Download Herbicide Mode of Action - Pesticide Education Program

Cecil Tharp, MSU Pesticide Education Specialist
Do you know what mode of
action means?
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Do you know the mode of action of all
pesticides you use?
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What is the mode of action?
Mode of Action
Sequence of events from absorption into
the plant until the plant dies
Systemic herbicides – translocated from
site of entry to area in plant where they
are active. Glyphosate
 Contact herbicides - kill only the area
where they are applied. Paraquat

Further Break Down into Groups
Weed Science Society of America has
classified all modes of action into
groups.
 These groups are usually on the
pesticide product label. This is for
Milestone.

What does a group 4 mode of
action mean?


Use the handouts available.
This would be a growth regulator
herbicide; or a synthetic auxin
herbicide.
 Causes abnormal growth, extended
leaf terminals, curled leaves and
eventual death
 These target broadleaf weeds
 Many products including 2,4-D,
dicamba, picloram, clopyralid,
aminopyralid, aminocyclopyraclor,
milestone etc.
Common Leaf Symptoms from PGR
Exposure
normal
cupped and blistered from
PGR exposure
We have many modes of action

ACCase inhibitors (Group 1)
 Inhibit ACCase enzyme in grasses. Stunting,




browning of grasses,
Used primarily in broadleaf crops
Some may be used to selectively control grasses in
grass crops.
Poast, Puma, Select
Sethoxydim, clethodim, fenoxaprop
Aromatic Amino – Acid Synthesis
Inhibitors
Group 9.
 Glyphosate
 Inhibits amino acid synthesis.

 Slow process that may take weeks.
 Broadspectrum, yellowing of plants, stunting
Plateau or Escort Injury Symptoms
Stunting
Chlorosis of
youngest tissue
ALS Inhibitors
Inhibit amino acid synthesis
 Sulfonylurea herbicides

 Vulnerable to low pH conditions
Also imazapic, imazamox, imazethapyr
 Resistance and cross-resistance is
common.
 Targets a wide variety of grasses and
broadleafs while protecting crops
(selective; read label)

Do you mix your modes of
action?
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Who Cares?

Why do we need to group our modes of
action?
Plants, Diseases and Insects often develop
resistance to modes of action.
History of Weed Resistance
The first reported case of herbicide
resistance in the United States was
reported in the 1950’s. Field bindweed
resistant to 2,4-D was reported in Kansas
in 1964.
 The greatest number has been observed
with the ALS inhibitors (imidazolinones,
pyrimidinylthiobenzoates, sulfonylureas,
triazolopyrimidines).
 25 herbicide families have confirmed
resistance to various weeds.

Glyphosate No Longer Effective in
Managing Kochia
From 1984 – 2013
 Resistant to:

 Photo-System II – 5 (atrazine)
 ALS Inhibitors – 2 (chlorsulfuron, etc.)
 Sythentic Auxin – 1 (dicamba, fluroxypyr)
 EPSP Synthase Inhibitor – 9 (glyphosate)
Wild Oat Resistance
Cell elongation inhibitors (8)
 Lipid inibitors (8)

 Thiocarbanates
 Difenzoquat

ALS Inhibitors (2)
 Imazamethabenz methyl

ACCASE inhibitors (1)
 Clodinafop
 Diclopop-methyl
 Tralkoxydim
Resistance to Persian Darnel,
Green Foxtail and

Persian Darnel
 ACCASE inhibitors (1)
○ Diclofop-methyl

Green Foxtail
 ACCase inhibitors (1)
○ Diclofop-methyl
○ Sethoxydim

Horseweed
 EPSP synthase inhibitors (9)
○ Glyphosate
How does the plant develop
resistance?
How can you combat resistance?

Mix modes of action from application to
application
 Don’t stay with only one mode of action

Or use multiple modes of action in a tank
mix
 Some pesticide products already deliver multiple
modes of action
Use highest rates
 Use multiple control tactics or integrated
pest management.

Are there other reasons to
combine modes of action?
Tank Mixing Multiple Products

Can cause unexpected outcomes
 Enhancement
○ Increases efficacy by adding adjuvants
 Synergism
○ By combining chemicals increased efficacy of
either product is achieved
 Antagonism
○ Efficacy is lowered by combining the
chemicals
 Incompatabilities
○ 2,4-D and glyphosate in hard water situations
Common Mixing Issues

WPs and ECs: they can form a putty or
paste with an oily layer that floats on the
top of the tank.
 Many physical incompatibility problems
Combinations of pesticides (ECs, for
 example) are mixed with liquid fertilizers.

 A few pesticides are available in special
fertilizer-grade formulations that reduce
incompatibility problems.

Some poor mixtures result in lower efficacy
from water issues.
Alkaline hydrolysis: High pH
Water
Dissociation is the breaking down of a complex molecule into simpler units.
 Efficacy often goes down when molecules are dissociated. Plants often won’t
absorb the chemical as well. At times this may totally inactivate the pesticide.
 pH 6 – 7 is ideal for most pesticides but it can vary.
 Weak Acid Herbicides are the most susceptible to alkaline (high ph)
dissociation
 Roundup® (glyphosate)
 Pursuit® (ammonium salt of imazethapyr)
 Liberty® (glufosinate ammonium).
 2-4D salt

Hardness of Solution Can Cause
Problems in Tank Mixes
Hardness is the
concentration of multivalent cations (positively
charged ions from minerals)
 Cations bind with negatively
charged pesticide
molecules

 pH and hardness work
together to reduce efficacy
 Forms insoluble salts
When is it a problem?

If the sum of the concentration (ppm) for the cations
exceeds 150 ppm action should be considered:
 2,4-D amine (> 150 ppm)
 totally deactivated at 500 ppm)







Dicamba (> 150 ppm)
Glyphosate (> 150 ppm)
Clopyralid (>150 ppm)
sethoxydim ‘Poast’ (>150 ppm)
Imazethapyr ‘Pursuit’ (>150 ppm)
Reduces efficacy of many
surfactants
Scale may plug sprayer
If you have a problem

Add an adjuvant containing sulfate or
organic acids as they bind with hard
minerals
 non-ionic surfactants increase efficacy
 Add ammonium sulfate (8.5 – 17 lb / 100
gallons)
Prior to tank mixing
Read the pesticide product
label on compatibility issues or
recommended tank mixes.
 Call your product
manufacturer for more
information prior to mixing.
 Last Resort: Can perform a jar
test

 Check for heat, gumming, foam
and scum
Order of Mixing

Always consult the label of each product
for product-specific mixing instructions
1. Fill the tank ¼ to ½ with carrier and
begin agitation
2. Add compatability or defoaming
agents if needed
3. Pesticides, in the order of WP, DF,
WDG, F, FL, ME, EC, S, SP
4. Crop oils or surfactants
Do you know what mode of
action means?
0%
0%
0%
ay
be
0 of 50
M
3.
No
2.
Yes
No
Maybe
Ye
s
1.
Do you know the mode of
action of pesticides you use?
0 of 50
0%
0%
No
2.
Yes
No
Ye
s
1.
Do you mix your modes of
action?
0 of 50
ly
na
l
as
io
0%
r
0%
Ne
ve
0%
Oc
c
3.
nt
ly
2.
Consistently
Occasionally
Never
Co
ns
ist
e
1.
Thanks
Cecil Tharp
Montana State University
Bozeman, Montana
(406)994-5067
[email protected]