The problem with that mechanism, when you’re using it on a farm or a lawn, is that humans use the same enzyme. The class includes chlorpyrifos, malathion, diazinon, parathion, and several dozen others. Organophosphates have the strongest research on harm to child brain development of any pesticide group, especially the work coming out of the UC Berkeley CHAMACOS study and parallel research at Mount Sinai. Chlorpyrifos is the cleanest example of how the US and EU treat the same chemical differently. The US banned it for residential use in 2001 and on food crops in 2021. The Eighth Circuit vacated the food-crop ban in late 2023; EPA reinstated the tolerances in February 2024, then in December 2024 proposed a new rule revoking food uses except for 11 specific crops. The legal status is still being worked out. The EU has banned chlorpyrifos outright since 2020.
Carbamates kill insects the same basic way organophosphates do (same enzyme, same blocking action), but the chemistry lets the enzyme recover faster, so the effect doesn’t last as long in either insects or humans. The class includes carbaryl (sold as Sevin, a residential pesticide that’s been around forever) and aldicarb. The toxic effects look broadly similar to organophosphates. Recovery from a poisoning event tends to happen faster.
Pyrethroids started as a natural product. Pyrethrins come from chrysanthemum flowers, and they’ve been used as insecticides for a long time. The pyrethroids are the lab-synthesized version, designed to last longer and work harder than the natural extract. The class includes bifenthrin, permethrin, deltamethrin, cypermethrin, and cyfluthrin. If you’ve used a household bug spray, treated your pet for fleas, or hired a residential mosquito or lawn service, you’ve used a pyrethroid. They work by disrupting how nerve cells fire. Acute poisoning of humans at residential doses is uncommon. What’s less clear, and still being studied, is what they do to developing brains in children when the chemicals are around constantly. CDC’s national health survey finds pyrethroid breakdown in the urine of roughly two-thirds of US adults.
Neonicotinoids came on the market in the 1990s. They were marketed as a safer, less acutely toxic alternative to organophosphates, which was true on that one specific dimension. The class includes imidacloprid, clothianidin, thiamethoxam, acetamiprid, and dinotefuran. They work by hijacking nicotine-style nerve receptors in insects. They’re also systemic, and that part matters a lot. Systemic means the plant pulls them up into its own tissue. The chemical ends up in the pollen, the nectar, the fruit, the leaves. That’s what makes them so effective against sap-feeding insects. It’s also what makes them catastrophic for bees and other pollinators, because the bees pick the chemical up directly from the flowers. The EU restricted three of the major neonicotinoids in 2018 based on the bee evidence. The US has restricted some uses but hasn’t banned the class. The question about what they do to developing human brains is an active research area, and the early findings aren’t reassuring.
Fungicides
Fungicides kill or suppress fungi and molds. They get sprayed on crops, on grain in storage, on seeds before planting. They’re also in lumber, paint, and household surfaces. The chemistry covers a lot of ground. The big commercial classes have names like triazoles, strobilurins, dithiocarbamates, and chlorothalonil, but most home consumers will never hear those words. Fungicides get less public attention than the other pesticide groups, and the research on what they do inside human bodies is less developed than for the herbicides and insecticides.
Mancozeb is a widely used fungicide sprayed on a long list of fruit and vegetable crops. Once it breaks down in the body or the environment, it becomes a compound called ethylene thiourea, which is a known animal carcinogen and disrupts thyroid function. The EU banned mancozeb in 2021 after EFSA, the EU’s food safety agency, concluded the risk to people and the environment couldn’t be ruled out at normal use levels. The US still allows it.
Chlorothalonil is the most-used fungicide in the United States, sprayed broadly on potatoes, tomatoes, peanuts, and turf. The EU banned it in 2019 after EFSA called it a likely human carcinogen and found that its breakdown products stick around in groundwater for a very long time. The US still allows it.
Triazoles (propiconazole, tebuconazole, difenoconazole) are systemic fungicides used on cereal grains and turf. The class is connected to a serious problem in medicine: it’s helping drive resistance to antifungal drugs in human pathogens, especially a mold called Aspergillus fumigatus, which can cause hard-to-treat lung infections in people with weakened immune systems.
Rodenticides and the smaller classes
Anticoagulant rodenticides (warfarin, brodifacoum, bromadiolone, difethialone) kill rats and mice by stopping their blood from clotting. The animal bleeds internally and dies. What gets less attention is that these chemicals are also highly toxic to anything that eats a poisoned rodent: owls, hawks, mountain lions, bobcats, the family dog, the neighbor’s cat. California restricted the strongest versions back in 2020, after years of documented wildlife deaths kept piling up. Federal restrictions are weaker. For most households dealing with rats or mice, snap traps are the cleaner option.
A few smaller pesticide categories are worth a brief mention. Molluscicides kill snails and slugs (metaldehyde, iron phosphate). Nematicides target the microscopic worms that live in soil. Fumigants are gas-phase pesticides used in grain storage and for indoor pest control (chloropicrin, methyl bromide, phosphine). And then there are the biopesticides, which are made from biological sources rather than synthetic chemistry. Bacillus thuringiensis (called Bt for short), neem oil, and spinosad are the most common ones. The biopesticide category is where most of the new development in organic-compatible pest control is happening right now.
Reading a pesticide label
The active ingredient is listed by chemical name on every legal pesticide product sold in the US, usually in a section called “active ingredient” with the percentage by weight. The brand name sits at the top of the label and tells you almost nothing about what the chemical inside will do.
The “signal word” on the label tells you the acute toxicity category: CAUTION (least toxic), WARNING (more toxic), DANGER (most toxic). The category only reflects how quickly the product can poison you. It doesn’t reflect long-term effects, effects on the environment, or whether the chemical interferes with hormones or might cause cancer. A pesticide can carry a CAUTION label and still be fully banned in Europe over long-term risks.
The EPA registration number, also on the label, can be looked up in the EPA’s pesticide product registration database. The National Pesticide Information Center, funded jointly by the EPA and Oregon State University, runs a free phone and email service that answers questions about specific products. NPIC is one of the more useful independent resources for anyone trying to understand what’s in a pesticide product.
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This page is reviewed periodically as new research emerges. Content reflects current scientific understanding of toxin-related health topics.
Last reviewed: June 2026