Chapter 9


In addition to the safety problems associated with the preparation and application of pesticides, there are several important problems related to pesticide use that should be understood by every applicator. These problems include pesticide drift, pesticide residues, phytotoxicity, destruction of beneficial species of animals and plants, resistance of pests to pesticides, and environmental pollution. There are many ways in which these undesirable effects can be reduced or eliminated. Each depends upon knowledge of the proper handling and use of pesticides, the components of the environment susceptible to contamination, the pesticides most likely to cause contamination, and preventive measures.


Except for ultra low volume (ULV) spraying to control adult mosquitoes, drift is an undesirable side effect associated with both aerial and ground pesticide applications. Spray drift is defined as airborne particles produced during application of a pesticide moving outside the intended treatment area. The severity of drift depends on the physical form of the material, the method of application, weather conditions, and to a lesser degree, movement of the substrate to which the product was applied (both soil and water).

Drift is a desirable and necessary part of an ULV application. In fact, pesticide labels specify that ULV applications must be done during weather conditions that favor pesticide drift (temperature inversion or lateral winds below 10 MPH). In a ULV application, the longer the effective drift of the product, the greater the efficacy.

For other pesticide applications, the formulation of the pesticide is a significant factor. Dusts are most likely to drift and granules least likely. High pressure sprayers are more likely to produce fine droplets that are more likely to drift than low pressure sprays.

A variety of other factors can affect the amount of drift. When spraying liquid formulations of pesticides, the nozzle and pump pressure have the greatest influence on subsequent drift. Improper or worn nozzles or excessive pressures cause the spray to be produced in a form which drifts readily.

The rate at which a drop of liquid falls through the air depends upon the size of the droplet. Very small droplets fall very slowly. These small droplets can drift for miles before they reach the ground. The method and amount of material applied also influences the hazard of pesticide drift. Small amounts applied by hand from the ground are rarely involved in drift problems. Spray from ground air blast sprayers is highly subject to drift. Aerial applications of large quantities of pesticides always present the possibility of significant drift.

A second form of drift occurs when pesticides evaporate during and after application. Certain herbicide formulations may volatilize and cause damage to plants miles from the point of application. A few herbicide formulations may drift as a result of evaporation following application; use of these may be restricted in many areas.

Drift should be avoided because:

l      It wastes resources, including pesticides, fuel, and technician time.

l      It spreads pesticides into the surrounding environment where they may become illegal residues on food crops, cause health problems, damage wildlife, and have other undesirable effects.

l      It can damage sensitive crops.

l      It has been the subject of many damage claims for crop losses. Drift can be a severe problem and should be taken into consideration before making any type of pesticide application

Whenever a pesticide chemical is applied, some of the chemical becomes a deposit on or in the treated crop, animal or object. The pesticide may remain in its original chemical form or it may be altered chemically by weathering, metabolic degradation or other processes. In any case, the quantity of material remaining is called a residue. Residues may result from direct application, from drift from nearby fields, from uptake from contaminated soil, or from other sources. In some situations residues are desirable and produce prolonged effective pest control, as in the control of certain public health and structural pests. In other situations, however, residues represent a source of unwanted and illegal contamination — for example, when residues exceed legally determined limits on food or feed crops at harvest.

Pesticide Residues

Pesticide residues are generally meant to include pesticides that are detectible in or on places other than their intended target. Fresh water reservoirs, stream bed sediments, and harvested food would be examples of places that would be tested for pesticide residues. Needless to say, if high levels of residues were found to occur in such situations, few would consider the test results to be a good thing. Pesticide residues are usually measured and tolerances expressed in parts per million (ppm) to parts per billion (ppb) on a weight basis. One ppm is one milligram in a kilogram, or one ounce of salt in 62,500 pounds of sugar, or one pound of pesticide in one million pounds of raw agricultural commodity. In some instances modern analytical chemistry techniques can test residue levels below one ppb.

The residue levels allowed on food crops at harvest are legally set by the federal and state regulatory agencies and are called tolerances. Tolerances are simply the maximum amounts of pesticide permitted to be present on or in raw agricultural commodities. These tolerances represent levels of pesticide residues which scientists have determined may safely remain on the food crop without injury to the consumer. Tolerances vary according to the pesticide and the crop.

The California Department of Pesticide Regulation regularly inspects and analyzes samples of fruits, vegetables, feeds, dairy products, meat and other produce to be certain pesticide residues do not exceed legal tolerances.

When pesticide tolerances are found to be exceed legal tolerances, the agricultural commodities involved may be seized and destroyed. Ordinarily, such situations would arise from the application of agricultural pesticides on crops, but it could happen even where pesticide applications are not specifically targeted at a crop pest, such as the application of pesticides on rice fields for mosquito control.

Before allowing the use of a pesticide on food crops, EPA sets a tolerance. If no tolerance has been set for that pesticide on that crop, the pesticide cannot be legally applied on the crop. Some pesticides may be considered "safe" by EPA, and they would be exempted from a tolerance.


Persistence of Compounds

Most organochlorine pesticides (e.g., DDT, chlordane) are very persistent. Most of the organophosphates (e.g., parathion, malathion) and pyrethroids are much less persistent. Pyrethrins, and carbamate pesticides are nonpersistent. Some factors that influence the persistence of a chemical and the possibility that residues may remain are:

l      The amount of chemical applied

l      The formulation

l      The pH (acidity or alkalinity) of the water diluent and of the target tissue, soil, or water.

l      The nature of the surface to which it is applied

l      Exposure to weathering from wind, rain, etc.

l      Chemical breakdown from high temperatures and humidity

l      Photochemical reactions from sunlight

l      Biological reactions.

If public health pesticides are applied properly and in accordance with label restrictions for applications around food crops residues on or in the crop should never be a problem.

Certified Organic Crops and Farms

Organic farming is a form of agriculture which does not permit the use of synthetic fertilizers and pesticides, plant growth regulators, livestock feed additives, and genetically modified organisms. As far as possible, organic farmers rely on crop rotation, green manure, compost, biological pest control, and mechanical cultivation to maintain soil productivity and control pests.

Organic agricultural methods are internationally regulated and legally enforced by many nations, based in large part on the standards set by the International Federation of Organic Agriculture Movements, an international umbrella organization for organic organizations established in 1972. The United States Department of Agriculture also tracks organic policies and procedures nationally.

Vector control technicians working near these farms need work closely with the landowner to prevent vectors from coming from the property, and to avoid jeopardizing the organic status of the crop. If a pesticide excluded for use on organically produced commodities is accidentally applied to an organic crop, the crop may no longer qualify to be sold as organic. If this occurs in connection with a vector control operation, the producer can pursue a settlement from the vector control program for his loss. While that particular harvested crop may no longer be considered organic, the farm will still qualify as "certified organic". If there is repeated contamination by any party, the farm will lose its organic certification and must wait at least 3 years before it may apply for organic certification again.

With the growth of organic farming in California, vector control operations will face increasing challenges in applying pesticides in the vicinity of organic farms. This will involve developing innovative methods of preventing vector problems from occurring on and adjacent to organic farms while respecting the landowner's desire to maintain organic practices. The increasing acreage of organic rice in the Central Valley is an example of a particularly difficult challenge.


Phytotoxicity is the injury or death of a plant due to exposure to a chemical. Plants may be injured or killed by various kinds of chemicals, including salts, fertilizers, or pesticides. Sometimes, plant injury is intentional, as when an herbicide is applied to a weed. In other cases, the plant injury is an accidental side effect of pesticide use. Phytotoxicity can effect any part of a plant, including roots, stems, foliage, blossoms, or fruit.

The degree of phytotoxicity caused by pesticides may vary in response to a number of factors. Some toxicants (active ingredients) are particularly damaging to plants. Other components of the pesticide mixture, such as the diluent, may cause plant damage. The plants themselves may vary in susceptibility to injury by various chemicals. The phytotoxic reaction may vary with the species of plant, with the age of the plant, or with the weather at the time of exposure.

The manner in which a chemical is applied may determine whether or not injury will occur. For example, excessive pump pressures while spraying may cause physical injury to the plant or drive the chemical into the plant tissues. Excessive concentrations of a chemical may cause plant damage. Certain combinations of pesticides may cause phytotoxic reactions. Some pest problems require two or more chemicals combined in the tank mix. Mixtures are used commonly with great success. However, some chemicals are not compatible with others and one of the results of this incompatibility may by severe phytotoxicity. Many herbicide labels list other products with which they may be combined.


The efficient production of many crops, including fruits, vegetables, forage, and seeds, would not be possible without the activities of honey bees and other pollinating insects. In California, the beekeeping industry maintains millions of honey bee colonies. Each year honey bees pollinate billions of dollars worth of crops in the state.

Over the past 100 years or so the honey bee industry has sustained serious losses from pesticide applications. Recently; pesticide poisoning has come under suspicion for reductions in honey bees in the USA. Some agricultural pesticides used currently are known to pose a significant hazard to bees. Vector control applications are ordinarily done in a manner that minimizes risk to bees and other beneficial insects.

In addition to pesticide exposure, honey bee colonies are at risk from a variety of parasites, pathogens, marauding mammals, and other factors. To protect these valuable insects from losses due to pesticide poisoning it is necessary to know where colonies are located before starting a pesticide application, and to protect them in some way. Moving colonies to areas far from the possibility of drift and avoiding the use of pesticides known to be especially toxic to bees are two ways to minimize damage. If public health pesticide applications are planned for areas known to be close to bee colonies, it is also a sound policy to warn hive owners in advance of the applications so they have an opportunity to protect them.

There are other pollinating insects that may suffer damage from pesticide applications, such as alkali bees and other wild bees. Wild bees are not in hives, but are present in a variety of nest types. This complicates their protection from pesticides.

Other insects are beneficial because they prey on or parasitize pests. There have been many studies on the effects on non-target organisms of pesticides in a variety of settings. Your local extension specialist can provide you with copies of reports of this kind of research.

By definition, pesticides that harm non-target organism populations significantly are non-selective. If use of a non-selective pesticide is considered essential, it must be justified based on the relative benefits balanced against the relative harm. In the case of public health pesticides, the threat to human health is a necessary consideration.

The ideal pesticide would be selectively nontoxic to bees and other beneficial organisms, while toxic to a specific pest. Few products available for adult mosquito control meet this ideal, but several larval mosquito control products and many herbicides are selective. For adult mosquito control and other pesticide applications, the best compromise must be found.


For vector control technicians, protecting domestic bees is primarily a concern when doing ULV adult mosquito control. The pesticides most commonly used for these applications (pyrethrins and pyrethroids) are toxic to bees. However, they are applied in minute quantities (often less than 1 ounce per acre of total volume of material) during the evening or early morning when bees are inactive. Taking the reasonable precaution of turning off the sprayer while passing the hives should be adequate to prevent any mortality in the bees from the product.

Bees are readily poisoned by organophosphates and many agricultural pesticides. When a pesticide known to be harmful to bees is used near bee hives or to any cropland where honey bees are working, special procedures must be followed. Under California law beekeepers may request notification of intended pesticide usages, and the applicator is required to provide such notice.

Vector control agencies that have signed the Cooperative Agreement with the California Department of Public Health are exempted from regulations that require notification of pesticide applications. Despite this regulatory exemption, vector control agencies should work closely with local beekeepers to protect their bees. Vector control technicians and agencies should also remember that they are not exempt from potential damage claims for lost bees or hives, nor are they protected from landowners claiming indirect damage to crops. It is thus only reasonable for vector control agencies to keep bee keepers apprised of ULV mosquito control applications so keepers can move or cover hives if they choose.

In some areas centralized private organizations operate a beekeeper notification program. Bee notification maps are maintained and each day copies of beekeepers' requests for notification from the County Agricultural Commissioner are received. Then interested bee keepers are notified by a single telephone call of all intended applications within one mile of their hives.


Pesticide resistance is the ability of pests to avoid the lethal effects of pesticides. Certain populations of pests use one or more different physiological or behavioral defense mechanisms to withstand doses of pesticides that previously were lethal to the pests. This can happen through spontaneous mutations in populations resulting in genes that confer pesticide resistance, or because a small proportion of the population carries a gene for pesticide resistance naturally. In either case, resistance develops gradually to the point where pesticide applications begin to fail after repeated exposure to the same pesticide. This is because the parts of the population that carry the gene for susceptibility are killed off, and soon, a disproportionate segment of the population carrying the gene for resistance predominates. This can be an unintended effect of using pesticides. Resistance in numerous pests of public health importance has occurred to a variety of pesticides. For mosquitoes and flies, resistance to organochlorines and organophosphates has been particularly common.

Selective pressure is the repeated exposure of a population of pests to treatments of the same pesticide over time resulting in a change in the genetic makeup of that population. In this case, the population is selected to favor resistant genes at the expense of susceptible genes, and the population becomes resistant to that pesticide. Because of the nature of population genetics, the population never becomes completely resistant, but the frequency of individuals have susceptible genes becomes very small.

Knowing the mechanisms of development of pesticide resistance is important to developing strategies to avoid creation of resistance in pest populations. The basic principle is the preservation of susceptible genes in pest populations, and the endeavor to do this is named pesticide resistance management.

Usually, when a pest population becomes resistant to one pesticide it can still be controlled by other pesticides, especially pesticides in a different family of chemicals. Occasionally, resistance to pesticides other than the pesticide responsible for resistance may occur. This is called cross-resistance. Its occurrence is usually seen among chemically related pesticides where the mode of action is identical or very similar.

Recognizing Resistance

Not all pest control failures are the result of resistance. Improper pest control practices may be at fault. However, if the material was timed and applied properly at the recommended rate and no other important factors (such as unfavorable weather) have interfered with the pesticide application, resistance should be considered.

Early signs of resistance may sometimes be recognized in the field. These include increasing difficulty in controlling a pest, increasing numbers of formerly minor pests, and increasing trouble with insect-transmitted disease. Developing resistance can be very subtle and may go unnoticed for a time; it may appear in certain locations or breeding sites. Suspected resistance should be reported to your supervisor immediately since early detection may make it possible to delay resistance by the application of counter measures.

Resisistance management

Based on the genetic principles of development of pesticide resistance in pests, a number of principles have evolved over the years that when implemented can either delay resistance, or avoid it entirely. Some of these principles are:

l      Avoid under-dosing in pesticide applications. If this is done repeatedly it encourages survival of individual pests carrying genes for resistance, especially when the effects of the gene are not absolute (protects only partially).

l      Do not always treat a given population with the same pesticide. Switch to other products periodically. This is called pesticide rotation.

l      Test populations of vectors for evidence of resistance, and when it is detected switch to alternate pesticides.

l      Avoid slow-release applications where pest populations are exposed for long periods of time to sub-lethal doses of one pesticide.

l      Combine pesticide applications with other forms of pest management such as biological control, habitat alteration, and use of biorational pesticides. The use of biorational pesticides is not a guarantee that resistance to these products will not occur, but resistance to biorational pesticides have been far less common than to conventional pesticides.


Unwanted pesticide chemicals in air, soil, water and vegetation are a form of environmental pollution. Accumulation and storage of pesticides in plants and animals from long lasting insecticides such as DDT was once a form of environmental pollution. This bio-storage and bio-accumulation of pesticides does not occur with modern insecticides that degrade rapidly under ambient conditions and are metabolized quickly by living organisms.

As the population of the world increases, our environment is becoming more and more polluted. Pesticide use contributes to this condition, especially in large-scale agricultural operations. Care in the use and disposal of pesticides will aid in reducing environmental pollution. In order to accomplish this, people who use pesticides should have an understanding of the fate of these chemicals after they have been released into the environment.

Whenever pesticides are used some proportion of the amount applied fails to reach the target. Furthermore, the material eventually reaches the general environment in either its original form or in the form of a breakdown product. Each pesticide has its own unique chemical and physical properties and therefore its own unique behavior in the environment. However, when the effects of pesticides on the environment are considered as a whole, it is possible to make a few generalizations.

Atmospheric Pollutants

Pesticides may enter the atmosphere by several methods. They may be blown away with soil particles in cultivation or as smoke from burning materials. A major source of atmospheric pesticide pollution is from improper application of sprays and dusts. Spraying during windy periods or with the wrong or improperly maintained equipment are major contributors to this problem. Once in the atmosphere, pesticides are either degraded to other compounds or trapped in rain, fog or dust. They eventually fall to earth.

Soil Pollutants

Very few persistent pesticides are used in vector control operations any more. Because some weed control projects may involve the use of long-lasting herbicides, public health pesticide applicators should be familiar with the ways persistent pesticides can cause pollution of soils.

Many pesticides are applied in areas with vegetation of some kind. If persistent pesticides are applied to plants, some of the material reaching the ground during the application and some of the material contained in harvest residues may be incorporated into the soil. Long-lasting pesticides also may be applied directly to the soil to control insects, weeds, fungi and nematodes. In this case most of the material reaches the ground immediately and in unchanged form; then it may be incorporated into the soil through agricultural operations.

When excess or repeated applications of inorganic or very persistent organic pesticides are made, soil residues can build up until they become a severe problem. Residues of hundreds of pounds per acre have accumulated in some soils. Pesticides in the soil may cause illegal residues in root crops or they may be translocated into the tops of plants. They may also leach into nearby surface or groundwater supplies or they may cause undesired phytotoxicity

Once in the soil, organic pesticides may be rapidly broken down by natural processes or they may remain unchanged for years. Pesticides in soils break down through chemical reactions which depend on the structure of the soil, its moisture content, its pH, salinity and other factors. Many organic pesticides are broken down by the action of microbes in the soil. Microbial decomposition depends on the temperature, moisture, and organic matter in the soil, as well as on the chemical nature of the pesticide itself. Cultural practices can have an important effect on the longevity of pesticides in soils. Proper cultivation and irrigation practices can speed removal of unwanted pesticides from soil. Pesticide labels often contain useful information on prevention of soil pollution problems associated with the material.

Water Pollutants

Pesticides occur in water as intentional and unintentional additives. Intentional applications are made to control aquatic organisms including mosquitoes and gnats, algae, snails, weeds and "trash" fish. Use of pesticides in water presents special hazards to plants growing in or irrigated with the water, to fish and other animals living in the water,. Specific hazards include non-target toxicity and biological oxygen demand created by decaying vegetation. Fish, animals and people who might drink or bathe in the water may be at risk. Therefore, it is extremely important that those planning to apply pesticides directly to bodies of water be completely familiar with and follow label directions pertaining to any material being used in or around water - including posting notice of the application when required by the product label.

Unintentional water pollution has resulted from drift, drainage from treated soils, accidental applications, unintentional spills, and from sewage effluents. Pesticides in the atmosphere are washed down by rain and reach streams, lakes, and the ocean. Other pesticides reach the surface waters through runoff from soils and from industrial waste disposal. At times pesticides have been accidentally sprayed onto bodies of water, where they cause extensive pollution. Such contamination is usually preventable. Pesticide spills are usually of limited local importance and, because of their emergency nature, are frequently dealt with very effectively. Sewage effluents are generally associated with manufacturing processes and are of little direct concern to the pesticide applicator.

Biotic Pollution

The direct effects of pesticides on wildlife depend on the kind and formulation of the pesticide, the target pest species, and on the species of wildlife exposed. For example, pyrethrins are highly toxic to fish, but virtually non-toxic to mammals or birds. Direct effects on wildlife also depend on the exposure of the animals to the pesticide; pesticides with long residual actions may cause wildlife losses for extended periods. On the other hand, pesticides with short residual effects may cause large losses but only for a short time.

The largest potential for indirect effects to wildlife is from vegetative changes that can improve or degrade habitat for many species of wildlife. Changes in vegetation or insect populations can significantly alter the food resources available to wildlife species.

Recently, residual accumulations of PBO, a synergist commonly used to increase the effectiveness of pyrethrins and pyrethroids, has been detected in stream bottom sediments. Although non-toxic itself, PBO may have the potential to make other pesticide residues in stream bottoms more toxic to aquatic organisms. This has raised the question of a potential role for PBO as a harmful stream pollutant.

Minimizing Environmental Effects

While each of us wants to protect the environment, we need to remember that pesticides are necessary tools in managing vector and pest populations. Each pesticide label contains a warning of known environmental effects of the active ingredient. Careful attention to label directions will aid you in the selecting and using of all pesticides in ways that minimize adverse environmental effects.

Remember — apply pesticides:

l      Only to identified pests

l      Only when necessary

l      Only where they are needed

l      Always at rates permitted by the label

l      Only in situations allowed by the label

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