Unit 8: Toxicity, Resistance,
Residues, Licensing
Chapters 13-16
Unit 8: Toxicity, Resistance,
Residues, Licensing
• Unit 7 Objectives:
– Discuss precautions and risks
– Prevention and treatment concerns
– Guarding against residues
– Licensing concerns and procedures
Unit 8: Toxicity, Resistance,
Residues, Licensing
• Toxicity of Herbicides
– Toxicity – capacity of a substance to produce
injury
• Effects
– Immediate (acute)
– Cumulative (chronic)
– Depends on:
» Exposure
» Dose
» Herbicide
» Duration
Unit 8: Toxicity, Resistance,
Residues, Licensing
• Other factors
– Species (animal or plant)
» Age, sex, nutritional status
– Route of exposure
– Human Toxicity
• Manufacturers required to test for toxicity levels
– Tested on experimental animals
» Mice, rats, rabbits
• Expression of acute toxicity
– LD50
» Lethal dose in mg/kg BW
» Kills 50% of test population
Unit 8: Toxicity, Resistance,
Residues, Licensing
• Label indicates extent of toxicity w/ signal words
– What are they?
– Signal word applies to most serious method of exposure
– Danger-Poison
» Herbicides w/ endothall or paraquat
» Wear goggles, face shield, rubber gloves, rubber
apron, avoid breathing spray mists
– Danger-Corrosive
» Irreversible eye or skin burns – wear goggles or face
shield, rubber gloves, apron
» Eyes - Flush w/ water 15 minutes, get medical
attention promptly
» Skin – wash thoroughly, seek medical attention if
irritation occurs
Unit 8: Toxicity, Resistance,
Residues, Licensing
– Warning
» Moderate toxicity through oral, dermal, or inhalation
exposure
» Warnings and risks are listed thoroughly
– Caution
» Any label that doesn’t have Warning or Danger on
the label
» Low oral, dermal, or inhalation toxicity risk
» Little/no irritability of eyes or skin
– Environmental Toxicity
• Toxicity regarding fish and wildlife
– Labels may warn of specific toxicities
Unit 8: Toxicity, Resistance,
Residues, Licensing
– Fish, groundwater, etc.
– Read the labels thoroughly, see the tables in the book
for specific ingredients
• Weed Resistance to Herbicides
– Resistance is a continual problem
• Incidence has increased the last decade
– Populations within a species of weed can develop
resistant tendencies not observed in the typical
population
– First reports of resistance in early 1950’s (dandelions,
wild carrots)
Unit 8: Toxicity, Resistance,
Residues, Licensing
– >170 weeds reported to have resistance qualities
– Herbicide resistance
• Inherited ability of plant to survive and reproduce following
exposure to chemical
• Normal lethal dose
– Herbicide tolerance
• Inherent ability of a species to survive and reproduce after a
chemical treatment
– Resistance – plant is resistant when species is
usually susceptible
– Tolerance – species has never been susceptible to
the herbicide
Unit 8: Toxicity, Resistance,
Residues, Licensing
– Cross-Resistance
• Resistance to a herbicide the plant may not have
previously been exposed to
• Similar mode of action to a previous herbicide
– Multiple-Resistance
• Resistance to more than one class of herbicides
• Different modes of action
• More than one basis for resistance may be
involved
Unit 8: Toxicity, Resistance,
Residues, Licensing
– Ex – developing resistance to atrazine and
simazine in continuous corn
• Cross resistance
• Multiple resistance
– Origin of Resistance
• Mutation Theory
– Genetic mutation occurs following herbicide application
– Mutation confers resistance to the plant
– Not widely accepted, not much evidence to support
Unit 8: Toxicity, Resistance,
Residues, Licensing
• Natural-selection Theory
– Regarded as most plausible theory
– Herbicide resistant types have always existed in low
populations
– Only those who survive and reproduce will make seed for
the future
– “Survival of the fittest”
• If same herbicide is used each year, resistant
varieties will survive and flourish
– Relying on the same herbicide creates more resistance
– 3 components
» Herbicide w/ single site action
» Repeated use of same herbicide
» Absence of other control measures
Unit 8: Toxicity, Resistance,
Residues, Licensing
– Basis for Weed Resistance
• Alterations in the target sites of the herbicide
– Herbicide activity is site specific in the plant
» Disrupts a particular plant function/process
– Some alteration may interrupt chemical ability to work
effectively
– Most resistance is developed in this manner
• Enhanced Metabolism of the Herbicide
– Plant “digests” the chemical to detoxify itself
– May inactivate the herbicide before it has the chance to
work
Unit 8: Toxicity, Resistance,
Residues, Licensing
– Management Strategies to Minimize
Herbicide-Resistant Weeds
• Goal is to prevent the selection of herbicide
resistant weeds
• Don’t necessarily want to develop a new herbicide
for each new weed problem
– Improve management techniques
• Strategies:
– Scout regularly to identify resistant weeds
» Record and response quickly to changes in weed
populations
» Restrict resistant weed development
Unit 8: Toxicity, Resistance,
Residues, Licensing
– Rotate herbicides w/ different sites of action
» No more than 2 applications consecutively
– Apply herbicides w/ multiple action sites
» Tank mixes, prepackaged, sequential mixes
» Chemicals must have substantial activity against
resistant weeds
– Cautious use of herbicide resistant or tolerant crops
» Still honor 2 consecutive application rule
– Combine mechanical and herbicide controls
– Clean tillage and harvest equipment to prevent crosscontamination
– Pay attention to RR, highway depts., etc. and their
control methods
» May need to help them adjust to prevent a resistant
development
Unit 8: Toxicity, Resistance,
Residues, Licensing
• Correct Diagnosis of Resistant Weed problems
– All other causes of herbicide failure are eliminated
» What might they be?
– Other labeled weeds are effectively controlled
– History of continuous use of the same herbicide
– Weed was controlled effectively in the past
• Identified by just a few weeds left standing after
herbicide should have controlled
– May be confined to small patches
– Patches may grow
– Lets review the scenario on pg. 336
Unit 8: Toxicity, Resistance,
Residues, Licensing
• Herbicide Persistence and how to Test for
Residues in Soils
– Factors Affecting Herbicide Persistence
• Herbicides applied for season-long control
• Length of time herbicide remains active called “soil
persistence” or “soil residual life”
– Anything that affects the herbicide may affect persistence
• Factors affecting herbicide persistence
– Soil factors
– Climatic conditions
– Herbicidal properties
Unit 8: Toxicity, Resistance,
Residues, Licensing
– Soil Factors
• 3 categories
– Physical
» Soil composition - % sand, silt, clay
» Organic matter content
– Chemical
» Soil pH
» CEC
» Nutrient status
– Microbial
» Type and abundance of soil microorganisms
» How can this affect herbicides?
Unit 8: Toxicity, Resistance,
Residues, Licensing
• Soil composition affects phytotoxicity & persistence
– Adsorption, leaching, volatilization
– Soils high in: clay, organic matter
» Higher potential for herbicide carryover
» Increased adsorption to soil colloids
» Decreased leaching & volatilization
» Results in decreased initial plant uptake & herbicidal
activity
» More herbicide held in reserve – may injure future
crops
Unit 8: Toxicity, Resistance,
Residues, Licensing
• Triazines
– Very affected by soil pH
» Lesser amounts of herbicide are held to soil colloids
at higher pH
» Remain in soil solution (only herbicide in soil solution
can be taken up by the plant)
• Chemical/Microbial breakdown slower at higher pH
• CEC
– Some herbicides more available in the presence of some
cations
– Others “tied up”
Unit 8: Toxicity, Resistance,
Residues, Licensing
• Soil Microorganisms
– Types and amounts determine rate of herbicide
decomposition
– What factors affect microbial activity?
– Climatic Conditions
• Factors involved:
– Moisture
– Temperature
– Sunlight
• Positive relationship between herbicide
degradation and temp & moisture
– Why? What is the primary culprit?
Unit 8: Toxicity, Resistance,
Residues, Licensing
• Cool/dry conditions create possibility for carryover
• Wet/mild winters increase likelihood of herbicide
persistence
• Photodecomposition
– May occur in surface applied herbicides
– Soil incorporation will decrease the risk
– Herbicidal Properties
• Important factors
– Water solubility
» Determines leaching potential
» Where might the herbicide go?
– Soil adsorption
» Those adsorbing strongly have greater potential to
persist, less likely to leach
Unit 8: Toxicity, Resistance,
Residues, Licensing
– Vapor Pressure
» Determines its volatility (changing from
liquid/solid/gas)
» Increases w/ temp
» Certain herbicides must be incorporated to avoid
gaseous losses
– Susceptibility to chemical/microbial degradation
» Ex. 2,4-D decomposes very quickly, atrazine – slow
» Dependent on herbicide makeup and microbial
population
Unit 8: Toxicity, Resistance,
Residues, Licensing
– Avoiding Herbicide Carryover
• Apply correct rates of herbicide for the soil type
and weed problem
– Lowest possible rate that produces the desired effect
» Determine acreage accurately
» Measure chemical accurately
» Proper calibration
» Uniform application
• Proper method and time of application
• Amount of tillage
– Tillage encourages herbicide decomposition
Unit 8: Toxicity, Resistance,
Residues, Licensing
– Minimum and No-till leave more herbicide near the soil
surface
– Persistent herbicides may affect susceptible crops
– Higher herbicides rates often used in reduced tillage
systems
» Some tillage may help dilute persistent herbicides
• Herbicide combinations
– Reduces risk of carryover
– Mixing will reduce application rates of both herbicides
while broadening control spectrum
– Some herbicides may interact to cause crop injury, if
carryover in enough amounts
Unit 8: Toxicity, Resistance,
Residues, Licensing
• Plant extraction also removes much herbicide from
the soil
• Crop rotation
– Awareness of carryover risks and following crop
susceptibility will minimize injury
– Testing for Herbicide Residues
• Soil chemical test
– Can be expensive
– Completed in a laboratory
• Bioassay
– Cheaper
– Field test
• May help make crop rotation adjustments if
carryover injury risk is present
Unit 8: Toxicity, Resistance,
Residues, Licensing
• Soil Collection & Preparation
– Early to mid-spring before planting
– Representative sample from no more than 20 ac
» Take separate samples from areas where excessive
residues are expected (turnaround points & end
rows)
– Sample at 6” depth
– Sample an area not suspect (“check” area)
» Nearby fencerow
» Untreated area
– Submit for analysis ASAP after sampling to improve
accuracy
» Refrigerate if samples can’t be submitted promptly
Unit 8: Toxicity, Resistance,
Residues, Licensing
• Bioassay
–
–
–
–
May predict potential crop injury
Cheap
Doesn’t measure amount of herbicide present in the soil
Field Bioassay
» Plant a test strip in the field
» Realistic?
– Indoor Bioassay
» Same procedure for soil collection
» Air-dry samples
» Plant test crop in pots and let them grow in a warm
area (greenhouse, inside your home, etc.)
» May be more realistic?
Unit 8: Toxicity, Resistance,
Residues, Licensing
• Testing for Specific Herbicide Groups
– Triazine Residues
» Atrazine, Princep
» Oat bioassay works best
» Place in warm location w/ ample sunlight
» Symptoms should occur within 10-14d
» Characterized by chlorosis, then necrosis of leaf
tissue
– DNA Residues (dinitroaniline)
» Treflan, Prowl, Pendimax
» Sorghum or corn root bioassay
» Wrap in moist paper towel 2-3d
» Allow to germinate then plant
Unit 8: Toxicity, Resistance,
Residues, Licensing
» Observe root formation at 10-14d
» Looking for inhibited root development – stunting,
stubbing, lack of root hairs, leaves that don’t unroll,
thickened hypocotyl
– Imazaquin, Imazethapyr, Chlorimuron Residues
» Scepter, Backdraft, Pursuit, Pursuit +, Extreme,
Lightning, Classis, Canopy XL, Synchrony STS
» Affect root and shoot development
» Symptoms – inhibited root development, stunting,
leaf striping (interveinal chlorosis)
» Use same sorghum/corn bioassay
» Observe at 14-21d
Unit 8: Toxicity, Resistance,
Residues, Licensing
– Command Residues
» Inhibits production of photosynthetic pigments
» Emerge lacking green color
» May appear as chlorosis or bleaching
» Test w/ oats and wheat
» Observe at 10-14d
» Affected plants will be white
• Pesticide Licensing Requirements & RUP
Listings
– IL Pesticide Act Licensing Requirements
• License required by anyone who purchases or
applied RUP’s
Unit 8: Toxicity, Resistance,
Residues, Licensing
• Anyone applying a RUP or general use pesticide in
the course of employment
• One using a general pesticide on their own
property is exempt
• Dept of Ag certifies and licenses
– U of I Extension provides training and study materials
• Dept of Public Health certifies those using
pesticides in/on manufactured structures
– Nuisance bird control, rodents, insects, etc
– Not covered in this manual
Unit 8: Toxicity, Resistance,
Residues, Licensing
• Types of Licenses
– Private Applicator License
» Required for people applying pesticides to an ag
commodity on property they own/control
» Must pass Grain Fumigation exam in order to
fumigate their own bins
» $15 for 3 yr. license (w/ or w/out G.F. cert.)
– Pesticide Dealer License
» Individual selling RUP
» Test, license, fee required
» Keep records of RUP sales for 2 yrs.
» Commercial Applicators & Structural Pest Control
Operators are exempt from the test & fee, must
register as a dealer
Unit 8: Toxicity, Resistance,
Residues, Licensing
» $100 for 1 yr license
» Certification exam good for 3 yrs
– Commercial Applicator & Operator License
» Those who use or supervise the use of general or
RUP for hire
» $45 for 1 yr applicator license, $30 for 1 yr operator
license
» Exam good for 3 yrs
– Public Applicator & Operator License
» Those who use or supervise general or RUP as an
employee of a state agency, municipality, or other
governmental agency
Unit 8: Toxicity, Resistance,
Residues, Licensing
» No fee, certification good for 3 yrs
– Commercial Not-for-Hire Applicator & Operator License
» Those who use/supervise the application on an
employer’s property as part of their responsibilities
» No fee, license good for 3 yrs
• Applicator vs. Operator License
– Applicators
» Responsible for purchasing, storage, handling, use
» Each organization must have at least one person
licensed as an applicator at each facility location
» Usually: owner, supervisor, foreman, etc.
» May supervise operators
Unit 8: Toxicity, Resistance,
Residues, Licensing
» Pass general standards exam (100 questions) –
general of aerial general
» Pass one or more category exam (50 questions ea) –
specialized topics, depends on sites where company
will use pesticides
» Complete the license application – after passing
exams, send to Dept of Ag within 90d, must retest
after 90d
» Provide certificate of insurance – property coverage,
must also be sent w/ application
– Operators
» User of pesticides on the job site
» Tied to applicator’s license
Unit 8: Toxicity, Resistance,
Residues, Licensing
» Cannot be licensed w/out being under an applicator
» Operator cannot legally apply for a different licensed
applicator
» Operator can only apply under direct supervision of
an applicator, only on areas covered by applicator’s
licensure
» Operator cannot legally apply if applicator is out-oftown or unavailable
» Provide proof of employment by licensed applicator
» Pass general standards exam
» Complete license application
Unit 8: Toxicity, Resistance,
Residues, Licensing
• Technical Categories of Licensure
– Each category requires specific professional knowledge
– Applicator license must encompass all areas in which
they might apply a pesticide
– Aquatic
» Weed control in standing/running water
– Demonstration & Research
» Use during research or teaching of
pesticide/equipment use
– Field Crop Pest Control
» Use in corn, SB, forages, etc.
Unit 8: Toxicity, Resistance,
Residues, Licensing
– Forest Pest Control
» Use in forest, forest nurseries, forest seed producing
areas
– Fruit Crop
» Fruit & nut crops
– Grain Facility
» Noncommercial use in/around grain elevators or
grain holding facilities, transport facilities, etc.
– Grain Fumigation
» Use by private applicators to treat stored grain on
own property
– Livestock
» Livestock application or in barns
Unit 8: Toxicity, Resistance,
Residues, Licensing
– Mosquito Control
– Ornamental
» Trees, shrubs, ornamental plantings
– Plant Management
» Portable plants used for interior landscaping
– Regulatory Pest
» By government employees for control of regulated
pests
– Right-of-Way Pest Control
» Weed control on noncrop sites – parking lots, roads,
etc.
– Sewer Line Root Control
– Seed Treatment
– Soil Fumigation
– Turf Pests – turf & sod farms
– Vegetable Crops
Unit 8: Toxicity, Resistance,
Residues, Licensing
– Restricted Use Pesticides
• Can only be sold to licensed applicators
• Tracked carefully
• New products carefully tested and classified
– Record-Keeping Requirements
• Requirement of all licensed applicators
– Under the 1990 Farm Bill
– In IL since the 1988 Pesticide Act
• No specific form, any form will work as long as all
data is complete
Unit 8: Toxicity, Resistance,
Residues, Licensing
• Private Applicators
– Record w/in 14d of application
– Maintain for at least 2 yrs
– Records must include the following:
» Brand, product name, EPA registration #
» Total amount applied
» Location of the application
» Size of the are treated
» Crop, commodity, stored product, site to which RUP
was applied
» m/d/yr of application
» Name and certification # of applicator who
applied/supervised
Unit 8: Toxicity, Resistance,
Residues, Licensing
– <1/10 ac application (spot application) – less stringent
data collection required
» Greenhouses and nurseries excluded from spot
application
• Commercial Applicators
– Must supply records to the customer w/in 30d of RUP
application
– Maintain records 2yrs from application date
– Requirements
» Brand, product name, EPA registration #
» Amount applied
» Use site
» m/d/yr of application
Unit 8: Toxicity, Resistance,
Residues, Licensing
• For more information:
– Contact your local U of I Extension office