Insecticide Overview
Insecticides by category are found here
A bigger list of insecticides is found here.
An overview of insecticide chemistry can be found here.
You can look up lots of information on just about any insecticide at EXTOXNET.

Classification
Insecticides are classified by their chemical basis.  We recognize the following classification:

Organochlorines.  At one time, there were many chlorinated hydrocarbon insecticides such as DDT, aldrin, endrin and chlordane. Throughout the 1970s and 80s, the EPA banned most chlorinated hydrocarbons from sale and use in the United States because these insecticides persisted in the environment and increased in the fatty tissues of animals. Many insect populations have developed resistance to these insecticides.  You still encounter them in developing countries.

Organophosphates.  The OPs were discovered in Germany during World War II research on nerve gas poisons. They are still used that way (e.g. sarin).  They are generally more toxic to vertebrates than the chlorinated hydrocarbons, but they tend to be less persistent in the environment. Some of the more common OPs are chlorpyrifos, diazinon, malathion, and parathion.

Malathion

Carbamates. In 1951, carbamate insecticides were introduced into the world market. Common carbamates include carbaryl (Sevin), carbofuran (Furadan), propoxur (Baygon®), and bendiocarb (Ficam®). Carbamates were initially used against insects that developed resistance to chlorinated hydrocarbons and OPs. However, some pest insects have now developed resistance to these carbamates.

Synthetic Pyrethroids. There have been dozens of synthetic pyrethroids identified and synthesized. A few include fenvalerate, permethrin, tetramethrin, cypermethrin and tefluthrin. (Notice the "thrin" part of the name). Chemists have made these compounds more stable and more persistent than natural pyrethrum. A synergist, like piperonyl butoxide (c.f. below), is usually added to the synthetic pyrethroid to further increase its effectiveness. One characteristic of using synthetic pyrethroids is that insects become more excited and active. This impacts their use for structural pests such as brown recluse spiders and cockroaches.  Like the botanical pyrethrum, synthetic pyrethroids have fast knockdown activity against flying insects and low mammalian toxicity. Pyrethroids are very toxic against fish so precautions must be taken to cover fish tanks and disconnect filters, at the very least. Synthetic pyrethroids are the active ingredients in most of the in-home aerosols that you can buy Raid®, Black Flag®, etc).  Exposure to synthetic pyrethroids may trigger asthmatic attacks in persons who have respiratory problems. This may limit their use in sensitive areas.

Botanicals (e.g. nicotine, rotenone).  These are compounds derived from plants.

Insect Growth Regulators (IGRs).  These compounds take advantage of insect-specific physiology.

New compounds (Novel Chemistries).  Newly developing approaches to insecticide chemistry.

Route of Entry

Residual insecticides (persistent) remain active in amounts sufficient to kill pests for at least a week, several weeks, or even years after application. These residual insecticides act by keeping a toxic insecticide residue on a surface that the insect will contact. Conversely, non-residual insecticides (nonpersistent) breakdown rapidly after application. Residual insecticides are useful when insects are a continual problem or when you need to make an application in advance of an expected problem.

Contact insecticides control the pest on contact. Contact insecticides are applied directly to the insect which is killed after is comes in contact with the insecticide. Very little toxic residue remains on a surface after spraying a contact insecticide. Most aerosols and foggers are contact insecticides.

Systemic Insecticides enter the insect's host plant or animal and translocate/circulate throughout the host.  When an insect feeds on the host, it ingests enough insecticide to be lethal.

Stomach poisons are insecticides eaten by an insect so that the poison enters the stomach and then is absorbed into the body.  Many baits are stomach poisons.  Boric acid is an excellent stomach poison.

Modes of action:  Many act on the nervous system.

Chlorinated Hydrocarbons -- All target the nervous system
Benzene ring-based CHC's (DDT, methoxychlor).  Interrupt axonal transmission causing nerves to "fire" multiple/many times in response to one stimulus.

Cyclodienes (Mirex, toxaphene, Dieldrin, heptachlor) and hexachlorcyclohexane (BHC, Lindane).  Interrrupt normal synaptic transmission of nerve signals by causing pre-synaptic terminals to over-release acetyl choline.

Organophosphates and Carbamates.  Both groups inhibit acetyl cholinesterase.  Examples of

Pyrethroids -- Mode of action depends on presence/absence of an a-cyano constituent.  This constituent affects the target site.  Both block neuronal transmission.  Although the exact target site is unknown, it likely involves the sodium channels of axonal transmission.

Botanicals -- Different modes of action

Nicotinoids (nicotine, nornicotine) -- Blocks acetyl choline receptor.  This is different from the OPs & carbamates suggesting that synthetic nicotinoids may hold promise.  Marketed under the name Black Leaf 40.

Rotenoids (rotenone) -- Derived from roots of Derris spp. plants.  Not neurotoxic.  Instead, interferes with respiration at the cellular level by inhibiting the action of cytochrome-b.

Azadirachtin (Neem).  Disrupts numerous physiological processes specific to insects (ecdysone mimic, inhibits chitin formation, feed deterrent [prevents swallowing], mating disruptant).  Target of much current research.  One product, Margosan-O, is marketed in this country for use in greenhouses, commercial nurseries, forests, and homes.

Insect Growth Regulators
Acylureas (Benzophenylureas).  Most products end in "uron":  Diflubenzuron, Chorfluazuron, Novaluron, Hexaflumuron, etc.  All products inhibit chitin formation which affects insects in one of two ways:  disrupts ecdysis and displaces mouthparts so insects can't feed.  Generally effective only on immatures (they're the ones that are synthesizing chitin).  Viewed as relatively less susceptible to resistance development.

Endocrine-based insecticides.  May be juvenile hormone (JH) analogs, anti-JH or formamidines.

JH analogs are terpenoid ethers-- Methoprene, hydroprene, phenoxyphenoxy carbamate.  Mostly interfere with larval-pupal transformation (malformations, extra instars).

Anti-JH, precocene.  Forces immature insects to molt prematurely.  If they survive to adulthood, they are sterile.

Formamidines (Chlordimeform, Amitraz) target the octopamine receptor causing diverse forms of behavioral abnormalities.  These abonormalities include repellent, feeding deterrent/inhibition, mating disruption, disorientation, quiescence.

Novel Chemistries
Fermentation Products  -- Cannot be readily synthesized directly but are produced by bacteria in fermentation vats.
Avermectins -- produced by the soil bacterium, Streptomyces avermitilis.  Interferes with interneuron function.  Mainly used as anti-helminth in livestock.

Spinosad -- from Saccharopolyspora spinosa.  It is a neurotoxin that acts as a acetylcholine agonist like nicotine but through another, unknown mechanism.

Phenyl-Pyrazoles (Regent, fipronil) -- interferes with exchange of cholride ions by neurons.

Pyrroles (Alert, chlorfenapyr): The pyrroles are compounds that act as metabolic toxins and work by uncoupling oxidative phosphorylation in the mitochondria. They have translaminar activity, and are toxic both by contact and ingestion to chewing and sucking arthropods.

Chloronicotinyls (Imidichlorpid, Admire, Provado) -- action like nicotine.

Synergists

Synergism -- One chemical increases the biological activity of another.  Often used in insecticides.  Most work by interfering with enzymes that would otherwise degrade an insecticide before it could do its job.  Two groups of synergists:

Methylenedioxyphenyl (MDP) compounds -- microsomal oxidase inhibitors (MOIs).  Best known of these is piperonyl butoxide.  Other MOIs, with different chemical structures, are also available such as the imidizoles.  Note that the phosphothioate insecticides (like malathion) are activated by oxidases so that these synergists would actually interfere with their insecticidal properties.

Hydrolytic Enzyme Inhibitors.  Compounds that inhibit esterases involved in metabolism of insecticides with phosphate or carboxyl esterases can be potent synergists.  Some of the best known materials of this type include EPN (O-ethyl-O-p-nitrophenyl phenylphosphonothionate), TOCP (tri-o-cresyl phosphate), trisubstituted aliphatic and aromatic phosphates, and a number of non-insecticidal carbamates.

Formulations

See the slide set at: urbanprinciples.ifas.ufl.edu/Formulations.ppt

Insecticides are not normally applied as a pure chemical.  Instead, they are formulated in a manner that facilitates efficient and safe use.  "Formulation" refers to the specific physical form of the insecticide, along with any additives, and can be a pressurized gas, liquid, or dry.  Pressurized gasses are used primarily for fumigation (soil and structures).  Liquid formulations are intended to be applied either as an aerosol or pressurized spray.  Dry formulations include: dusts, powders, granules, treated seed/bait pellets, encapsulated and cubed.  Some formulations, especially those made for in-home applications for homeowners, are ready for use. Others must be diluted (usually with water) to a correct concentration before application. The directions on the label will tell you how to use any given pesticide formulation.

An active ingredient (AI) is the specific chemical in a pesticide product that "does the dirty work". An active ingredient is mixed with less toxic inert ingredients and are listed on the label.  It is important to realize that some active ingredients are often formulated in more than one way. For instance, chlorpyrifos, a commonly used insecticide, can be formulated as an emulsifiable concentrate, a wettable powder, an aerosol, a bait and a dust.

Normally, the concentration of an active ingredient is combined with a mnemonic associated with a formulation.  For example, carbaryl (Sevin) in a 50 % wettable powder is listed on the bag as Sevin 50WP.  If it were formulated as granules containing 10 % carbaryl, the bag would read Sevin 10G.

Liquid Formulations

1.  Emulsifiable Concentrates (EC).  Water is added to an emulsifiable concentrate which forms a smooth mixture of the pesticide, solvent and the water carrier. The inert ingredients are often highly refined oils and other solvents.

Emulsifiable concentrates that are formulated for household pests usually have lower concentrations of active ingredients. Some ready-to-use ECs have already been diluted by the manufacturer before you buy them and are to be used without further dilution. This increases the safety so the applicator doesn't need to mix concentrated liquids.

Advantages of ECs are that they can be applied as crack and crevice treatments and provide some residual control.  The petroleum solvents in EC formulations may stain carpets, fabrics and wallpapers so their use in homes needs to take this into consideration.

ECs are important formulations and some can be purchased from discount, drug, grocery and hardware stores. Professional-use EC formulations can be purchased from local pest control companies advertising sale of pest control supplies to the public. However, EC formulations, marketed for professional pest control technicians, will be in a concentrated form and will need to be mixed in a spray tank with water.

2.  Flowables (F or L)A flowable, or liquid, can be mixed with water to form a suspension in a spray tank.

3.  Microencapsulated (ME or MEC).  In microencapsulated formulations, the insecticide is contained inside very small spheres. This microencapsulation allows the insecticide to escape very slowly through the sphere wall, increasing the length of the effective application.  Created by first making an emulsion which results in tiny insecticide droplets, adding monomers, then causing the monomers to join together as a polymer (using heat or a catalyst).  Lacking anything else to form around, the polymers will coat and encase the insecticide droplets.

Microcapsules     Capsule on a hair

Left:  Electron micrograph of insecticide microcapsules.  Right: A microcapsule on a human hair.

4.  Low Odor (LO).  There are some insecticides formulated to reduce the chemical smell that might be offensive to the public. They can be used in sensitive environments, like supermarkets, restaurants and public buildings so that patrons will not be offended. For example, Dursban LO® has less odor than other formulations containing chlorpyrifos.

5.  Aerosols (A).  The active ingredient in an aerosol formulation is dissolved in a volatile petroleum solvent with pressure from a gas propellant. Common, over-the-counter aerosol formulations have a low percentage of active ingredient. These aerosols are most effect when the liquid contacts the insect directly, and are not very effective when used against insects that are hiding in cracks and crevices. There are some crack and crevice aerosol formulations, marketed for pest control technicians, that contain higher percentage of the active ingredient, and can be quite effective when used as crack and crevice treatments.

The main advantage of over-the-counter aerosols is that they are easy to use and readily available. Disadvantages are that the container must be held upright during operation, the residual control is very poor, they are relatively expensive for the amount of insecticide they contain, and the solvent vapors are readily airborne and can be harmful to the applicator.

Aerosol products that contain higher percentages of active ingredients are only available through specialty outlets that sell professional-use pest control supplies to the general public.

6. Ultra-low volume (ULV) -- Highly concentrated formulation intended for use in special application equipment that can meter out minute quantities of insecticide.  Often defined an 8 or more lb/gal active ingredient.  Equipment calibration and droplet size is critical.  Common in mosquito control.

6.  Foggers.  An insecticide fogger or insecticide bomb is a total-release aerosol that will discharge its entire contents in a single application. They are almost exclusively used for structures (homes, greenhouses, etc.) During fogging, the occupants and pets should leave and remain away for a few hours. Consult the label for the re-entry time and follow all precautions before using a fogger. Aerosols and foggers produce very small droplets that settle on furniture, the floors and countertops. These small droplets are easily inhaled by humans and pets. Fogging is NOT the same as fumigation. Fumigation uses deadly insecticidal gases and can only be done by professional, certified applicators.

Foggers are easy to use and readily available. Unfortunately, foggers only kill exposed insects. Like aerosols, insects hiding in cracks and crevices are protected and so are not killed by fogger insecticides.

Dry Formulations

1.  Dusts (D).  Dust formulations contain an active ingredient plus a powdered dry inert substance like talc, clay, nut hulls or volcanic ash. The inert ingredients allow the dust formulation to store and handle well. In the home, dusts should be used only in locations where the inhabitants will not stir the dust, move it around, or inhale it. Appropriate places for dust applications are wall voids, behind baseboards, in enclosed spaces under kitchen counters, above dropped ceilings and in unused attics. Dusts always must be used dry and stored in a dry place.

2.  Wettable Powders (WP or W).  These are dry, finely ground, powdery formulations. They look like dusts, but a wetting agent has been added to the other ingredients to help them to mix with water. Wettable powders form a suspension rather than a true solution when added to water. Good agitation is needed to prevent settling of the insecticide on the bottom of the spray tank. When a wettable powder formulation is sprayed on a porous surface, like wood, the water will penetrate the wood, but the powder stays on the wood surface. This give the greatest possible residue (and residual activity) left on the surface, but the powder itself may be visible and unsightly.

3.  Soluble powders (SP or WSP): A soluble powder formulation is made from an active ingredient in powder form that dissolves in water.

4.  Granules (G or GR).  The pesticide is incorporated into tiny beads made of clay or other material, such as mixed fertilizer-herbicide products for lawns. 

5.  Pellets (P, PE, or PS) -- Differs from granule formulation in that all particles are the same shape (usually spherical) and weight thereby increasing application precision.

6.  Water Dispersible Granules (WDG or DF) -- Also referred to as Dry Flowables (hence the DF).  This formulation superficially looks like pellets but is intended to be mixed with water instead of applied as pellets.  Once it's mixed with water, the pellets break into wettable powder and behave like the WP formulation.  Formulating a WP in pellets like this is commonly done for those insecticides that have an inhalation or other exposure hazard.  The pellets in WDG or DF formulations typically have much higher concentrations of insecticide than do the G or P formulations.

7.   Baits (B). A bait formulation is an edible or attractive substance mixed with a stomach poison. Baits are marketed over-the-counter in bait stations that are easy to use and reasonably safe to humans. Bait stations should be placed near places where the insects live, in locations where the baits will not be a menace to nontargets. One disadvantage of using some baits alone is that control may be slow.  Another disadvantage is that some products do not seem to suppress large insect populations.  Baits are an ideal companion to other types of control and work best when food available to the target insect population is limited (i.e. sanitation is good).

Special Purpose Formulations

1.  Capsule Suspensions (CS) -- A micro-encapsulated formulation held in a suspension. 

2.  Public Health (PH) --  Not an actual formulation but, instead, a certification that a pesticide is appropriate for public health use.  Some of the labels, though, have a PH designation in a manner that looks like a formulation.

3.  Technical -- Pure active ingredient in its natural state.  Usually available only from the manufacturer and encountered only by researchers.

Adjuvants

An adjuvant is an approved product which is added to an insecticide spray formulation to improve its properties, eg better spreading, sticking or penetration. They can also be used to reduce application rates by improving activity or improving product performance in adverse conditions.  "Adjuvant" is a broad term and includes surfactants,  anti-foaming agents, stickers and spreaders, drift control agents, penetrating agents and buffers.

Surfactants or spreading agents lower the spray particle surface tension allowing droplets to spread out on plant surfaces instead of combining to form large droplets subject to run-off. They are especially useful when spraying hard-to-wet surfaces such as waxy citrus leaves, armored scale insects, mite colonies with webbing or hairy-surfaced weeds. Some pesticide products are formulated with surfactant included.  The benefit surfactants provide is offset, to a degree, by the increased drift hazard they cause. Reducing the surface tension of the spray solution permits it to break up into finer droplets, which are more likely to drift off target (cf. drift control agents below)..

Surfactants are probably the most widely used type of adjuvant. For more information about this group, go here.

Anti-foaming adjuvants are a special kind of surfactant.  Some formulations will create foam in some spray tanks and this foam can interfere/prevent normal spray operation. This is usually a result of both the surfactants used in the concentrate formulation and the type of spray tank agitation. This foam can be reduced or eliminated by a small amount of foam inhibitor. Foam is an emulsion of air in water. A variety of surfactants will destabilize these air/water emulsions but the most commonly used one is a silicone/carbon polymer. This is sometimes included in wetter-spreader adjuvant formulations or is available as a separate product that can be squirted directly into the foam on the top of the liquid in the spray tank.

Stickers and extenders function to lengthen pesticide performance life by increasing product tenacity or  weathering qualities of materials sprayed on plant surfaces. These frequently are combined with surfactants and are available as "spreader-stickers."

Drift control agents are adjuvants that help reduce the risk of drift. Pesticide drift is off-target spray deposit and off-target damage. Drift is primarily a function of droplet size and wind; droplets with diameters of 100 microns (0.1 mm) or less contribute the bulk of the drift off site from the treated fields. Drift control adjuvants increase the viscosity and the "tensile" strength of water and decrease the proportion of smaller drops in a spray system. They will also increase the average drop size resulting in fewer drops per square inch of leaf surface, but it will still be the same rate of deposit of pesticide in pounds per acre. Some research has indicated that certain drift retardants may degrade due to the shearing action of recirculation after several passes through an agricultural sprayer pump.
Use of these adjuvants is especially important in areas adjacent to residential areas. Lo-Drift, Nalco-Trol and Drift Proof are examples of drift control agents.

Penetrating agents dissolve the waxy layer that protects the surface of leaves. This speeds up absorption with foliar treatments. Lower application rates used with these adjuvants may provide the same control as higher rates made without them; more chemical enters the plant before breaking down or washing off. Examples of penetrating agents include Arborchem and kerosene.  These products are mainly used in herbicides.

Buffers maintain the proper pH of a pesticide spray; usually to decrease breakdown of pesticides in alkaline water. Organophosphate insecticides are particularly sensitive to high pH.  For example, Cygon (dimethoate) loses 50 percent of its pest control power in just 48 minutes when mixed in water of pH 9. At a pH of 6, however, it takes 12 hours for degradation to progress to that extent.

A Note on Adjuvant Use:  Many products can be used as adjuvants but, as a general rule, only products marketed specifically as agricultural spray adjuvants should be used.  In particular, do not purchase products made for household use. For example, soaps and detergents are essentially surfactants. However, these are not generally effective because they contain low concentrations of surfactant (10-20%) compared to agricultural products (50-95%). They may also combine with hard water to form scum and precipitates the affect spray performance. They can also cause excess foaming in the tank.