lecture 9 pesticides: regulations, chemistry, toxicology i...

ES/RP 532 Applied Environmental Toxicology of 30

ESRP532 Lecture 9.doc Fall 2004

Lecture 9 Pesticides: Regulations, Chemistry, Toxicology

September 29, 2004

I. Regulatory Definition of PesticidesA. Pesticide (defined by law--the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA),

passed 1947, Title 40 CFR (Code of Federal Regulations, Section 162.3):1. Any substance or mixture of substances intended for preventing, destroying, repelling,

or mitigating any pest (insect, rodent, plant or animal life or viruses, bacteria, or othermicroorganisms, except viruses, bacteria, or other microorganisms, on or in living manor other animals, which the Administrator declares to be a pest); in other words, a pestis:a. Any insect, rodent, nematode, fungus, weed, orb. any other form of terrestrial plant or animal life or virus, bacteria or other micro-

organism (except viruses, bacteria, or other micro-organisms on or in living man orother living animals), which the [EPA] administrator declares to be a pest.

2. Any substance or mixture of substances intended for use as a plant regulator, defoliant,or desiccant;

3. Definition in (1), therefore does include disinfectants;a. Indeed, if you look on a can of Lysol, you will see the active ingredient (the older

cans contained orthophenylphenol) with an EPA Registration No. (which mustaccompany all approved pesticide active ingredients).

4. The major groups thus include herbicides, insecticides (also acaricides & nematicides),fungicides, plant growth regulators, rodenticides, molluscicides, piscicides, disinfectants.

B. Fertilizer1. Macro and trace nutrients added to soil, or in some cases to plant foliage, to maximize

the production of plants (crop, ornamentals, lawns, etc.)2. Not regulated by EPA with regards to labeling requirements or application rates;

requires no permits or licenses to make an application;a. However, consequences of use of any fertilizer as it pertains to effects on water

quality will be under the jurisdiction of the EPA through both the Safe DrinkingWater Act and the Clean Water Act.

3. States may regulate labeling of fertilizers to protect against adulteration, which caninclude putting in too much or too little of a nutrient;a. Canada regulates the heavy metal content of fertilizers in addition to the essential

elements.b. WA State now has regulations for maximum permissible heavy metal content,

modeled after the Canadian standards

II. Use of PesticidesA. Pesticides have certain advantages in crop protection that make their use very convenient,

efficient, and cost-effective (i.e., the number one tool in crop protection) (from Metcalf, R.L., and W. H. Luckmann. 1975. Introduction to insect pest management. John Wiley &Sons, N.Y.)1. For most cropping systems and in some cases insect-vectored diseases, pesticides are

the only practical technology (i.e., other technologies are not available, unproved, or donot work efficiently)

2. Pesticides have rapid curative action in preventing loss of crop yield or protectinghuman/animal healtha. Thus, they can be used in an emergency;b. Furthermore modern pesticides are biodegradable; thus, they disappear from the

agroecosystem (at least the residues become toxicologically insignificant--a point ofsome controversy)



3. Pesticides offer a wide range of properties, uses, and methods of application to pestsituationsa. Many different types of products (chemistry); some selective, some broad spectrum;b. Many modes of application and formulations available

4. Economic return-cost ratio for pesticide use is generally favorablea. Depending on the crop, this ratio can range from $4-$29 returned for every $1

spent.b. The economic return-cost ratio goes down when

1. Price of crop decreases but pesticide cost is fixed;2. A product is used and pest populations are not at a level that will cause economic

damage3. Development costs for a new product are high

B. Costs of Pesticide Development and Registration1. R&D costs have been estimated to be as high as $70 million. To acquire a registration

probably costs over $100 million.2. Reregistration Fee estimated at $150,000 per product (i.e., not active ingredient but

formulated product); does not include cost of any new studies that may be required3. Product maintenance fee estimated at $35,0004. Economics of pesticide development and marketing (Leng, M. L. 1991. Consequences

of re-registration on existing pesticides. pp. 27-44 in “Regulation of Agrochemicals:A Driving Force in their Evolution, G. J. Marco, R. M. Hollingworth, and J. R. Plimmer,eds., Am. Chem. Soc., Washington, D.C.)a. It may take 15 years from discovery for a pesticide product to attain a positive cash

flow; by year 20 after discovery, the patent would have expired (the patent is usuallyobtained around 3 years after discovery of a new product)

C. Some advocacy groups repeatedly proclaim that pesticide use is on a treadmill of ever risinguse, however,...1. Generalizations are dangerous!!

a. There is a tendency to equate the amount of pesticide use with hazard, but these arenot evenly remotely related if one considers basic toxicological and risk assessmentprinciples.1. For example, the most heavily used (in terms of pounds) pesticide in orchard

crops are petroleum oils, largely used as a dormant spray for insect control.Many types of petroleum oils are certified for organic agriculture, and they areeven used for post bloom control of pests in organic orchards.a. Another example is the use of sulfur as a fungicide; on a poundage basis it

is one of the most heavily used fungicides, but certain formulations are alsocertified for organic agriculture.

2. Amounts of pesticides used will vary depending on acreage cropped, weather,and pest outbreaks.

3. For trends in pesticide use over the last decade, specifically for field crops (corn,cotton, potatoes, rice, soybeans, and wheat) and various fruit and vegetable crops,see the NASS (National Agricultural Statistical Service) web site(http://usda.mannlib.cornell.edu/reports/nassr/other/pcu-bb/). (see Table 1 forselected statistics)

Table 1. Change in acreage and pesticide use for corn, potato, and apple (1991 vs. 2002/2003)Corn * 1991 2002 ChangeAcres Planted X 106 76.2 79.1 2.9% Acres Treated with Herbicides 94 89 -5 Pounds X 106 211 147 -64% Acres Treated with Insecticides 30 24 -1



Pounds X 106 23.3 6.1 -17.2

Potato 1991 1999 ChangeAcres Planted X 106 1.20 1.20 0.0% Acres Treated with Herbicides 79 93 14 Pounds X 106 2.2 2.2 0.0% Acres Treated with Insecticides 91 93 2 Pounds X 1000 3.1 2.8 -0.3% Acres Treated with Fungicide 69 95 26 Pounds x 106 2.7 7.8 5.1% Acres Treated with Others (Fumigants,Defoliants) 45 45 0 Pounds x 106 39 108 69

Apple 1991 2003 ChangeAcres Planted X 106 0.35 0.31 -0.04% Acres Treated with Herbicides 42 42 0 Pounds X 106 0.31 0.39 0.08% Acres Treated with Insecticides 99 94 -6 Pounds X 1000 12.7 9.3 -3.4% Acres Treated with Fungicide 83 90 7 Pounds x 106 4.7 4.9 0.2% Acres Treated with Others (Plant GrowthRegulators, Pheromones) 57 20 -37 Pounds x 106 0.07 0.13 0.06* Data adjusted by proportion of total corn acreage surveyed in 2002, which was 65%.

III. The Down Side of Pesticide UseA. Despite their benefits, pesticides have limitations that make their indiscriminate use unwise

1. Noted Problems With Pesticides (note: the first three listings are considered by manyscientists to be the most important problems; the change in pesticide chemistry over thelast two decades has demoted the importance of the last two listings, although these arestill very controversial with regard to their significance)a. Worker exposure and poisoning (especially acute toxicity)

1. Pesticide Incident Reporting and Tracking panel in WA Stateb. Development of pest resistance; limits the available control technologiesc. Reduction of natural enemies and resurgence of pest problems; elevation of

importance of damage by “secondary” pestsd. Adverse effects on environmental health

1. Fish/bird kills2. Bioaccumulation in food chain3. Endocrine disruptive effects

e. Human health (general population)1. Chronic toxicity (cancer)2. Accidental or intentional acute poisoning

B. Calculating An Economic Cost (Environmental and Social Costs) to Pesticide Use



1. Total estimated environmental and social costs from pesticide in the U.S. (according toPimentel, D., et al. 1993. Assessment of environmental and economic impacts ofpesticide use. pp. 47-84 in “The Pesticide Question: Environment, Economics, andEthics., Pimentel D. and H. Lehman, ed., Chapman & Hall, NY.) (Table 2).

Table 2. Estimated health, environmental, and social costs of pesticide use.Costs Million $ per YearPublic health impacts 787Domestic animals deaths and contamination 30Loss of natural enemies 520Cost of pesticide resistance 1400Honeybee and pollination losses 320Crop losses 942fishery losses 24Bird losses 2100Groundwater contamination 1800Government regulations to prevent damage 200TOTAL 8123

IV. History of Pesticide Regulations (reference: Johnson, J. M., and G. W. Ware.Pesticide Litigation manual. Clark Boardman Company, Ltd., New York.)

A. Regulation of chemicals to protect the public from exposure has been traced back to themid-1800’s (~1863, “An Act for the More Effectual Condensation of Muriatic Acid inAlkali Works”) in England. In the U.S., the Pure Food & Drug Act (1906), the MeatInspections Act (1906) and the Insecticide Act (1910) were early statutes aimed at chemicalsand health concerns.1. However, these acts really prevented consumer fraud and protected the legitimate

manufacturers of pesticides (originally called economic poisons) and drugs.2. The Acts prevented the manufacture or transportation of adulterated or misbranded

pesticides and drugs within the United States or its territories.a. They did not provide for registration of products, nor testing requirements.

However, the Pure Food and Drug Act was intentioned to protect health.b. The Acts did require specific labeling information, namely it prohibited false or

misleading information on labels; The Insecticide act required specific labelinformation for some categories of pesticides, i.e., those that contained arsenic (theamount had to be stated); furthermore, inert ingredients had to be stated.1. Some recognition of environmental effects was contained in a provision that

stated a product was considered adulterated if it was intended for use onvegetables and yet proved to be injurious to those vegetables.

3. The Insecticide Act was little used; there were hardly any commercial pesticidesavailable.

4. These acts were forerunners of respectively, the Federal, Food, Drug, and Cosmetic Act(FFDCA--1938), and the Federal Insecticide, Rodenticide, and Fungicide Act (FIFRA--1947)

B. The Pure Food and Drug Act and the Insecticide Act functioned in parallel to regulatepesticide products and in a rudimentary way protect human health from adulterated foods.1. Both the Pure Food and Drug Act and the Insecticide Act had relied on designation of

products as misbranded or adulterated to keep inefficacious or undesirable productsout of the market.a. An unanticipated issue raised by the Pure Food Act was determining when, or at

what level, treatment with a pesticide rendered a food product adulterated.



b. There was an early recognition that pesticides could render a food unsafe, but therewas no mechanism for removing that food from the marketplace.1. The Secretary of Agriculture, in order to prove that a food treated with a

pesticide was adulterated, had to prove the food was injurious to health.However, pesticide residues did not cause acute injury at the time.a. Even today, residues that are above the so called tolerance or standards for

legal sale are extremely unlikely to cause an acute, adverse response;b. Furthermore, pesticides were recognized as being necessary to the

production of crops.2. Thus, the two acts were actually in conflict (Figure 1).

Pure Food & Drug Insecticide Act

Health Protection

Prohibit Misbranding and Adulteration

Ensure Efficacious Product

Adulteration Standard Placed the Laws in Conflict

Figure 1. Conflicting objective of the two original laws covering pesticide use.

c. To remedy the dilemma, the FFDCA (1938) provided that a food was adulterated ifany amount of poisonous material (including pesticides) were present. However, afood was not considered adulterated by a pesticide if the pesticide was required forits production--thus was born the concept of benefit versus risk. In such a case, theAdministrator of the Food and Drug Administration (FDA) was authorized toestablish tolerances or legal (permissible) levels of pesticide residues.1. Consider whether the state of the art of analytical chemistry would have even

been able to detect residues at this time.2. The tolerances were supposed to be maximum legal residues; although they

were supposed to be developed to ensure safety, we will discuss how they arenot really safety standards when we talk about how tolerances are developed inthe lecture on food residues and dietary exposure.

d. The FFDCA was impractical and never worked well. Between 1939 and 1947 (theyear FIFRA was passed) only one tolerance was established (for fluoride-basedpesticides on apples and pears) and was challenged in court. However, in the1920’s, concern about lead arsenate residues on apples led to the establishment of a“tolerance” for export to markets in Great Britain.

1. Although the numbers of pesticides began exploding during WWII, toolittle was known about them to even set tolerances for safe levels.

C. The growing number of new pesticides, which were very effective but whose risks wereunknown presented a dilemma. In this atmosphere, Congress began work on FIFRA, whichwas passed in 1947 with great support from the manufacturers and users (Figure 8).1. FIFRA repealed the Insecticide Act.2. Rodenticides and herbicides were also covered (as well as insecticides and fungicides).3. Pesticides had to be registered with the Secretary of Agriculture.4. Each pesticide sold must include specific label or package information.

a. This is the heart of the law, requiring name and address of manufacturer, aningredient statement, and directions for use which were “adequate for protection ofthe public”



b. Assumption that an adequate label would allow safe use5. Did not impose testing requirements, neither for efficacy or safety; however, the

Secretary could request a full description of all tests if he felt they were needed;a. Secretary could register a product “under protest”b. Secretary could cancel a product if he felt it was necessary to protect the public

6. The main effect of the law was to allow USDA to track pesticides, i.e., keep an adequateinventory.

7. Governed pesticide regulation until 1972, when the law was significantly overhauled.

Historically Parallel Regulations

Pure Food & Drug Act(1906)

Insecticide Act(1910)

Federal Food, Drug &Cosmetic Act (FFDCA,1938) Federal Insecticide,

Fungicide & RodenticideAct (FIFRA,1947)

registrationlabelling

residue tolerances

Figure 2. The parallel nature of pesticide laws and their main objectives.

D. Note that health concerns were left under the jurisdiction of the FFDCA (thus FDA ratherthan USDA); following FIFRA, the FDA began to develop tolerances for pesticides on foodcrops (Figure 2).1. Evidence for establishing tolerances were taken at public hearings.2. Unworkable system; (the first tolerances were not actually established until 1955);3. New law was being drafted to improve process; first draft required manufacturers to

prove safety, which brought great protest;4. Law finally amended in 1954 (Miller Amendment)--essentially required health and

safety data to be analyzed before a pesticide could be used on a food crop; such datawould be used to develop the tolerance (Figure 3).a. The intent of the law was to withhold registration until the tolerance was established-

-thus, although the Miller Amendment did not modify FIFRA, its effect was to tietogether two laws for regulation of pesticides.1. A manufacturer who had registered or sought to register a pesticide under

FIFRA for use on food crops was required to obtain a certificate of usefulnessfrom the Secretary of Agriculture and then to petition the Secretary of Health ,Education, and Welfare to establish a tolerance for pesticide residue, or toexempt the substance from compliance.

2. Petition had to contain:a. Name and chemical composition of pesticide;b. Application procedures;c. Data from investigations on safety;d. Results of residue testing;e. Methods for removing excess residue;f. A proposed tolerance and other grounds in support of the tolerance.

3. Any food sold containing residue in excess of tolerance was deemed adulterated.b. Both FIFRA and Miller Amendments passed with consensus of manufacturers,

agricultural interests, and the public.



5. In 1958, FFDCA was amended again by the Food Additive Act containing a provisionknown as the Delaney Amendment (Figure 9). The law’s intent was to regulate foodadditives purposefully added to foods (i.e., processed foods). Pesticides wereexempted as long as they did not concentrate during processing (i.e., to levels above thetolerance for the raw agricultural commodity, which was established under theprovisions of the Miller Amendment). If the pesticide residues did concentrate, aprocessed food tolerance would be required under the provisions of the Food AdditiveAct.a. However, the Delaney Amendment in the Food Additive Act stated that an additive

cannot cause cancer in laboratory animal testing, in which case it would not bepermitted. If a pesticide was deemed oncogenic, then it also could be regulatedunder the Delaney clause. In other words, as the law was written, an “oncogenic”pesticide could not have a processed food tolerance, if one was needed as a result ofresidues concentrating to levels above the raw food tolerance.

b. So a paradox was created (called by the National Academy of Sciences the DelaneyParadox); an oncogenic pesticide could obtain a raw agricultural commoditytolerance and be used widely, but the residues could not legally be present inprocessed food.1. To get around this paradox, the EPA had chosen to use “de minimis” risk,

which is a negligible risk defined as no more than 1 in a million excess cancersin the population.

Historically Parallel Regulations

Pure Food & Drug Act(1906)

Insecticide Act(1910)

Federal Food, Drug &Cosmetic Act (FFDCA,1938)

Federal Insecticide,Fungicide & RodenticideAct (FIFRA,1947)

Miller Amendment(1954)

Food Additives Amendment(Delaney Clause) 1958

Figure 3. Important amendments that overhauled the FFDCA.

6. The Delaney Amendment was rescinded recently with the passage of the Food QualityProtection Act during August, 1996 (discussed below)

E. After passage of FIFRA, many state legislatures enacted their own pesticide laws; mostlyregulated use rather than manufacture and sale.

F. The regulation of pesticides tool place by the overlapping of responsibilities of two federalagencies. Opinion in the early 1960’s held that the laws were ineffectively administered;President Kennedy had appointed a commission to study the need for revisions to the lawsprior to public interest stirred by Silent Spring. Concern over environmental effects alsobegan to be voiced prior to publication of the latter book.

G. By 1969, Congress commissioned a study of the effect of pesticides on the environment;the study group was known as the Mrak Commission.1. One of the first results from the study was the removal of pesticide regulation

responsibility from the USDA to the new EPA (created in 1970 under the Nixonadministration).



H. DDT had really been the center of the storm of controversy over pesticide use during the1960’s; this controversy probably spawned the major overhaul of FIFRA known as theFederal Environmental Pesticide Control Act of 1972 (FEPCA).1. FIFRA became an environmental statute: manufacturers were required to demonstrate

that a product could be used without “unreasonable adverse effects on theenvironment.”

FFDCA Amended(Food & Drug Admin.-FDA)

FIFRA(USDA)

EPA(1970)

Federal Environmental PesticideControl Act (FEPCA, 1972)FDA

(food residues)

Shared Responsibilities

Figure 4. Creation of the EPA to oversee pesticide law and major overhaul of FIFRA toaddress environmental concerns of pesticide use.

I. Provisions of FEPCA1. New requirement for registration and reregistration of all products within four years

(this deadline never met; law amended again in 1978 to set new deadlines, and severaltimes thereafter);a. An applicant for registration had to show that the pesticide would “perform its

intended function without unreasonable adverse effects on the environment...”2. Use of any registered pesticide in a manner inconsistent with its labeling became a

crime;3. New system of classifying pesticides--restricted vs. general use;

a. Restricted use pesticides designated as one which could cause “unreasonableadverse effects on the environment, including injury to the applicator...”, ifadditional regulatory restrictions were not imposed.1. Under this system, only certified applicators could apply restricted use

pesticides2. This provision necessitated implementation of training and certification

programs by the states; largely accomplished through universities in programsknown as Pesticide Applicator Training (PAT).

b. General use pesticide could be sold to the general public (for ex., in supermarkets,nurseries, etc.) without a requirement for licensing or certification.

J. The re-registration process was observed to be a failure; by 1988, over 600 activeingredients required reregistration. In 1988, FIFRA amended again (under Reagan)requiring completion of reregistrations by 1997. Products included in the process werethose registered prior to 1984.1. Reregistration rationale

a. Historically required but never carried out by USDA;b. Pesticides registered prior to 1984 met different set of safety standards and testing;c. Need for institution of GLPs (Good Laboratory Practices) for tracking data validity



1. This provision of testing raises the costs significantly. It is essentially asophisticated book keeping system requiring documentation of literally everyfacet of a lab and field testing operation

2. The need for GLPs grew out of a testing scandal in the late 1970’s whenIndustrial Biotest, a Chicago testing company that conducted many of the animaltoxicity tests for manufacturers’ products, was found to have committed veryserious fraud in reporting of test results, as well as mistreatment of lab animalsand assorted “bad” practices.

2. Components of re-registrationa. Updating of the database--EPA was to prioritize information needs. Consideration

was given to pesticide use on food or feed crops, whether use could result incontamination of groundwater, fish or shellfish; whether there were missing data; orwhether farm worker exposure was likely.1. Thus, EPA had to determine the current data requirements and ensure data were

availableb. The data available had to be re-evaluated against current safety and testing criteria

and then EPA had to make determination of need for registration changes.c. After the evaluation process, EPA could require a modification in label or

cancellation of product.1. Note that when a pesticide is granted a tolerance and prior to registration, a label

is drafted to accompany the product.2. The pesticide label has information about the active ingredient, legal uses (crops,

sometimes specific pests), rates of application, personal protective equipment(worker protection), restrictions on use (no drift, no application near water) andproper disposal.

3. Violation of provisions on the label is a crime.d. Companies had to submit new data as required by EPA; they had 4 years with an

extension possible to conduct new studies if needed.K. Note that other environmental laws like the Clean Water Act and the Safe Drinking Water

Act affect pesticides, but these have a different legislative history and could be invoked whenpesticide residues in water exceed certain standards or if an industry manufacturing or usingpesticides needed to discharge wastewater.

V. The Food Quality Protection Act of 1996 (FQPA) (access the law athttp://www.epa.gov/oppfead1/fqpa/)A. In 1993, the National Academy of Sciences released a report commissioned by Congress in

the late 1980’s. The report, Pesticides in the Diet of Infants and Children, became theblueprint for amendments to FIFRA and the FFDCA.1. The NAS report concluded that infants and children were not as well protected as adults

by the current pesticide regulations;2. Thus, the Academy recommended changes in exposure analysis and more focus on the

types of foods eaten by kids. It also recommended that other exposures besides foodbe aggregated.

B. The FQPA was supported by both industry and environmental advocacy groups because itsprovisions were designed to eliminate balancing of risks and benefits and make pesticideregulation entirely risk based, as were other laws regulating environmental contaminants.Furthermore, protection of children and infants would take center stage. In return, industrywanted the repeal of the Delaney amendment.1. With elimination the Delaney amendment, all food tolerances would be unified. In other

words, no distinction would be made between raw and processed commodities.2. Also, residues of putative carcinogenic pesticides would be permitted as long as they

met the standard of negligible risk. As a matter of fact, the negligible risk standardapplied regardless of the perceived health and environmental effects.



C. Although the FQPA really amends both FFDCA and FIFRA simultaneously, the mostimportant provisions from our perspective is how risk will be determined. How risk isdetermined is directed by new definitions in the law. (See Figure 5 for summary graphic ofthe interaction of the FQPA with FIFRA and FFDCA).1. Before the FQPA, the law dictated that tolerances would be set to “protect public

health”. The FQPA says that tolerances will be “safe,” i.e., “a reasonable certaintythat no harm will result from aggregate exposure.” Thus all pathways of exposure,food, water, residential home, garden, and lawn use would be aggregated together.a. Aggregate exposure includes exposure of a specific pesticide to all media: air, water,

soil, as well as food;2. A “safe” tolerance considers the following factors in addition to aggregate

exposurea. Exposure of infants & children and whether they are more susceptible to the effects

of a pesticideb. Threshold vs. non-threshold effects (i.e., is the pesticide a carcinogen; carcinogens

according to EPA logic have no thresholds)c. The potential for disruption of the endocrine system;d. Cumulative exposure;

1. Requires that food containing residues of multiple pesticides with the samemechanism of toxicity be added together in considering the amount of exposure.

D. How EPA ensures safety for pesticide residue exposuresFIFRA(1947)

FFDCA(1938)

Tolerance (“MRL”)

FEPCA(1972)

Labelling Registration

Risk Assessment

FQPA(1996)

Miller (1954)Delaney (1958)

Figure 5. Relationship of FQPA to FIFRA and FFDCA. FQPA has a profound effect on riskassessment policy that in turn affects tolerance setting and labeling.

1. Briefly, safety is determined from the various mammalian toxicology tests for acute andchronic toxicity (see testing requirements below).

2. In these tests, the most sensitive toxicological endpoint (i.e., the adverse biochemical orphysiological effect occurring at the lowest tested dose) is chosen. For this endpoint thedose causing no effect (No Observable Adverse Effect Level, NOAEL) is determined.

3. The NOAEL is divided by an uncertainty factor (100 if no extraordinary childsensitivity noted, or 1000 if child sensitivity noted) to obtain a reference dose (RfD,given in units of mg/kg/day). Any exposure due to residues from food, water, orresidential use must not exceed the RfD).

E. Consequences of the FQPA1. The FQPA mandated that all pesticide tolerances be reassessed under the new

requirements for children’s sensitivity, endocrine disruption, aggregate exposure, andcumulative exposure.a. In 1996, there were about 10,000 pesticide tolerances ; EPA was given until 2006 to

complete the assessments.



b. After EPA completes a draft re-assessment (i.e., risk assessment to determinewhether exposure exceeds the RfD), then the agency issues a Re-registrationEligibility Decision document (RED).1. REDs are available on the WEB at

(http://www.epa.gov/pesticides/reregistration/status.htm)2. EPA chose to examine organophosphate (OP) insecticides first, and then to tackle sol-

called carcinogenic pesticides.3. OPs were surmised to have the greatest potential for exhibiting differential effects on

children (i.e., children more sensitive at a given dosage (mg/kg body weight) thanadults).

4. As a result of the reassessments, uses have been cancelled or restricted. For example,a. Chlorpyrifos in the urban use formulation of Dursban will be completely off the

market by ~2003.b. Diazinon will no longer have urban uses.c. Methyl parathion uses was cancelled for orchard fruits.

F. Secrets of the FQPA1. The FQPA was written as a consumer protection law with no attention to worker

exposure and risk nor ecological effects and risk.2. However, during the re-registration process (i.e., the process of reassessing the safety of

the tolerance and development of the RED), EPA does examine worker exposure andecological effects.

3. Thus, EPA during the reassessment process can require changes in pesticide usecharacteristics to protect worker and ecological health.

4. Indeed, late during 2000, EPA was sued by a coalition of advocacy groups headed byNational Resources Defense Council (NRDC) for failure to fully implement the FQPA,especially the provisions for cumulative exposure and risk assessment.a. In the court sanctioned consent decree, the EPA is directed to specifically tend to

worker exposure and ecological effects issues in addition to the statutoryrequirements of the FQPA.

b. Ironically, pesticides (mostly the group known as organophosphates that arerunning in to problems with re-registration and are having uses cancelled orvoluntarily withdrawn by manufacturers, are doing so because of concerns overworker exposure (i.e., exceeds EPA’s level of concern) and secondarily ecologicaleffects.

VI. Tests Required Under Current Law (Code of Federal Regulations 40, Part 158)A. A. Number & Kind of Studies Required for Registration (classification according to Leng,

M. L. 1991. Consequences of reregistration on existing pesticides. pp. 27-44 in“Regulation of Agrochemicals: A Driving Force in their Evolution, G. J. Marco, R. M.Hollingworth, and J. R. Plimmer, eds., Am. Chem. Soc., Washington, D.C. );1. Product chemistry (26)2. Mammalian toxicology (27)3. Wildlife toxicity (27)4. Toxicity to nontarget organisms (15)5. Environmental fate (24)6. Drift (2)7. Residue in crops & analytical method (18)

B. Specific provisions of the 1988 amendments to FIFRA requiring data for registration--i.e.,data requirements as listed in the Code of Federal Regulations:1. product chemistry

a. Active ingredientb. Impurities

1. Some impurities may be toxicologically important; for ex. isomalathion (anisomer contaminant) is about 5 times more toxic than malathion



a. malathion LD50 = > 2500 mg/kgb. isomalathion LD50 = < 500 mg/kg

P S CH C OC2H5

OS

CH3O

CH3O

CH C OC2H5

O

CH C OC2H5

O

P S CH C OC2H5

O

CH3O

OCH3S

malathion isomalathion

c. Analytical method for active ingredientd. Certifiable concentrationse. Physical & chemical characteristics

1. Useful for defining necessity for other studiesa. high partition coefficient could indicate bioconcentration potential;b. vapor pressure is a consideration in setting worker reentry intervals;c. viscosity & miscibility is important to setting acceptable labeling for tank

mix and spray applicationsf. Production processg. Formulation processh. Dupont has been fined by EPA for a Benlate fungicide formulation contaminated

with the herbicide atrazine, even though it is not clear that the contamination actuallycaused crop damage or the contaminant even existed in the stocks that were used(Pesticide & Toxic Chemical News, Oct. 5, 1994).

2. Residue chemistrya. Used to estimate exposure of general population to pesticide residues in foodb. Used to set and enforce tolerancesc. Information required:

1. Chemical identification and composition of product2. Amounts, frequency, and timing of application3. Amount of residues remaining on food4. Analytical method adequate for enforcement

a. Note that the analytical method developed for analyzing active ingredientcontent in a formulated product may be very different (i.e., simpler) than theanalytical method needed to analyze for environmental residues

5. Practical method for removing excess residues3. 3. Environmental fate

a. a. Generic studies required1. Degradation2. Metabolism3. Mobility4. Dissipation5. Accumulation

b. Rationale:1. Used to assess toxicity to humans through exposure to residues remaining after

application either in treated areas or from consuming contaminated food;2. Used to assess the presence of widely distributed and persistent pesticides in the

environment which may result in loss of usable land, water, or wildlife resources3. Used to assess potential exposure of nontarget organisms like fish and wildlife;4. Used to estimate expected environmental concentrations in specific habitats

where threatened or endangered species are found



c. Practical significance1. Availability of residues to rotational crops2. Irrigation water might contain residues3. Setting of realistic field reentry intervals4. Protection of potable water supplies5. Workers may be exposed to degradation products

4. Hazards to humans & domestic animalsa. Acute toxicity

1. Defines hazard of handling product; can answer questions of how hazardousdermal exposure is in relation to oral or inhalation exposure;

2. LD50 measurements by various routes of exposureb. Subchronic toxicity

1. Helps define doses to be used for chronic toxicity2. looks at range of effects that might occur from doses that are not acutely toxic

but occur repeatedly over a limited time frame3. Usually a 90-day study

c. Mutagenicity1. Assess potential to affect the cell’s genetic material using a battery of different

tests2. Compounds that damage DNA directly represent true hazards of

carcinogenicity; likely to be dropped from further development3. Findings of mutagenic potential can be used to assess heritable effects,

oncogenicity, or other health effects;d. Chronic toxicity

1. Life-time feeding studies using maximum tolerated dose2. Endpoint is usually carcinogenicity (actually tumorigenicity) although other

effects are measurede. Teratogenicity (birth defects)f. Reproductive effects (effects on fertility)

1. These are multigenerational studies; the pregnant rat is fed the pesticide mixedinto the diet; effects on offspring are measured; ability of offspring tosuccessfully mate and reproduce are also examined

g. Metabolism1. Aids extrapolation of data from animals to humans;2. Development of poisoning antidotes

h. Note that under mammalian toxicology we would now include endocrine disruptorscreening studies

5. Field reentry protection studiesa. Monitoring data under actual exposure conditionsb. Combined with data from studies of toxicity and residue dissipationc. Practical significance: occupational exposure is the highest risk for health hazards

6. Pesticide spray drift evaluationa. Measure droplet size spectrum using formulationsb. Drift evaluation under field conditionsc. Practical significance: precautionary labeling to protect nontarget crops

7. Hazards to nontarget organismsa. Determination of pesticidal effects on birds, mammals, fish, terrestrial and aquatic

invertebrates, and plants;b. Tests include short-term acute, subacute, reproduction, simulated field, and full field

studies;c. Testing is hierarchical or tiered--if tests lower in the tier indicates a problem, then the

next test in the tier will be performed (progression is from basic lab tests on one endto applied field tests on the other end);



d. Toxicity information is compared with measured or estimated pesticide residues inthe environment to assess potential impacts and decide whether further studies arewarranted;

e. Long-term field studies, which include reproductive, life cycle, and plant field studiesmay be required if predictions of possible adverse effects in less extensive studies(i.e., lower in the tier) cannot be made, or when the potential for adverse effects ishigh.

8. Product performancea. Ensures that pesticide products will control the pests listed on the label;b. Unnecessary pesticide exposure to the environment will not occur as a result of the

use of ineffective products;c. Includes specific performance standards to validate the efficacy of pesticides used in

the public health area (e.g., disinfectants, rodenticides)

VII. Major Groups of PesticidesA. The major groups thus include herbicides, insecticides (also acaricides & nematicides),

fungicides, plant growth regulators, rodenticides, molluscicides, piscicides, disinfectantsB. Note that pesticides generally have several nomenclatural designations;

1. Common chemical name (the technical active ingredient)a. endosulfan (MW = 406.9)

2. Formulation names containing the active ingredient (commercial product)a. Thiodan, Thiofur, and others

3. “Official” chemical nomenclature (i.e., approved by the IUPAC--International Unionof Pure and Applied Chemistry)a. 6,7,8,910,10-hexachloro-1,5,5a,6,9,9a-hexahydro-6,9-methano-2,4,3-

benzodioxathiepin 3 oxide (HOLY COW!!)C. We will focus on chemistry and toxicology of insecticides and herbicides because these

have been the most contentious with respect to environmental contamination and perceivedadverse health effects1. However, fungicides are a very diverse group of compounds with very low mammalian

and wildlife toxicity;a. Environmental contamination by fungicides has not been an issue, but the EPA

considers quite a few of them potential carcinogens; this becomes an issue withresidues in food;1. Many fungicides may test out as oncogenic because the MTD (maximum

tolerated doses) can be very high (because of low acute toxicity), thus promotingmitogenesis, which raises the probability of tumors forming.

D. Insecticides1. Chlorinated hydrocarbons (essentially contain only C, H, Cl with some exceptions)--

also known as the “hard” or persistent pesticidesa. DDT and metabolites and analogs (technical DDT is actually a 3:1 mixture of the

p,p’ isomer and the o,p’ isomer); all registrations of DDT suspended circa 1973.1. Isomers and metabolites of DDT

a. p,p’-DDT, DDE, DDD

p,p'-DDTCl

CCCl3

H

ClDDE

C

Cl

CCl2

ClDDD

CH

Cl

CHCl2

Cl

b. o,p’-DDT, DDE, DDD (only o,p’-DDT shown)



o,p'-DDT

CCCl3

H

Cl

Cl

c. dicofol (still registered; used as an acaricide)

Cl

CCCl3

Cl

HO

Dicofold. methoxychlor (still registered)

CCCl3

H

H3CO OCH3methoxychlor

2. Chlorinated cyclodienes (essentially chlorinated hydrocarbons like DDT butcharacterized by a bicyclic structure.a. Aldrin, Dieldrin (dieldrin is the oxidative metabolite of aldrin; called an epoxide)

1. All registration suspended circa 1974

aldrin dieldrin

Cl

Cl

Cl

ClCl Cl

Cl

Cl

Cl

Cl O

ClCl

b. Other analogous structures that were commercialized insecticides include heptachlor,chlordane, and endrin (all registrations suspended).

c. Endosulfan (example of a sulfur heteroatom; still registered O

OS O

Cl

Cl

Cl

ClClCl

endosulfan



3. Lindane (historically misnamed as BHC, which was an acronym for benzenehexachloride, another contaminant of many secondary products as well as a fungicide inits own right but is now banned).a. Should be properly called hexachlorocyclohexaneb. Has several isomers, but only the gamma isomer is insecticidalc. The alpha and beta isomer formulation contaminants are much more persistent and

likely to be found as an environmental contaminant.

ClCl

Cl

Cl

Cl

Cl

Cl

Cl

Cl

Cl

Cl

Cl

Cl

Cl

Cl

ClCl

Cl

gamma hexachlorocyclohexane (lindane)

alpha hexachlorocyclohexane beta hexachlorocyclohexane

d. The chlorinated hydrocarbons, i.e., mainly DDT and chlorinated cyclodienes,frequently show up as contaminants in all media (air, water, soil, biota).

4. Organophosphorus (OP) and carbamate (CB) insecticidesa. OP’s are phosphate esters; CB’s are esters formed from carbamic acid;

NN

N

O

SP

CH3O

CH3O S

azinphos-methyl (Guthion)

carbaryl (Sevin)

O CO

NH CH3

CH3O

CH3OP O C CHCOCH3

OCH3O

mevinphos (Phosdrin)

CH3-S-C-CH=N-O-C-NH-CH3

CH3

CH3

aldicarb (Temik)

O

b. These compounds degrade quickly in the environment and are probably the mostwidely used group of insecticides;

c. Some members of these groups have very high acute toxicities and represent aconsiderable hazard to farm workers;



d. Others are fairly innocuous if handled carefully and can be bought “over thecounter;”

e. The incredibly high toxicities of a few members of the OP group to some aquaticinvertebrates places the ecological safety criterion literally at ppt levels.

f. OPs occasionally show up in water (especially from urban watersheds and morefrequently on food.

g. Dependency of structure (notably the leaving group on toxicity characteristics) CH3CH2O

CH3CH2OP

S

NO2

parathion

NO2

S

PCH3CH2O

CH3CH2O CH3

fenitrothionLD50 oral = 3 mg/kg LD50 oral = 250 mg/kgLD50 dermal = 6.8 mg/kg LD50 dermal = 1300 mg/kg

5. Pyrethroids (these are esters; modified synthetic derivatives of the natural pyrethrum,which contains a mixture of pyrethrins, located in the flower heads of Chrysanthemumcinaerofolium.a. These were to be the third generation pesticides, having very low toxicity to

mammals and birds, no bioaccumulation, and rapid environmental degradation;unfortunately they have very high toxicity to aquatic organisms, and one of the firststable, synthetic agriculturally useable compounds, permethrin, was deemed as apotential carcinogen by EPA

1S trans permethrin

CH3H3C

C-OC O

O

H

HCCl

ClC

1R trans permethrin

H

H

CH3H3C

CCl

ClC

OC-OO C

CCl

ClC

H3C CH3H H

OC-OO

C

1S cis permethrin

CCl

ClC

H3C CH3H H

OC-OO

C

1R cis permethrinb. Permethrin is an example of a synthetic pyrethroid insecticide. One characteristic of

many of the pyrethroids is the optical activity, or stereoisomers around anasymmetric carbon atom. Although the structures may look similar, the 1R, cisisomers are the most insecticidal. There is also preferential metabolism of theisomers in the environment and in the biota.



E. Herbicides1. This group is incredibly diverse with respect to chemistry, but several groups have been

particularly contentious from the viewpoint of contamination and putativecarcinogenicitya. triazines

1. Symmetrical triazines (atrazine, simazine, cyanazine); asymmetrical triazine(metribuzin)

atrazine (Aatrex)

CH3CH2

NN

N

Cl

NHNHCH3CHCH3

NN

N

(CH3)3

O

SCH3

NH2

metribuzin

2. Chloroacetamides (also called chloroacetanilides)a. alachlor, metolachlor

C2H5

C2H5

CH2-O-CH3C-CH2-Cl

N

O

alachlor

3. Phenoxyacetates (weak acids; but there are ester analogs also)a. 2,4,5-T (the most infamous, now banned);b. 2,4-D (main active ingredient in common home lawn herbicides like “Weed &

Feed”);1. Comes in several forms, including the dimethyl amine salt which quickly

dissociates to the acid (2,4-D)

dimethyl amine salt of 2,4-D

OCH C OH

O

Cl

Cl

CH3

CH3NH

2,4-D

OCH C OH

O

Cl

Cl

c. The other two ingredients in Weed & Feed include dicamba (a chlorinated benzoicacid) and mecoprop (a phenoxyacetate type)

COHO

OCH3

Cl

Cl

dicamba

Cl

CH3

OOH

OCCHH3C

mecoprop (MCPP)



4. Dinitroanilinesa. trifluralin, pendimethalin

NCH2CH2CH3CH3CH2CH2

NO2O2N

CF3

trifluralin5. Aminophosphonic acids

a. glyphosate (Roundup)—the herbicide used on Roundup Ready transgenic crops

glyphosate (Round-Up, Rodeo)

O

OHCCH2HO

HOO

PCH2 NH

6. Bipyridiliniums (cationic in nature)a. paraquat; diquat

N+ N+ CH3CH3 2 Cl-

paraquat (Gramaxone) --dichloride salt

7. Sulfonylureas (incredibly low use rates; ~10-20 g/ha; most other types of herbicidesapplied as high as 1-2 kg/ha (1-2 lbs./acre)a. chlorsulfuron, thifensulfuron, metsulfuron

NN

N

CH3

OCH3

NHOCNH

O

OS

Cl

chlorsulfuron (Glean)

8. Imidazolinones (low use rates)

N

C OHO

HN

N

O

CH3C

CH3

CH3H

imazapyr



9. Herbicides in the triazine and chloracetamide groups frequently show up in water, butnot in food.

F. Comparative Acute Toxicity of Major Insecticide and Herbicide Groups

Chlorinated Hydrocarbons & CyclodienesCompound Rat oral LD5 0

mg/kgRabbit DermalLD5 0 (mg/kg)

Fish LC5 0 µg/L, trout

Bird LD50 mg/kgCalif. Quail

DDT 87 1931 4.1-11.4 595DDD 3400 4000 70DDE 880-1240 32Chlordane 400-700 580 8.2-135 14.1Dicofol 575-2000 4000 111 1.89Endosulfan 18 74 1.1-2.9 80-160 (pheasant)Methoxychlor 5000 2820 11-61 >2,000

Organophosphates and CarbamatesCompound Rat oral LD5 0

mg/kgRabbit Dermal LD50 (mg/kg)

Fish LC5 0 µg/L, trout

Bird LD50 mg/kgCalif. Quail

parathion 3 6.8 1500 16.9chlorpyrifos 135 2000 3 68.3phorate 1.6 2.5 13 7.1 (pheasant)azinphosmethyl 13 250 20 74.9 (pheasant)diazinon 300 379 16000 4.33 (pheasant)malathion 885 4000 170 167 (pheasant)

carbaryl 307 2000 1300 >2000carbofuran 8 2550 280 4.15 (pheasant)aldicarb 0.9 >5 880 2.58propoxur 95 >1000 3700 30

HerbicidesPesticide LD5 0, rat oral

mg/kgLD5 0, rabbit dermal (mg/kg)

LC5 0, trout(ppm)

LC50 avian (mg/kg)(

atrazine 1869 >3100 24 >2000 (pheasant)simazine 5000 >3100 >100 8800 (quail)cyanazine 149-835 >1200 9 >2400 (mallard)alachlor 1200 >1200 1.4-2.4 >2000 (mallard)metolachlor 2780 10000 2.0 100002,4-D 699 2000 (rat) 44.5 472 (pheasant)2,4,5-T 300 17.2 >2000 (mallard)chlorsulfuron 5545-6293 >3400 >250 250glyphosate 4320 >7940 86 3850 (quail)paraquat 150 236 >100 199 (mallard)trifluralin >10000 >2000 0.03-0.10 >2000 (mallard)

VIII. Subchronic & Chronic ToxicityA. In the acute toxicity tables above, the data for rats are used as surrogates for predicting

effects in humans.



1. However, we’re more interested in the NOAEL than the LD50 when assessing risk ofhuman health following exposures. The table below give a comparison ofacute/subchronic and chronic NOAELs/LOAELs (expressed as mg/kg/day) for twocommonly used OP insecticides, azinphos-methyl and chlorpyrifos, with the herbicidesatrazine, 2,4-D, and glyphosate. All data are taken from the EPA Registration EligibilityDecision Documents (REDs), except for 2,4-D, which is taken from a Federal RegistrarNotification for a time-limited tolerance.a. Note that different endpoints are used depending on the nature of the sensitive toxic

reaction;1. For example, the enzyme acetylcholinesterase and the extent of inhibition are

the toxicological endpoints used to characterize hazards of azinphos-methyl andchlorpyrifos.a. The acute NOAELs were based on a single oral dose used in an acute

neurotoxicity study.b. The chronic NOAELs were based on acetylcholinesterase activity inhibition

during either a one-year dog dietary exposure study (azinphos-methyl) or aweight of evidence approach among dog and rat studies (chlorpyrifos)varying in duration from 90 days to two years.

2. For atrazine, the subchronic NOAEL was determined from a developmentaltoxicity study wherein delayed ossification of certain cranial bones was observedin fetuses at a dose of 70 mg/kg.a. On the other hand, the chronic NOAEL was chosen from a six-month

exposure study wherein preovulatory luteinizing hormone (LH) surge wasattenuated at a dose of 3.65 mg/kg/day. LH surge was used as a biomarkerthat could be indicative of hypothalamic function disruption.

3. The subchronic NOAEL for 2,4-D was based on an acute oral exposure (singleexposure) wherein neurobehavioral effects were noted at a dose of 227mg/kg/day.a. The chronic NOAEL for 2,4-D was based on a dog dietary exposure study

(1 year) wherein at 5 mg/kg/day, the animals exhibited blood chemistrychanges and liver/kidney histopathological lesions.

4. The glyphosate subchronic study did not observe a NOAEL (thus thedesignation <63 mg/kg/day) is used; the study was a 90-day dietary exposureand the effects noted were on blood chemistry.a. The chronic NOAEL for glyphosate was based on a rabbit developmental

toxicity study wherein the females exhibited diarrhea, nasal discharge, andexcessive mortality at a dose of 350 mg/kg/day.

Pesticide

Acute orSubchronic

NOAEL

Acute orSubchronic

LOAELChronicNOAEL

ChronicLOAEL

ChronicRfD

ChronicPAD

azinphosmethyl 0.3 1 0.149 0.688 0.00149 0.00149chlorpyrifos 0.5 1 0.03 0.22 0.0003 0.00003atrazine 10 70 1.8 3.65 0.018 0.00182,4-D 67 227 1 5 0.01 0.01glyphosate <63 63 175 350 2 2

2. Note in the table above that the reference dose (RfD) is based on the NOAEL (thechronic RfD is shown) divided by an uncertainty factor of 100.a. For pesticides in which EPA is concerned about infant/children sensitivity, an extra

10-fold uncertainty factor is applied to the RfD to establish a PAD (PopulationAdjusted Dose).



B. Owing to EPA’s insistence on using MTD dosing and an assumption of a linear thresholdfor induction of carcinogenicity, numerous pesticides are classified as either C or B2carcinogens1. Interestingly, however, the more acutely toxic a compound, the less likely it is classed as

a carcinogen; the obverse holds--the less acutely toxic, the more likely the classificationas a carcinogena. Indeed, an NAS (through the NRC’s Issues in Risk Assessment committee) study

has shown that the MTD and tumorigenic potency are linearly related2. Thus, few OPs and the more toxic carbamates are classified as carcinogens; however,

many fungicides and herbicides have been deemed carcinogens (for ex., atrazine,simazine, alachlor, commonly used fungicides like captan and benomyl)

IX. MetabolismA. With the exception of DDT and a few other chlorinated hydrocarbon insecticides, all other

pesticides should be considered readily biodegradable;1. Thus, exposure to most of these compounds results largely in most of the compound

being enzymatically oxidized or hydrolyzed, followed by conjugation and excretion; anystorage is temporary; for example:a. 98% of a radioactive methoxychlor dose fed to mice was recovered in excreta

within 24 h.b. 50% of administered dose of Dicofol excreted in urine in 24 h.c. In rats, 77% of a total inhaled dose of chlordane was retained (thus this is another

compound like DDT, which is metabolized very slowly and thus bioconcentrates inadipose tissue);

d. 2,4-D is excreted unchanged in urine within 24 hours of exposureB. Contrary to perception, DDT is also metabolized; unfortunately the rate of metabolism and

thus excretion is very slow, allowing bioconcentration in adipose tissue;1. The amount of DDT stored in the tissues is gradually reduced if exposure to the

compound is discontinued or diminished. Note that storage of DDT and DDE reachesa steady state, i.e., continued feeding does not result in ever increasing storageconcentrations.

2. A study with rats showed that much of an oral dose of p,p’-DDT was secreted into bileand excreted in the feces. Only 1-3% of the oral dose was excreted in the urine within 5days; the urinary form was a glucuronide conjugate of DDA (DDT “acid”); the fecescontained 10-25% of the dose with 6% unchanged DDT, 30% was DDA conjugatesand 64% as DDD;a. DDE is another metabolite of DDT and is the main storage (in adipose tissue) form;

hydroxylated metabolites of DDE have been found in the feces.3. Mice and hamsters excreted 13-21% of an oral dose in the urine within 5 days; the

major urinary metabolites were DDA conjugated with glucuronic acid, glycine, alanine,or serine. A small amount of free DDA was observed in the urine.

4. In humans, DDT is absorbed from the intestinal tract and converted mainly into DDEand DDD; DDD is further metabolized to DDA; DDA is excreted into the urine, butDDT and metabolites are mainly excreted via the bile into the feces.

5. DDT and DDE are highly persistent in adipose tissue and elimination half-lives from fathave been estimated to be about 2 yrs. The depletion rate of DDT and metabolites fromthe body are in the order p,p’-DDA > p,p’-DDD >o,p”-DDT > p,p”-DDT > p,p”-DDEa. Depletion of DDTr (residues of all DDT metabolites) from humans is believed to

be slower than from monkey, dog, and rat.6. Rats treated with o,p’-DDT extensively metabolize it to ring hydroxylated products;

similar reactions occur in humans, but the rate is much slower than in rats;



CHCl ClCCl3

CHCl ClCHCl2

p,p'-DDT p,p'-DDD

CHCl Cl

COOH

p,p"-DDA

conjugation with alanine, glycine, serine,or glucuronide; then excretion

CHCl Cl

CCl2

p,p'-DDE

various hydroxylated metabolites,for ex.,

CHHO Cl

CHCl Cl

HO

CCl2

CCl2

X. Mode of Action (MOA) for Acute ToxicityA. Of all the environmental contaminants, the pesticides are perhaps the most studied from the

perspective of environmental chemistry and toxicology1. With respect to the mode of action, i.e., how toxicity is manifested through biochemical

interactions, pesticides are well studied because they must work against pests yetprovide a margin of safety to the user

B. Insecticides1. For the most part, the insecticides that have been most environmentally contentious are

toxicants with biochemical modes of action in the nervous system2. The ultimate biochemical receptors are at different sites on the axon, either pre or post

synapse3. Because normal nerve physiology is interfered with, a discussion of normal nerve

physiology is necessary to understand mode of action of commonly used insecticidesC. Normal Nerve Physiology overview: (See figure on p. 24)

1. Nerve cell morphologya. neuronb. cell body/nucleusc. axond. dendritee. synapsef. myelin

2. Cell membrane and the resting potential--cell membrane is semipermeable; allows someions to freely diffuse, but other ions cannot; (see upper right hand side of figure onnext page)a. Cell membrane is freely permeable to potassium but not to sodium;b. In addition to sodium and potassium, chloride and organic anions are present;c. The difference in distribution of the ions at equilibrium creates the resting potential,

which is about -56 mV (millivolts) of electrical potential;



1. The inside of the cell is negative with respect to the outside, i.e., it isPOLARIZED

d. The concentration of sodium and chloride is much greater on the outside of the cellthan on the inside;

e. The concentration of potassium and organic anions is much greater on the inside ofthe cell than on the outside

3. Depolarization and the Action Potential (see upper right hand side of figure on nextpage)a. When the nerve is stimulated, proteins in cell membrane (sodium gates) open up to

allow sodium to pass to the inside of the membrane. Shortly thereafter, potassiumpasses out of the cell.

b. The resting potential is depolarized and then hyperpolarized in the opposite directionto about +40 mV.

c. This change in electrical potential causes the appearance of the action potential, anall-or-none electrical response, that is transmitted along the length of the nervethrough a mechanism known as the local circuit current (i.e., a wave ofdepolarization).

4. Repolarization and Return of Resting Potentiala. After the electrical current passes a specific place along the membrane, the sodium

gates close;b. Potassium efflux slows;c. The membrane potential rapidly falls to its polarized resting state

5. Synaptic Transmission (see lower left hand side of figure on next page)a. At the terminus of the axon, there is a tiny gap (synapse) to the next nerve cell (or

axon);b. At the synapse, transmission of the nervous electrical energy is changed to chemical

energy;c. A neurotransmitter chemical called acetylcholine is released from packets (vesicles)

in the presynaptic membrane;d. The acetylcholine molecules diffuse into the synapse;e. Some of the acetylcholine molecules interact with receptors embedded in the post-

synaptic membrane;f. Interactions with the receptors than causes a depolarization in the post synaptic

membrane by the local current circuit mechanism across the rest of the nerve;g. The chemical transmission of the nerve signal across the synapse is terminated by

enzymatic breakdown of the excess acetylcholine molecules in the synapse.1. Enzyme = acetylcholinesterase2. Neuromuscular junction of vertebrates is where acetylcholine-based chemical

transmission occurs; in insects, acetylcholine is used only in the central nervoussystem, but glutamate is used at the neuromuscular junction.



6. Inhibition and Modulation of Signal Transmissiona. Some neurotransmitters inhibit nerve-nerve or nerve -muscle transmission;

1. GABA (gamma amine butyric acid) interacts with receptors on the post-synapticmembrane to keep the membrane in a hyperpolarized state, making generation ofan electrical signal improbable.

b. Some nerves terminate on organs without a specific synaptic junction1. These release chemicals called neuromodulators that modify the

neurotransmitter signals of adjacent nerves;2. For ex., neuromodulators could produce finer gradations of movement in a muscle;3. An example of a neuromodulator at certain neuromuscular junctions in insects is

called octopamine.D. Effects of Chlorinated Hydrocarbon Insecticides on Normal Nerve Cell Physiology

1. Organochlorines (other than DDT and related compounds) for ex., endosulfan, anddieldrin are GABA antagonists.a. Dieldrin may require further metabolism to a trans-diol form

2. DDT (& Dicofol) inhibits Na gate closingE. Effects of OP and CB Insecticides on Normal Nerve Cell Physiology--inhibition of

acetylcholinesterase.1. CB insecticides are considered reversible inhibitors of AChE;2. OP insecticides for the most part are reversible inhibitors, but the enzyme is dealkylated

much slower than with CBs.



a. OP insecticides can undergo an aging process, cleaving off one of the alkyl groupsof the P, leaving the enzyme phosphorylated for very long periods of time. Diethylgroups are much slower to age than deisopropyl groups.

b. OP insecticides in the thio form, i.e., P=S, have to be activated (i.e., metabolized) tothe oxon (P=O) form to be significantly bioactive; (see next table, especially datafor demeton and phorate)

P SCH3CH2OCH3CH2O

SCH2 S C CH3

CH3

CH3

terbufos (Counter)

CH3CH3

CH3CCH2 S

CH3CH2OCH3CH2O

SPO

terbufos oxon3. In other words, the Km for the oxon form is much lower than the Km form for the thio

form.

Cholinesterase (ChE) Inhibition and Toxicity of Organophosphorus Insecticides and Metabolites(refer to section X E) (From Felsot & Pedersen (1991, Am. Chem. Soc. Symp. Ser. 459)

Compound House Fly Head ChE Inhibition(I50, moles x 10-6)

LD 50 (µg/fly)

Malathionoxon

200.0046

Not determined

Demeton, thionosulfoxidesulfoneoxon

oxon sulfoxideoxon sulfone

2203.600.830.0241.100.12

Not determined2.01.20.78.73.7

Demeton, thiolosulfoxidesulfone

3.501.500.60

Not determined0.81.2

Disulfotonsulfoxidesulfoneoxon


>100703.503.501.500.60

Not determined

Phoratesulfoxidesulfoneoxon


253.700.040.500.400.10

1.55.54.51.15.51.5

4. Typical symptoms include headache, giddiness, nervousness, blurred vision, weakness,nausea, cramps, diarrhea, and discomfort in the chest.

5. Typical signs include sweating, miosis, tearing, salivation, excessive respiratory tractsecretion, vomiting, cyanosis, papilledema, uncontrollable muscle twitches followed bymuscular weakness, convulsions, coma, loss of reflexes, and loss of sphincter control.The last four signs are seen only in severe cases but do not preclude a favorableoutcome if treatment is prompt and aggressive.a. Antidotes are available--atropine and 2-PAM;



1. These essentially function as competitive inhibitors of the AChE enzyme, butthese are hydrolyzed much more readily than the insecticides

F. Herbicides (four MOA among many possibilities)1. Inhibition of photosynthesis (triazines)2. Agonist of auxin (plant growth hormone) (phenoxyacetates, i.e., 2,4-D)3. Inhibition of amino acid synthesis (sulfonylureas)4. Inhibition of mitosis (dinitroanilines [trifluralin])

XI. Physicochemical Properties and Phase-Transfer Partition Coefficients for SelectedPesticidesA. Notes that the half-life (T1/2) [time to 50% disappearance] is shown for future reference,

but this value is highly variable and only gives an idea of relative persistence in soil.

Chlorinated Hydrocarbon and Cyclodiene InsecticidesPesticide Cw

(ppb)Kow(log)

Koc(log)

V.P.(mm Hg)

KH(atm.m3/mol

T1/2 Soil(years)

DDT 1.2-5.5 4.9-6.9 5.1-6.3 1.7x10-10 1.29x10-5 5.4-10DDE 40-65 5.7-7.0 5.4 1.6x10-5 1.22x10-3 2.7DDD 20-90 5.1-6.2 5.4 4.7x10-6 2.2x10-5 2.7Aldrin 17-180 5.2-7.4 2.6-4.7 2.3x10-5 5.0x10-4 1

Dieldrin 80-200 3.7-6.2 4.1-4.6 1.8x10-7 5.8x10-5 2.4Endosulfan 280 3.62 3.4 1x10-5 1.9x10-5 0.14

Lindane 21,300 2.8-4.1 3.3 1.7x10-5 2.5x10-7 1.1Methoxychlor 40 3.3-5.1 4.9-5.0 0.33cis-Chlordane 51 5.93 5.4-6.0 8.3x10-5 8.75x10-4 1

trans-Chlordane 8.7-9.7 5.5-6.0 9.8x10-6 1.34X10-3 1

Organophosphorus and Carbamate InsecticidesPesticide Cw

(ppm)Kow(log)

Koc(log)

V.P.(mm Hg)

KH(atm.m3/mol

T1/2 Soil(days)

parathion 10-15 2.2-3.9 2.5-4.2 4x10-4 8.6x10-8 18chlorpyrifos 0.5-1.3 3.3-5.3 3.8-4.1 1.9x10-5 4.2x10-6 60-100phorate 20 2.9-3.9 2.5-2.8 8.4x10-4 6.4x10-6 68-82azinphosmethyl 10-44 2.7-2.8 2.5-3.5 1.6x10-6 7.5x10-9 13diazinon 44-71 3.0-3.8 3.0-3.3 8.5x10-5 1.1x10-7 32malathion 141-164 2.4-2.9 2.61 1.25x10-6 4.89x10-9 1

carbaryl 72-130 2.3-2.8 2.0-2.6 6.6x10-6 1.3x10-5 10carbofuran 291-375 1.6-2.3 2.0-2.3 2x10-5 3.9x10-8 11-65aldicarb 6000 0.7-1.1 0.9-1.7 3.5x10-5 1.5x10-9 9-70propoxur 1740-2440 1.5-1.6 0.5-2.0 9.8x10-6 1.3x10-9 30

HerbicidesPesticide Cw

ppmKowlog

Koclog

KHatm m-3/mol

V.P.mm Hg

T1/2soil, days

atrazine 28-33 2.3-2.8 2.0-2.7 3.0 x 10-9 3.0 x 10-7 42-113simazine 3.5-5 1.9-2.3 2.1 3.4 x 10-9 6.1 x 10-9 45-100cyanazine 171 1.8-2.2 1.6-2.6 3.0 x 10-11 1.6 x 10-9 12-25alachlor 242 2.6-2.9 1.6-2.3 6.1 x 10-8 3.1 X 10-5 6-23



metolachlor 530 2.9-3.5 2.9-3.5 9.2 x 10-9 1.3 x 10-5 15-1002,4-D 890 1.5-4.9 1.7-2.7 2.0 x 10-2 4.7 x 10-3 152,4,5-T 220 0.6-3.4 1.7-2.3 4.9 x 10-8 3.8 x 10-5 14chlorsulfuron 60 (pH 5)

7000 @(pH 7)-1.0 1.02 3.6 x 10-16 2.3 x 10-16 28-42

glyphosate 12,000 -1.60 3.4-3.7 1.4x10-10 7.5x10-6 <60paraquat 620,000 2.4 6 0 125-181trifluralin 4 5.1-5.3 2.9-4.5 4.8 x 10-5 1.1 x 10-4 132

XII. Environmental Chemistry--General OverviewA. Chlorinated Hydrocarbon Insecticides

1. Note that compounds like DDT, Aldrin, Heptachlor, and Chlordane have extremely longhydrolysis half-lives, which would indicate tremendous stability in aquatic systems. Forexample:a. The hydrolysis half-life of chlordane is >197,000 years; the hydrolysis half-life for

endosulfan is 218 hours.2. Major Environmental Degradation Pathways of Chlorinated Hydrocarbon Insecticides

a. DDT------->DDE (other metabolites: DDD, DDA, dicofol)b. Aldrin--------------->dieldrinc. Heptachlor------------>heptachlor epoxided. Endosulfan------------>endosulfan Sulfate, endosulfandiole. Chlordane-------------->chlordene, heptachlor, heptachlor epoxide

3. DDT and DDE residues are still prevalent on lands wherever it was used; for ex. inorchards, where it was used very heavily because of the need for spraying numeroustimes in a growing season, the residues are significant (ppm levels); furthermore, theresidues are still volatilizing.a. A long-term experiment was conducted at the Univ. of Calif. in Riverside; plots were

treated with DDT in 1971; air and soil residues were measured after application(following the growing season) and again in 1994. {Spencer, W. F. and et al. 1996.DDT persistence and volatility as affected by management practices after 23 years. J. Environ.Qual. 25:815-821.)

b. Note the differences in DDE/DDT residues ratio observed in soil between the yearsof treatment (DDTr, which includes all soil transformation products, are shown onthe left hand side of the graph).



c. In the previous graph, note that the majority of residues of DDTr are attributable toformation and persistence of DDE residues.

d. The air residues detected in 1971 above the plots were largely due to DDE residues;even in 1994, DDE still constitutes a large proportion of the air residues althoughthe contribution is much lower than in 1971.

B. Both organophosphorus (OP) and carbamate (CB) insecticides are esters; thus, one wouldexpect high biological reactivity, and in fact, these groups of compounds are highlybiodegradable. (Remember, however, that the tradeoff for some of them in commerce is anincredibly high acute toxicity).1. Note that organophosphate and carbamate insecticides are very susceptible to hydrolysis

at pH≥8.0; below this pH, the compounds are relatively stable in buffered water.However, addition of sediment or mud has shown to cause enhanced degradation ofcertain OPs like parathion.a. Below is an example of chlorpyrifos hydrolysis and oxidation

N

ClClHO

ClC2H5O

C2H5O

S

P- OH

N

ClCl

ClC2H5O

C2H5OP-O

S

N

ClCl

ClC2H5O

C2H5OP-OO

+ +C2H5O

C2H5O

O

P- OHN

ClClHO

Cl

trichloropyridinoldi-ethoxythiophosphate di-ethoxyphosphate trichloropyridinol

b. Another important pathway of loss of OPs from water may be volatilization



2. When carbamates and OPs hydrolyze, they lose their biological activity.3. In soil, many OPs and CBs are biodegraded and substantial quantities of CO2 are

released; there can also be a significant formation of bound residues4. Some OPs are oxidized in the environment; they maintain their biological activity.

a. It was discovered in California orchards under dry conditions in the presence ofozone and dust, that parathion residues on leaves were oxidizing to paraoxon, thetrue anticholinesterase agent. This phenomenon may have been responsible forincreased toxicity to workers. Not much paraoxon, if any is formed in soil.

C. All the currently registered herbicides are biodegradable; most of them are partiallytransformed and end up as bound residues.1. Little CO2 is produced from molecules like atrazine, alachlor, metolachlor, trifluralin, and

2,4,5-T (which is suspended)2. Substantial quantities of CO2 can be produced from mineralization of 2,4-D; this

herbicide is subject to enhanced biodegradation3. Atrazine is probably the most ubiquitously occurring herbicide in the environment

a. It is moderately persistent in soil, and until recently it was not thought to bemineralized to any extent.1. However, soils that have been treated repeatedly in the field with atrazine now

can be shown to be capable of rapidly mineralizing atrazine (i.e., CO2 isproduced during its transformation).

b. Atrazine is the number one most used herbicide in corn production, and thus tens ofmillions of pounds are used each year.1. Its water solubility (~35 ppm) would predict little capability of movement in soil,

but it’s Kd is comparatively small.2. Thus, due to high use and small Kd, atrazine is the most frequently detected

pesticide residue in ground water and surface water.3. Research over the last 10 years has shown that not only is atrazine frequently

detected in water, but its de-alkylated metabolites are also detected. Thesemetabolites are microbial transformation products.a. Hydroxy atrazine is also found in soil, but is believed to be the product of an

abiotic reduction.