Chapter 2


Pesticides are defined as substances or mixtures of substances intended for controlling, preventing, destroying, repelling, or attracting any biological organism deemed to be a pest. Insecticides, herbicides, defoliants, desiccants, fungicides, nematicides, avicides, and rodenticides are some of the many kinds of pesticides.

Classification of Pesticides

Pesticides may be classified in a number of ways; these classifications can provide useful information about the pesticide chemistry, how they work, what they target, etc. Following are brief descriptions of some commonly used classification systems.

By Chemical Nature

One traditional classification of pesticides places them in one of two groups: organic and inorganic. Organic pesticides are based on chemicals having carbon as the basis of their molecular structure. The chemicals in organic pesticides are more complex than those of inorganic pesticides, and usually do not dissolve easily in water. Inorganic pesticides are simpler compounds. They have a crystalline, salt-like appearance, are environmentally stable, and usually dissolve readily in water. The earliest chemical pesticides were inorganic, and included substance such as sulfur and lime.

The vast majority of modern pesticides contain an organic chemical. There have been hundreds of pesticides developed based on organic chemicals, often with oxygen, phosphorus, or sulfur in their molecules, in addition to their basic carbon structure.

Organic pesticides can be subdivided into two additional groups: the natural organics, and the synthetic organics. The natural organic pesticides (sometimes just called "organics") are derived from naturally occurring sources such as plants. Rotenone and pyrethrum are examples of natural organic pesticides.

Synthetic organic pesticides (usually just called "synthetics") are produced artificially by chemical synthesis. This group comprises most "modern" pesticides (i.e., discovered or used as insecticides post-World War II), and includes DDT, permethrin, malathion, 2, 4-D, glyphosphate, and many, many others.

By Target Pest Species and Pesticide Function

Pesticides are sometimes classified by the type of pest against which they are directed or the way the pesticide functions. Table 2.1 is an example of this sort of classification.

INSECTICIDES and acaracides: Classification by Chemistry

Insecticides are designed to control insects, and acaracides control ticks and mites. In public health applications they are most commonly used to control mosquitoes, flies, ticks, mites, lice, and fleas. Since insecticides and acaracides are often the same pesticides, they are not discussed separately here.

Organochlorines (=chlorinated hydrocarbons) represent the one of the first group of pesticides synthesized, and include the well-known insecticide DDT. Although DDT is still legally used for vector control in some areas of the world (particularly where malaria occurs), DDT's registration for nearly all uses was suspended by the EPA many years ago, and consequently, it is no longer used in the United States. Most other organochlorines used for arthropod control, including chlordane, dieldrin, and lindane, have met similar fates.

Table 2.1 Classification of insecticides.


Target Pest / Function


Mites, ticks






Attracts insects or birds






Plant leaves


Disrupts water balance in arthropods



Growth regulator

Regulates insect and plant growth








Snails, slugs






Vertebrate predators


Repels vertebrates or arthropods




Woody vegetation


Organophosphates Although a few organophosphate (OP) formulations remain available for vector control, their use has dramatically decreased because of resistance to OPs, the potential for non-target effects, and the development of alternative products. Members of this group contain phosphorous in their molecules. Products currently labeled for vector control include naled, malathion, and some formulations of dursban. Organophosphates are considered by most to pose a greater human health risk for pesticide applicators than other families of pesticides.

Carbamates are chemically similar in structure to organophosphates, but whereas OPs are derivatives of phosphoric acid, carbamates are derivatives of carbamic acid. Pesticides in this group used for vector control in California include carbaryl (Sevin®) for dusting rodent burrows to control fleas, propoxur (Baygon®) for use against insect pests, and certain brands of bee and wasp control sprays. Carbamates also pose a relatively high risk for human poisoning. Some carbamates are herbicides.

Pyrethrum is a natural organic insecticide that is derived from plants in the genus Chrysanthemum. There are about 30 species in the genus, most of which use the generic name as their common name. The insecticide is produced by grinding of the flowers, thus releasing the active components of the insecticide, called pyrethrins. The main active constituents are pyrethrin I and pyrethrin II plus smaller amounts of the related cinerins and jasmolins.

Insecticides containing pyrethrins are neurotoxic to nearly all insects. They are harmful to fish, but are far less toxic to mammals and birds than many synthetic insecticides and are non-persistent, breaking down easily on exposure to light. They are considered to be amongst the safest insecticides for use around food.

Pyrethrin-containing insecticides are used widely in California for vector control. They are broadly labeled, and can be used in both rural and urban areas in a variety of habitats. Pyrethrins are usually mixed with PBO (piperonyl butoxide), which acts as a synergist. Synergists are materials that are not necessarily pesticidal by themselves, but have the effect of increasing the toxicity of insecticides with which they are mixed. Without PBO, insects treated with the same dose of pyrethrins would be knocked down, but would eventually recover.

Fig. 2.1 Chrysanthemum coronarium in the Tel Aviv garden (from Wikipedia).

Recent studies suggest that PBO may have a longer residual life in aquatic systems than previously thought, possibly acting to increase the toxicity of other chemicals to benthic organisms (i.e., animals inhabiting the sediment layer of aquatic environments). Additional studies should clarify the significance of vector control applications to these populations.

Pyrethrins are highly toxic to fish and their direct application to water is restricted.

Pyrethroids are synthetically produced molecules that are chemically similar to pyrethrins. Pyrethroids are not persistent. At rates applied for vector control, they break down quickly in sunlight, and are rarely present after just a few days. The mode of action of pyrethroids is the same as that of pyrethrins. Most pyrethroids are also synergized with PBO. Several generations of pyrethroids have been produced, with the latest formulations being effective at extremely small doses. Some of these new compounds may not break down as readily in sunlight as do pyrethrins, and in some cases pyrethroid synergists may not markedly improve their effectiveness.

Pyrethrins and pyrethroids are now among the most common public health pesticides used in California, especially for the control of adult mosquitoes. Their use now far outstrips that of conventional synthetic pesticides such as organochlorines and organophosphates.

Biorationals (biorational pesticides or biopesticides) are a group of pesticides that are considered relatively non-toxic to humans and are also environmentally safe. The EPA defines biorationals as "certain types of pesticides derived from such natural materials as animals, plants, bacteria, and certain minerals." Most pesticide specialists interpret the word "derived" broadly, and include synthetic pesticides that resemble natural substances. Ware and Whitacre, in The Pesticide Book, refer to biorationals as 21st century pesticides, and point out that the term has no single or legally clear definition. Biorationals can be separated into two groups: (1) biochemical (hormones, enzymes, pheromones, and natural insect and plant regulators) and (2) microbial (viruses, bacteria, fungi, protozoa, and nematodes). Among public health pesticides, methoprene (Altosid®) would fall into the first group, Bacillus thuringiensis israelensis (Bti) into the second. Because there is presently no strict legal definition for the term biorational, many will realize that some products, such as Bti, will satisfy the definitions for both biorational and biological control agent.

The action of biochemical biorationals is based on the interruption of natural growth processes of arthropods. They are not particularly selective among arthropod species, but generally have extremely low toxicity for vertebrates, including people. Insect growth regulators (IGRs), chitin inhibitors, plant growth regulators, and chromosterilants are included in this group. For vector control, the use of methoprene (an insect growth regulator) in mosquito control far outweighs all other uses. For many years after their introduction, IGRs were considered immune to the development of resistance by target pests. However, resistance has now been detected against some IGRs in specific locations in California and elsewhere. Diflubenzuron (Dimilin®) is a good example of an IGR that mosquitoes have shown resistance to.

Microbial pesticides kill arthropods either by toxins released by microbial organisms, or by infection by the organisms. Two common pesticides that fit within this group include the bacterial toxin produced by Bti, and the live bacteria, Bacillus sphaericus (Bs). Products containing both of these bacteria are used against mosquito larvae, with Bti being effective in killing black fly larvae as well. Most microbial pesticides are more selective than biochemical pesticides.

Materials Applied To Water Surfaces

Petroleum oils are refined from crude oil and in vector control are used both as carriers for insecticides, and more directly (as active ingredients) when mixed with a surfactant and applied to the water surface (i.e., Golden Bear 1111®) as a suffocating agent against mosquito larvae and pupae. Measured by weight, petroleum oils are by far the most common active ingredient (>90%) used for vector control in California

Alcohols are also used as surface control chemicals against mosquito larvae (i.e., Agnique®). These materials act by reducing surface tension of the water, eventually leading to the drowning of mosquito larvae or pupae.

Water surface control insecticides have several advantages over conventional insecticides. Oils and alcohols will kill mosquito pupae as well as larvae, and because there action is more physical than biochemical, they do not lead to development of pesticide resistance. The disadvantage of these products is they kill non-target organisms that either breath at the water surface (e.g., small aquatic beetles), or that depend on surface tension of the water (e.g., water striders).

Other Classifications Of Pesticides

Pesticides can also be classified by how they enter the target organism or where they act.

Stomach toxicants enter an insect's body through the mouth and digestive tract, where they are absorbed into the insect's body. Stomach poisons are acquired during feeding. In vector control, this category includes bacteria, or their toxins, applied to the water where filter-feeding mosquito or black fly larvae will consume the poison. These microbial insecticides kill by destroying the midgut (or stomach) of the larvae. Ants, cockroaches, and other pest insects with chewing mouthparts can be controlled by incorporation of insecticides into baits of various types. The site of insecticidal action varies. Rodents are also often controlled for using ingested anticoagulants. They die from internal bleeding, the result of loss of the blood s clotting ability and damage to the capillaries. Because anticoagulant baits are slow in action (several days following the ingestion of a lethal dose), the target animal is unable to associate its illness with the bait eaten. As a result, bait shyness usually does not occur. This delayed action also has a safety advantage because it provides time to administer the antidote (vitamin K1) to save pets, livestock, and people who may have accidentally ingested the bait.

Contact toxicants generally enter the pest or plant's body either by exposure to water treated with the chemical or direct contact with an aerosol (e.g., adult mosquitoes flying into an insecticidal "fog"), or by exposure to some treated surface, such as leaves. Like most insecticides, these poisons act upon the nerve and respiratory centers of arthropods. Most adult mosquito control products are contact toxicants.

Fumigants are volatile compounds that enter the bodies of insects in a gaseous phase. There are no longer any fumigants registered for use for control of public health insect pests in California. (But are sometimes used in rodent control)

Systemic toxicants are absorbed by plants, pets, or livestock and are disseminated throughout the organism via the vascular system. When a pest organism feeds on the plant or animal, they ingest the toxicant. Some toxicants are quickly lethal to the pest; others work to prevent the pest from maturing. Application in vector control is typically used for tick and flea control on pets, as well as dog heartworm prevention.

Chemical repellents used in public health applications prevent bloodsucking insects such as mosquitoes, black flies, and ticks from biting humans, livestock, or pets. The most widely used chemical used in repellent formulations to protect people is dimethyl toluamide, or DEET. In recent years a variety of formulations have been developed to minimize DEETís unpleasant characteristics when used as a skin repellent, and also to lengthen the duration of its effectiveness. Since the end of the 20th century, several repellents with an entirely new chemical basis have been released, some rivaling the effectiveness of DEET. (Mechanical repellents, such as high frequency emitters, marketed for insects or other pests are generally unproven and ineffective)

Formulations with Combinations of Pesticides

When two or more chemicals can be mixed safely, or used in combination, they are said to be compatible. These combinations can be a pesticide mixed with another non-pesticide chemical, or can be two or more pesticides that are combined in the same tank mix. The reasons for combining pesticides are:

1. To increase the effectiveness of one of the chemicals. As mentioned above, this is called synergism. The material added to increase the effectiveness of the primary chemical is called a synergist. The synergist may not necessarily be pesticidal by itself, but increases the effectiveness of the pesticide with which it is combined. The best-known example of the use of a synergist is the addition of piperonyl butoxide (PBO) to pyrethrum, pyrethrins, or some synthetic pyrethroids. Without the addition of PBO, flying insects may be knocked down by these insecticides, but will later recover and fly away.

2. To provide better control than that obtained from one pesticide. Applicators sometimes combine active pesticides to kill a pest that has not been effectively controlled by either chemical alone. Many combinations are quite effective, but in most cases it is not known if the improved control is a result of a synergistic action or an additive effect of the combined chemicals on different segments of the pest population. One should always check the label prior to verify the safety and legality of mixing pesticides.

3. To control different types of pests with a single application. Frequently, several types of pests need to be controlled at the same time. It is usually more economical to combine the pesticides needed and make a single application.

When two or more pesticides cannot be used in combination, they are said to be incompatible. Some pesticides are incompatible because they will not mix, others because even though they mix, they do not produce the desired results. Some combinations of chemicals result in mixtures that produce an effect which is the opposite of synergism. This effect is called antagonism and may result in chemical reactions which cause the formation of new compounds. In other cases incompatibility may result in separation of the pesticide from the water or other carrying agent. If one of these reactions occurs, one of the following may result:

l      Effectiveness of one or both compounds may be reduced.

l      Precipitation may occur and clog the screen and nozzles of application equipment.

l      Various types of phytotoxicity may occur.

l      Excessive residues may result.

l      Excessive runoff may occur.

Another less familiar, but extremely important, undesirable effect of combining certain pesticides is potentiation. Some of the organophosphorous pesticides potentiate (or activate) each other as far as animal toxicity is concerned. In some cases, the combination increases the toxicity of a compound that is normally of very low toxicity, so that the result is a compound that is highly toxic to people, other animals or plants.

Some pesticide labels indicate known compatibility problems. Some pesticide formulations are prepared for mixing with other materials and are registered for pre-mixes or for tank mixes. If this is true, it will be so indicated on the label.

Prior to tank mixing a combination of chemicals, refer to the compatibility charts that are available through your pesticide dealer or from various other sources. You should also remember that organophosphates, and some herbicides are more persistent in the environment and multiple applications several days apart may result in excessive residue, phytotoxicity, or livestock poisoning.


Vector control agencies frequently use herbicides to kill plants, or inhibit their growth when the plants either contribute to vector ion, or prevent technicians from being able to control vectors efficiently. Herbicides have been used in association with vector control operations for many years. In the early 1900s organic materials such as iron sulfate, copper nitrate, and sulfuric acid were used. In the 1940s, 2,4-D, a synthetic organic chemical, was developed as a selective herbicide. Since that time, hundreds of herbicides have been synthesized.

Vector control technicians should take special care in mixing and applying herbicides, and in learning the proper safety precautions needed for their use. Herbicide mixing, storage, and application can pose significant occupational health risks. Also, herbicides often present greater long term environmental risks than other pesticides, particularly to groundwater. Herbicides can create significant economic damage to croplands in agricultural areas.

Herbicides can be separated into organic or inorganic materials. Organic herbicides have a carbon based molecular structure and usually act by altering the normal growth pattern of the plant. Organic herbicides may be further divided into two major groups—the petroleum oils and the synthetic organic herbicides. The petroleum oils, refined from crude oil, can be used as either herbicides or insecticides. When formulated as herbicides, they usually are applied without dilution. Synthetic organic herbicides are artificially created in laboratories, and are made up of carbon, hydrogen, often nitrogen, and other elements. Included among the common synthetic organic herbicides are 2,4-D, and glyphosate.

Inorganic herbicides are often in the form of a salt, or contain a metal that is toxic to plants, often preventing proper uptake of water or inhibiting movement of material across cell walls. Inorganic herbicides are chemical compounds which do not have a carbon structure. The inorganics include such common materials as salt, copper sulfate, sulfuric acid, and sodium chlorate. These herbicides are extremely persistent and have caused serious soil pollution problems in some areas. Many are restricted materials.

Herbicides are applied for control of plants in a variety of ways, and the way that they are used affects the target plants differently. Although herbicides are formulated and used in ways similar to insecticides, because they are applied to control plant growth, their modes of action tend to be very different.

Chemical Groups Of Herbicides

Devising a simple classification scheme for herbicides based on their chemical group is difficult. To classify herbicides by chemical group requires at least 20 different categories, only a few of which will be mentioned here.


Phenoxy herbicides are used in both crop and non-crop areas for control of most annual and perennial broadleaf weeds. Some commonly used phenoxies include 2,4-D, MCPA, dichlorprop (2,4-DP), and 2,4-DB (Butoxone® or Butyrac®).

These herbicides are primarily plant growth regulators and affect the actively growing tissue of the plant. The ester formulations of the phenoxies are relatively volatile and turn into a gas during hot summer days. Care should be taken not to use them around susceptible broadleaf crops and ornamentals.


Triazines are used to control annual grasses and broadleaf weeds. Some commonly used triazines include atrazine (AAtrex®), simazine (Princep®), and metribuzin (Sencor® or Lexone®). The triazines are primarily used as non-selective pre-emergent herbicides. Prometon (Pramitol®) can be used as a pre-emergent or post-emergent herbicide on non-crop land. Triazines affect plants by inhibiting their ability to photosynthesize. Triazines have been found as contaminants in groundwater in the USA.


Thiocarbamates are used for control of annual grass seedlings and broadleaf weed seedlings. EPTC (Eptam®) is a commonly used thiocarbamate. They inhibit the meristematic growth of plants, such as root and shoot tips and are applied as a pre-plant, soil incorporated treatment.

Ureas and Uracils

Ureas and uracils have several similar uses and their modes of action have many features in common. Diuron (Karmex®) and tebuthiuron (Spike®) are commonly used ureas, and bromacil (Hyva®) is a widely used uracil. These compounds are primarily applied to soil as pre-emergence herbicides, but they also provide post-emergence control for certain plants. The ureas and uracils affect plants by inhibiting their ability to photosynthesize.


Benzoic acid herbicides are used in both crop and noncrop areas for control of numerous broadleaf weeds and annual grasses. Banvel is a commonly used member of this group. The benzoics are effective when applied either to the plant foliage or to the soil and work as plant growth regulators that affect the actively growing tissues of plants.


The acetanilide herbicides are used for control of many annual grasses and broadleaf weeds. Common acetanilides include alachlor (Lasso®), acetochlor (Harness® or Surpass®), metolachlor (Dual®), and pronamide (Kerb®). This group can be applied either pre-emergence or pre-planting in crop areas.


One of the most recently developed groups of herbicides, the sulfonylureas are highly active compounds used at extremely low rates. They are used to control many broadleaf plants in small grain crops, pastures, and noncrop areas. Commonly used sulfonylureas include chlorsulfuron (Glean® and Telar®), triasulfuron (Amber®), sulfometuron (Oust®), and metsulfuron (Ally® and Escor®t).

These compounds are usually applied as foliar treatments; however, they also control newly emerging broadleaf seedlings. Chlorsulfuron and sulfometuron are sulfonylureas that are more persistent in nature and will carry over into a second year when applied in high-pH soils. Extremely low residues from wind drift or in wind blown soil can cause significant losses in certain crops including corn, potatoes, and sugar beets.


A new and important herbicide family is the imidazolinones. It includes imazethapyr (Pursuit®), imazamethabenz (Assert®), and imazapyr (Arsenal®). This group act as biosynthesis inhibitors within the actively growing plant. These are broad spectrum herbicides and may be used against grass, broadleaf annuals, biennials, and perennials, vines, brush, and trees. Care must be taken around trees as root uptake from soil may result in death.

Herbicides By Use

Herbicides can also be classified by their pattern of use. Examples are selectivity (selective versus non-selective); whether they are applied to soil or plant foliage; and by how the herbicide moves within the plant (systemic versus non-systemic). The important classification for vector control use is where an herbicide may be used (terrestrial, aquatic, ditch bank, etc.).

Classification can be further complicated by the differences in effects of herbicides at different dosages. For example, some may act to help regulate plant growth and production of the seed when applied at a low rate, while at higher rates, they will kill plants. Other herbicides are selective for broadleaf plants at low application rates, but are non-selective when applied at higher rates.

Selective herbicides

Selective herbicides can be used to control certain plant species without injuring others. This characteristic can be used to control weeds while avoiding harm to desirable plants. Other selective herbicides may affect foliage of plants while leaving plant roots unaffected. Still other examples of selective products are herbicides that can be applied to soil or water before (pre-emergence) or after (post-emergence) the active growing season of plants. Some of these products can control growth of all plants, others affect only certain species. Perhaps the most common selective herbicides are those that affect broad-leaf plants, but not grasses. This selectivity is based almost entirely on the shape and size of the target foliage. In these cases, some herbicide formulations will wet broad-leaved plants but run off on grasses.

The application of selective herbicides may kill only the parts of the plant actually sprayed. In this case they are considered contact herbicides. Complete weed kill using contact herbicides requires well-directed and properly applied sprays. Complete coverage of the weed is a must.

Some herbicides are applied to the leaves of plants and absorbed into their stems and roots (translocated) causing the death of the entire plant. Because species of plants vary in their susceptibility to these systemic herbicides, these herbicides are selective to some degree.

Examples of selective herbicides are 2,4-D, dicamba, and picloram. Pre-emergent herbicides with selective properties are atrazine, trifluralin, and oryzalin.

Non-selective herbicides

Some herbicides are non-selective and must be used with extreme caution. They are used primarily in situations where complete removal of vegetation is desired, such as on transportation rights-of-ways. Some commonly used nonselective herbicides include glyphosate, imazapyr, bromacil and paraquat.

Non-selective herbicides can be applied to foliage as contact herbicides or as translocated herbicides. They may also be applied to soils where they kill nearly all plants growing there. Some soil-applied herbicides are available as fumigants.

Even selective herbicides can damage desirable plants if not used in strict compliance with their labels. In this regard, accurate dilution and calibration of equipment is critical. Since the distinction between weeds and non-weeds is so subjective, designing sound weed control strategies requires considerable knowledge and planning.

Extra care must be taken in applying non-selective pesticides. Do not apply them in sloping areas or where soil may be taken for use in a different location.

Contact Herbicides

Contact herbicides are applied directly to the plant, and may affect only the part of the plant contacted. This type herbicide can be used for preventing growth of brush and tree limbs into pathways. Bromoxynil, paraquat, and diquat are examples of contact herbicides.

Systemic Herbicides

Herbicides that move from one part of the plant to another such as from the leaf to the roots are called systemic. These formulations are particularly useful for control of deep rooted perennial vegetation.

Systemic herbicides may enter the plant through the roots or the leaves then move via the plant's vascular system to affect the entire plant. Commonly used systemic herbicides applied to plant foliage include MSMA, glyphosate, dichloprop, 2,4-D, dicamba, picloram, and chlorsulfuron.

Simazine, diuron, pronamide, and EPTC are examples of soil applied systemic herbicides. Some herbicides including triazines and thiocarbamates will translocate through both processes; however, they primarily work through root uptake which is the recommended method of application.

Plant Growth Regulators

Plant growth regulators (PGRs) are herbicides used for regulating or suppressing the growth of a plant and/or seeds. PGRs may be very selective, preventing growth or seed production of certain grasses, brush, or annual broadleaf plants without affecting non-target grasses. PGRs usually are applied directly to the foliage of the target plant.

Mefluidide (Embark®), sulfometuron (Oust®), and fosamine ammonium (Kernite®) are examples of selective PGR's.


There are many factors that affect the results of foliar herbicide applications. Some of these are:

l The age of the plants treated.

l The season of the year of applications.

l The life cycle stage of the plants (budding, flowering, overwintering, etc.).

l The type of life cycle of the plant (annual, biannual, or perennial).

l The degree of maturity of the plant.

l The time of day of the application.

l Weather conditions at the time of application.

l The life form of the plant treated (woody, succulent, broad-leaved, grassy, etc.).

l The morphology of the plant treated (cuticle thickness, presence of leaf hairs, etc.).


Soil Characteristics

The physical and chemical characteristics of the soil as well as the climatic conditions will determine the effectiveness of a soil applied herbicide, the persistence of the herbicide in the soil, and the potential movement of the herbicide through the soil (leachability).

Both soils and herbicides vary in their polarity of their constituent particles. Both can be negatively or positively charged or have a neutral charge. This will affect the movement of herbicides thorough soil and also the persistence of the herbicides applied.

Soil texture also will influence movement and persistence of herbicides. Light soil types (sands and sandy loams) tend to have large pore openings between the particles that allow water to move down through the soil profile rapidly. This will promote the more rapid movement of herbicides through these soils, but more rapid leaching, and thus lower persistence.

Herbicides applied to heavy soils (clay loams and clays) behave in the opposite manner. They move slowly through these soils and tend to remain longer. Medium texture soils (loams and silt loams) respond in an intermediate fashion to herbicides applied.

Before application of an herbicide to a soil a pesticide technician should know the characteristics of the soil to be treated. This can be determined by a soil test. A local county extension office or a Natural Resources Conservation Service (NRCS) Office can furnish information on collection of soil samples for testing. Herbicide labels have recommended rates of application based on the soil texture. The texture of soil basically is determined by the percentage of sand, silt, clay, and organic material in it. Generally, heavier soils require higher amounts of herbicide for plant control than lighter soils.

Other factors that can affect herbicide applications to soils include that amount of organic matter in the soil, the degree of compaction of the soil, the moisture content of the soil, and whether an underlying hard-pan is present below the soil surface.

Herbicide Persistence

Other factors beyond those already discussed that affect herbicide persistence include the rate of application, soil temperature, exposure to sunlight, microbial and chemical decomposition, solubility of an herbicide, and precipitation. These factors also affect how fast an herbicide will be degraded, and how deep it will leach through the soil.

factors to consider in planning HERBICIDE applications

When choosing an herbicide to use for weed control, consider the vegetation that is close to the application site. Take precautions to avoid movement of herbicides into surrounding areas, especially if valuable vegetation is nearby.

Herbicide applications should be avoided when it is raining, or in areas where overland water flow is likely to occur. Applications should likewise be avoided in heavy winds. The danger of drift in high wind conditions is especially high in open areas with little protection from wind.

Volatile herbicides such as the 2,4-D ester formulation and dicamba will vaporize during hot summer days, and danger of herbicide drift will be greater under these circumstances. Danger from volatilization will be included on the pesticide label.

General recommendations on herbicide labels include:

l      Mix and apply herbicide formulations having a low volatility.

l      Apply herbicides using the lowest practical spray pressures.

l      Apply herbicides using the largest practical spray droplet size.

l      Apply herbicides when wind speed is low.

l      Do not apply herbicides during a temperature inversion (when air is coolest at ground level, gets warmer up to a certain height, and gets cooler from that point up).


Management of vegetation in and around lakes and ponds, other than limited application of algaecides and control of emergent vegetation, is beyond the scope of activities covered by a vector control technician license.

Active management of rooted lake or pond vegetation and/or fisheries is very complicated and takes specialized knowledge of organic and inorganic chemistry, micro-biology, fisheries biology, and a variety of environmental disciplines.

Vector control programs and technicians may not engage in lake and pond management, including control of aquatic vegetation, without additional training and certification.

In summary, herbicide activities by public health pesticide applicators should only target vegetation that directly contributes to vector production or prevents technicians from efficiently controlling vectors.


Nearly half of all species of mammals are rodents, but only a few are of public health importance. Domestic rats and mice are the primary targets of pesticide applicators in urban and suburban situations, but rodents also are associated with rural diseases such as plague and hantavirus diseases. Control of these kinds of diseases by use of rodenticides is impractical except in unusual circumstances. Other rodents that may present problems for public health agencies are squirrels, gophers, hares, and rabbits. These problems may involve the roles of the vertebrates as disease reservoirs, or may involve activities of rodents such as ground squirrels or gophers in damaging water impoundment dikes used for mosquito control.

Rodenticides may be typically classified as first or second generation. There are a few instances where the product may not fall under either heading. A first generation rodenticide requires higher concentrations (usually between 0.005 and 0.1%) and consecutive intake over multiple days so a lethal dose may bio-accumulate. There are considered less toxic than second generation agents. Second generation Rodenticides are applied in lower concentrations in baits (usually in order 0.001-0.005%) and are lethal after a single ingestion of bait. Second generation rodenticides are also effective against rodents that are resistant to first generation anticoagulants. The second generation anticoagulants are sometimes referred to as "superwarfarins".

The most widely used group of rodenticides is the coumarins. The best-known member of this group is Warfarin, which derives its name from the Wisconsin Alumni Research Foundation, where it was originally developed. Coumarins affect all mammals, including humans, by serving as blood anticoagulants. Coumarins kill rodents over time by two related effects. They inhibit prothrombin formation, thus disrupting clotting, and they also damage capillaries, resulting in internal bleeding.

Warfarin was very successful as a rodenticide when it was first introduced because rodents did not exhibit bait shyness because of the extended period of action of the coumarins. However, physiological resistance to coumarins has been reported in rats in some areas. Some newer coumarins have been developed (e.g., brodifacoum, bromadiolone) that will kill rodents in 4–7 days after a single feeding. These materials can be used where rodents are encountered that are resistant to conventional anticoagulants.

Indandiones is another group of rodenticides. Although indandiones belong to a different chemical class than the coumarins, they also are anticoagulants. Diphacinone, pindone, and chlorophacione belong to this group. Pindone was the first anticoagulant developed, and requires daily feeding to cause rodent death. Diphacinone will cause death after a single feeding. Both of these chemicals may induce bait shyness in rodents. Chlorophacione will also result in rodent death after a single feeding, and unlike most of the anticoagulants, does not cause bait shyness.

Benzenamines are chemicals that are not anticoagulants. The only rodenticide in this group is bromethelin (Vengeance®, Fastrac®, Gladiator®). These materials are particularly effective against Norway rats, roof rats, and house mice. When used with baits, rodents stop all feeding after a single dose, and death occurs shortly thereafter.

Cholecalciferol (Vitamin D3) is the active ingredient in the rodenticides Quintox®, Rampage®, and Muritan®. These materials cause calcification of soft tissues, which can be fatal to rats after extended feeding. Cholcalciferol is used in baits and is tasteless. It is less toxic to humans than most rodenticides, but may poison small pets.

Some rodenticides are extremely dangerous to all mammals, including humans and their pets, and must be used with extreme care by applicators. Compound 1080, sodium fluoroacetate, is one of the most poisonous pesticides known. This material has gained considerable notoriety in connection with coyote control programs. Government predator control programs are now the only permitted use of Compound 1080.

Strychnine is a botanical rodenticide. It is highly toxic to all warm-blooded animals. It is somewhat commonly used for gopher and other underground pest control when pets and people are not present. Elsewhere it is rarely used because of its high toxicity and its relative poor performance as a rodenticide in comparison with anticoagulants.

Because people, rodents, and many domestic animals and pets are closely related genetically, rodenticides have a high potential for accidental poisoning. This danger can be minimized by use of protected bait boxes, and as always, usage in strict compliance with the pesticide label.


Using pesticide formulations are another way to classify pesticides. Pesticides are nearly always applied in formulations containing other materials. This is true by almost all types of pesticides. Unformulated pesticides are referred to as technical grade, and these are used only by toxicologists and other pesticide chemists or biologists conducting tests on pesticide resistance or susceptibility to target and non-target organisms. Technical grade pesticides are either formulated by manufacturers or by commercial pesticide distributors. All formulations sold in the USA must be labeled with complete instructions and restrictions for use. One formulation of a pesticide may be legally applied for a certain purpose, but a different formulation of the same pesticide may not be. That is why it is critical to completely read and understand the label for all pesticide formulations, especially if an applicator is using it for the first time.

Formulation is done to improve safety, ease of handling, storage, ease of use, and effectiveness of pesticides.

Formulations are nearly always the form in which pesticides are obtained by vector control specialists, and it is the formulation that must be registered, have an EPA registration number, a label, and a Material Safety Data Sheet. The formulation of any pesticide is identified by a letter or letter combination on the label.

Formulations may undergo a final dilution with water or other diluent after being added to a spray tank or similar device. This is not considered formulation, and this final form is usually called the tank mix.

Some of the most commonly used formulations are:

Emulsifiable Concentrates (EC)

These chemicals consist of concentrated oil solutions of technical grade pesticides combined with an emulsifier added to permit further mixing with water. Emulsifiers are detergent-like materials that allow the suspension of very small oil droplets in water to form an emulsion. Emulsifiable concentrates are used widely in vector control operations, with final water dilutions typically being made in spray tanks. Tank mixes are usually milky in appearance. ECs are losing popularity somewhat with the rise in costs of petroleum products, and new formulations using plant-derived oils are being sought.

Wettable Powders (WP or W)

These dispersible powders are finely ground, dry powders consisting of active pesticide ingredients mixed with other ingredients to aid in mixing and dispersion. Wettable powders are intended for mixture with a liquid, usually water, for application by spray equipment. They are generally mixed with water to form a slurry before being added to the spray tank. In the tank they require continual agitation. WPs can be used for most pest problems and in most spray equipment. Bti is available as a WP. WPs are harder on equipment than some other formulations, and can cause rapid wear on pumps, gaskets, and spray nozzles.

Soluble Powder (SP)

These powders are similar to wettable powders, except that the active ingredient, as well as the diluent and all formulating ingredients are completely soluble in water. Uses of soluble powders are similar to those of wettable powders.

Dusts (D)

Pesticides formulated as dusts are finely ground mixtures of active ingredient and a carrier material. Dust formulations are intended for direct application without further mixing. Dusts are never used where drift is a potential problem. For this reason, herbicides are not formulated as dusts. In vector control, dusts are frequently used to control fleas and other ectoparasites on pets. They are also applied to rodent burrows and bait stations to control fleas in plague control operations.

Granules (G)

In a granulated formulation, the active ingredient is mixed with various inert clays to form particles of various sizes. Granules used in vector control operations are usually from 20 to 80 mesh in size. Granular formulations are intended for direct application without further dilution. Granular formulations require specialized dispersal equipment, and may be applied from the air or on the ground. They may be used with small hand-cranked units, or simply scattered by hand (with appropriate personal protection). Granular applications of pesticides are especially useful in treating mosquito larvae in locations where heavy vegetation would otherwise prevent the insecticide from reaching the water. They are also favored in situations where drift would otherwise be a problem.


Fumigants are volatile chemicals stored as liquids under pressure, or incorporated into a solid form with clay which releases toxic gas when combined with water vapor. The only vector-related uses of fumigants are for treating rodents and their associated ectoparasites underground. Fumigants are used for structural pest control and may include species that are considered vectors (e.g., cockroaches). Mothballs are fumigants.


Baits contain active ingredients that are mixed with a pest food or attractant. Principal uses include control of household pests such as ants, mice, rats, roaches, and flies; they are used outdoors to control birds, ants, slugs, snails, and agricultural pests such as crickets and grasshoppers.

Aerosols (A)

Aerosols, or "bug bombs" are pressurized cans which contain a small amount of pesticide that is driven through a small nozzle under pressure from an inert gas (called a propellant). Aerosols are often used in households. Organisms that may be killed using aerosols include weeds, flies, for a variety of greenhouse pests, and in structural pest control. The use of aerosols peaked during the 1990s before concerns for propellants consisting of chlorofluorocarbons were linked to damage to the ozone layer. Since then, aerosol can uses of all kinds have dropped significantly, although substitute propellants are continually being tested as replacements for chlorofluorocarbons.

Flowables (F or L)

A flowable liquid usually is mixed with water for use in a sprayer. It forms a suspension in water which requires continual agitation. Principal uses are similar to those of emulsifiable concentrates.

Water-Soluble Concentrate (WS)

These liquid formulations form true solutions in water and require no agitation once mixed. They are used in the same way as emulsifiable concentrates.

Ultra Low Volume Concentrates (ULV)

Ultra low volume concentrates (ULV) are sold as technical product in its original liquid form, or solid product dissolved in a small amount of solvent. They are applied using special aerial or ground equipment that produces a fine spray at very low application rates. Their main use in public health is as mosquito adulticides. The underlying principle of ULV is that an extremely small droplet of pesticide (~10-30 microns) is supposed to lethally strike a mosquito. Droplets that are larger are considered inefficient, wasteful, and can have undesirable environmental effects. ULV applications, when done correctly, are very effective and very safe to people and other non-target organisms.

Fogging Concentrates

Fogging concentrates combine a pesticide with a solvent, with the type of solvent depending upon the type of fogging to be done. These are formulations sold only for public health use to control flying insects such as flies and mosquitoes. These formulations are applied using special truck-mounted machines called foggers. Foggers are of two types: thermal foggers use flash heating of an oil solvent to produce a visible plume of vapor or smoke, and cold (ambient) foggers atomize a jet of liquid in a venturi tube under pressure from a high-velocity air stream. Cold foggers can use insecticides combined with oil, water, or emulsifying agents.

Slow Release or Controlled Release Formulations

Some insecticides can be encased (encapsulated) in an inert material for a controlled release, resulting in decreased hazard and increased likelihood of the active ingredient reaching the target organism. Sustained-release mosquito larvicides are based on this principle. Previous uses of this general method using resin strips impregnated with dichlorvos (DDVP), a volatile organophosphate insecticide, for fly and moth control, are no longer approved for use. The most successful vector control products are those that provide slow release of bio-pesticides, although some critics of this approach claim that this encourages physiological resistance on the part of the target pests.

Other Formulations

There are other formulations that could be mentioned here, some of them very important, such as the formulations for impregnating clothing, bed nets, and curtains in tropical areas of the world for malaria control. There are other formulations, such as oil solutions and soluble pellets that are found mostly on hardware store shelves for home and garden use. Novel formulations are being evaluated continually, and some of these will probably eventually be adopted for vector control use.

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