Agricultural Chemicals
What is a
pesticide?
Pesticide Definition
 Agricultural chemicals: Pesticides
▫ economic poisons: control, kill, repel pests
History of Pesticides: Early
 Ancient Chinese:
used arsenic to
control insects
 Early Romans:
used common salt
for weed control and
sulfur for insects
History of Pesticides: 1800s
 Pyrethrin
▫ insecticidal
properties
discovered
 Derris elliptica roots
▫ fish poison
▫ isolated rotenone,
used for insect
control
Rotenone
History of Pesticides: 20th
Century
 Establishment of organic chemical
industry
▫ 1920s: petroleum oils to control spider
mites
▫ 1940s: chlorinated hydrocarbon (e.g. DDT)
and phenoxy acid (e.g. 2,4-D) insecticides
▫ 1950s: Triazine herbicides (e.g. atrazine)
▫ Late 20th century: synthetic pyrethroids
Definitions and Terms
 US production about
1.2 billion pounds
 Categories:
▫
▫
▫
▫
▫
fungicides
herbicides
insecticides
rodenticides
fumigants
Key Points
 Unique pollutants in that they are
designed to kill
 Ideally, objective is to be highly
selective
▫ destroy target without effects to nontarget organisms
 Realistically, benefit vs. risk to health
and environmental quality are weighed
Pesticide Formulations
 Active ingredient.
 Formulations: solid or liquid.
 Adjuvants: adhesives,
surfactant spreaders and
emulsifiers.
 Solvents: water, xylene or oil.
Terminology
 Emulsifier
 Carrier
 Adjuvant
 Active ingredient
 Spreader
 Surfactant
 Technical material
Other Classification
Categories
 USEPA Use
Categories
 Carcinogenicity
Classification
Use Category
LD50 (mg/kg)
Category I
≤ 1.0
Category II
1.0< x ≤ 500
Category III
500 < x ≤ 15,000
Classification
Carcinogenic to Humans
Likely Carcinogenic to Humans
Suggestive of Carinogenicity
Inadequate Data
Not Likely Carcinogenic to Humans
Relevant Laws and
Regulations
 U.S. Code of Federal Regulations (CFRs)
▫ Title 7: agriculture
▫ Title 21: food and drugs
▫ Title 40: environmental protection
Relevant Laws and
Regulations
 Insecticide Act of 1910
▫ Label contents accurate and product works
 Federal Insecticide, Fungicide and
Rodenticide Act of 1947 (FIFRA)
▫ Basic authority for pesticide regulation
▫ Evaluation of risks posed by pesticides
▫ Amended in 1988 for reevaluation of early
pesticides with current test protocols
▫ Enforced by the USEPA
Relevant Laws and
Regulations
 Clean Water Act (1972)
▫ Permit required for any discharged
pollutants
 Endangered Species Act (1973)
▫ Regulation of pesticide use surrounding
wildlife sanctuaries
▫ Concern for transport of chemical to non
target organisms
Relevant Laws and
Regulations
 Safe Drinking Water Act (1974, 1986,
1996)
▫ Maximum contaminant levels (MCLs) for
public drinking water systems established
 Resource Conservation and Recovery
Act (RCRA) (1996)
▫ Regulate transport, storage and disposal of
hazardous substances
Relevant Laws and
Regulations
 Comprehensive Environmental
Response, Compensation, and Liability
Act (CERCLA or Superfund) (1980)
▫ Cleanup hazardous waste sites
 Superfund Amendments and
Reauthorization Act (SARA) (1986)
▫ Increased stakeholder involvement and fund size
▫ Seek reimbursement for cleanup costs
Relevant Laws and
Regulations
 Worker Protection Standard
for Agricultural Pesticides
(1994)
▫ Designed to prevent
occupational exposures
▫ Employers provide safety
training, access to labels,
medical treatment
▫ Covers use on farms, forests,
nurseries, and greenhouses.
Relevant Laws and
Regulations
 Food Quality Protection Act (1996)
▫ Amendment to FIFRA and FFDCA, as
applied to food crops
▫ “reasonable certainty of no harm”
from residues found on food
▫ Aggregate
(multipathway/multimedia) risk
▫ Cumulative (multipollutant) risk
Organochlorine Insecticides
Organochlorine Insecticides
 Insecticides containing
chlorine
▫ e.g. DDT, heptachlor, dieldrin,
aldrin, lindane, chlordane
 Neurotoxicants
▫ Acute effects caused by
interference with nerve
transmission along axons
Organochlorine Insecticides
Major Groups of OCs
 DDT and its analogs
 Hexachlorocyclohexane
 Cyclodienes and similar compounds
 Toxaphene and related compounds
 Mirex and Chlordecone
Organochlorine Insecticides:
DDT
 1,1,1-trichloro-2,2-bis(p-chlorophenyl)
 First synthesized in 1874, but insecticidal properties not
determined until 1939
 US used during WWII to reduce vector-borne diseases (e.g.,
malaria, typhus)
 Widespread agricultural and private use followed owing
partly to its persistence
 Alters sodium-potassium transport across axonal membrane
such that repolarization does not occur
 Rachel Carson’s book Silent Spring published in 1962 led to
eventual ban of chlorinated pesticides in 1972
DDT Synthesis
DDT Toxicity
 p,p’-DDT
▫ LD50s:
−1.7 mg/Kg in honeybees
−113 mg/Kg in rats
−205 mg/Kg in Japanese beetles
−1296 mg/Kg in pheasants
−2000 mg/Kg in bullfrogs
 DDE and DDD
▫ Very few acute effects.
Human Toxicity
Environmental Transformation of
DDT.
DDE
 Most problematic of all
the derivatives of DDT.
 Very persistent.
 Bioconcentrates in biota.
 Main responsible for
egg-shell thinning in
several bird species.
Organochlorine Insecticides:
Issues
 Persistence
▫ Despite ban, continue to be found in the
environment
 Bioaccumulation Potential
▫ Transfer of residue through food chains +
environment.
 Lipophilicity
 Suspected Carcinogenicity
Organophosphorus Insecticides
Organophosphorus
Insecticides
 Phosphoric or thiophosphoric acid esters (OPs)
▫ e.g., ethylparathion, chlorpyrifos, parathion, malathion,
leptophos
 Neurotoxicants
▫ Acetylcholinesterase inhibitors, results in excess
acetylcholine in nerve tissue and effector organs
▫ Generally low mammalian toxicity (carboxylesterases)
▫ Delayed toxicity, organophosphorus-induced delayed
neuropathy (OPIDN)
− Paralysis of distal muscles 7-10 days after ingestion
 Relatively nonpersistent, susceptible to hydrolysis
Organophosphorous
Insecticides
Structure
Name
LD50 (mg/kg)
Ethyl Parathion
3 – 13
Diazinon
250 – 285
Chlorpyriphos
> 3000
Malathion
1000 - 1375
Carbamate Insecticides
 Esters of N-methyl carbamic acid (H2NCOOH)
▫ e.g., carbaryl, aldicarb, propoxur
 Neurotoxicants
▫ Acetylcholinesterase inhibitors
▫ Inhibition is rapidly reversed in comparison with
OPs
▫ High mammalian toxicity
 Relatively nonpersistent, readily hydrolyzed
Carbamate Insecticides
Structure
Name
LD50 (mg/kg)
Carbaryl (Sevin)
250 - 550
Propoxur (Baygon)
100
Aldicarb (Temik)
~1
Botanical Insecticides
 Extracts from plants
 Nicotine first used as
insecticide in 1763
 Oral LD50: 83 mg/kg
 Dermal LD50: 285
mg/kg
 Respiratory toxicant
Nicotine
Botanical Insecticides
 Pyrethrins (6 acids and esters)
▫ extracted from chrysanthemums
 Low mammalian toxicity
 Nonpersistent, degraded by heat/light
Pyrethroid Insecticides
 Designed from
pyrethrin structure
for added
photostability
 Two basic structures
▫ Cyclopropane ring
−cypermethrin
▫ No cyclopropane ring
−fenvalerate
Herbicides
 Used to control weeds, fairly specific
 Widespread usage (half of pesticides)
 Typically inhibit plant specific enzymes
▫ Low mammalian, fish, insect, bird toxicities
 Continual usage however leads to
environmental contamination
Herbicides: Agent Orange
 Combination of 2,4-D and 2,4,5-T
 Used during Vietnam War as defoliant
 TCDD identified as a toxicological
contaminant in mixture
▫ High mammalian toxicity (0.022 mg/kg)
Toxicity of TCDD
 Related to each individual congener.
 Toxicities may vary a lot among the
different congeners.
 Toxic equivalency factors:
▫ A representation of the fraction of TCCD
toxicity manifested by a group of
congeners found in a mixture.
▫ A mixture having 50% of each TCDD and
hexaCDD:
−total toxicity of 0.505 times the toxicity of
Herbicides: Triazines
 Atrazine, cyanazine, simazine
 Relatively nontoxic (LD50:
3,100 mg/kg)
 Readily transport to water
systems
 Possible human carcinogen
however
Herbicides: Paraquat
 Member of the bipyridyl family of
herbicides
 Highly water soluble but binds tightly to
soil particles
 Respiratory toxicant following ingestion
▫ LD50: 150 mg/kg
Fungicides
 Used to prevent crop loss and health effects
from toxins released by fungi
▫ e.g., chlorothalonil
▫ relatively nontoxic
 Dithiocarbamates
▫ e.g., mancozeb
▫ relatively nontoxic but hydrolyze to produce
carcinogens such as ethylenethiourea (ETU)
Rodenticides
 Used to prevent losses in grain and
other food storage facilities
 Generally restricted use only
▫ e.g., warfarin, an anticoagulant
−lack of blood clotting leads to bleeding to
death (LD50: 3.0 mg/kg)
Rodenticides: Others
 Fluoroacetamide: supplied as bait
pellets or grains
▫ LD50: 15 mg/kg, fast acting
 ANTU (a-naphthylthiourea)
 Strychnine
 Thallium salts
Fumigants
 Used to protect stored products (e.g.,
grains) and kill nematodes
 Hazardous due to potential for
inhalation exposure (restricted use only)
▫ e.g., methyl bromide
▫ LD50: 0.06 mg/L (inhalation, rat)
▫ Effects include respiratory distress, cardiac
arrest, and CNS effects
Biocontrol Pesticides
 Uses “natural” mechanisms for combat
▫ Parasites (Aphytis lepidosaphes, wasp)
▫ Predators (Neoseliulus califirnicus, spider
mite)
▫ Attractants (pheromones)
 Nontoxic to humans
Enhanced Seed
 Uses traditional breeding and
genetic engineering to
▫ tolerate herbicides
▫ resist insects and disease
 Bt (Bacillus thuringiensis)
gene inserted into plants
 Nontoxic to humans however
▫ Genetically modified seed or
organism (GMO) controversy
References
 1-Crosby, D.G. Environmental
Toxicology and Chemistry. 2000. Oxford
University Press, New York.
 2- Hodgson and Levi (Eds), A Textbook
of Modern Toxicology, Second Edition.
Appleton and Lange, Connecticut, 1997.
Additional Resources
 U.S. EPA on pesticides:
http://www.epa.gov/ebtpages/pesticides.htm
l
 Code of Federal Regulations:
http://www.access.gpo.gov/nara/cfr/
 The Endangered Species Act:
http://endangered.fws.gov/esa.html
Credits
 Lecture Slides contributed by:
▫ Dr. Ross B. Leidy
▫ Dr. Alicia Chaves
 Online lecture prepared by:
▫ Dr. Alicia Chaves
▫ Dr. Stephen Graham