Effective Pesticide Application
Reeves Petroff
Pesticide Education Specialist
MSU Extension
http://MTPESTICIDES.org
A Basic Understanding of
Effective Pesticide Application
Should not be Confusing !
Or Painful !
but it can be!!
The Pesticide Application Process
The Spray Solution
Atomization
Transport to Target
Impaction / Deposition
Post-Impact Drop Behavior
Spreading, Retention, Penetration, Translocation
Contact Action
Systemic Action
Biological Effect
Spray Solution - Water
•
Primary diluent
•
Large percentage of spray solution
volume
•
Can have a negative influence on
pesticide performance.
pH
Minerals
Acidity / Alkalinity (pH)
Affects:
a. The chemical stability of some pesticides
b. The dissociation and subsequent
penetration of weak-acid herbicides
c. The physical stability of the formulation
upon dilution
Chemical Stability of Pesticides
as related to pH
Many pesticides are unstable under alkaline conditions
that is pH levels above 7.0
Alkaline hydrolysis.
Stability is usually referenced in terms of “half-life”.
What is Meant by the Term
“Half-Life”?
It is the time required for degradation
to 50% of the original amount of the
pesticide
It is used as a standard to describe
the relative stability of a pesticide.
Examples of Stability
relative to pH
Insecticides
Bendiocarb
Carbaryl
Dimethoate
Trichlorfon
Fungicides
Captan
Iprodione
Half-Life
45 min pH 9
3.2 hrs pH 9
48 min pH 9
63 min pH 8
8.3 hrs pH 7
2 min pH 10
< 1 hr pH 9
Example of Instability
Alkaline Hydrolysis produced by high pH
Dimethoate
At high
(alkaline) pH,
_
OH ions attack the
molecule here
S
(CH3O)2 - P - S - CH2CONHCH3
Producing
S
(CH3O)2 - P - OH
+
H - S - CH2CONHCH3
Molecules that are inactive as insecticides
Chlorpyrifos
At _high (alkaline) pH,
OH ions attack the
molecule here
N
S
(CH3O)2 - P - O
Cl
Cl
Cl
N
S
(CH3O)2 - P - OH
OH
+
Cl
Cl
Cl
Examples of pH and Stability
Herbicides
Clodinafop
Diclofop
Flumiclorac
Half-Life
2.5 hrs pH 9
12 hrs pH 9
6 min pH 9
pH also affects the Ohio group of
herbicides
Weak Acids - OH
Examples of Weak-Acid Herbicides
Clethodim (Select)
Clopyralid (Curtail)
Dicamba (Banvel, Clarity)Endothal
Fluazifop (Fusilade)
Glyphosate (Round Up, Accord, Ranger, Glyphos)
Imazamox (Raptor)
Imazapyr (Arsenal)
Imazethapyr (Pursuit)
MCPA Amine
Metsulfuron-Methyl (Ally, Escort)
Paraquat
Picloram (Tordon)
Sethoxydim (Poast)
2,4-D Amine
Dissociation of a Herbicide Molecule
to an ionic form at Alkaline pH
Picloram (Tordon)
O
CL
CL
N
NH2
O
C-OH
CL
CL
CL
N
NH2
C-O
CL
At low pH
At high pH
Neutral Molecule
Ionic Molecule
_
Dissociation of a Herbicide Molecule
to an ionic form at Alkaline pH
2,4 - D
CL
CL
O
OCH2C-OH
O
OCH2C-O
CL
At low pH
CL
At high pH
Neutral Molecule
Ionic Molecule
_
Dissociation of a Herbicide Molecule
to an ionic form at Alkaline pH
CL
The effect of pH on Weak-Acid
herbicides
Want: Neutral or uncharged whole molecule
Don’t want: Charged or Ionic form
Movement of Ionizable (Weak-Acid)
Herbicides
Weak-Acid herbicides in an acid environment are not ionized and
can freely cross plant membranes, upon entering the alkaline
phloem (high pH) they will become ionized. This “traps” the
pesticide in the phloem and will subsequently be transported to
“active sites” within the plant.
Non-ionized pesticides freely
cross plant membranes
Leaves
pH 5
Xylem 150 cm / hr
Phloem – 90 cm / hr
Roots
pH 8
Non-ionized pesticides freely
cross plant membranes
Movement of Non-Ionized Herbicides
Once inside the plant, Non-Ionized herbicides such as atrazine can
move freely between xylem and phloem, the xylem moves more
rapidly than the phloem so the net movement is in the direction of
xylem.
Leaves
Xylem 150 cm / hr
Phloem – 90 cm / hr
Roots
Start
Non-ionized pesticides freely
cross plant membranes
Source: Nufarm
Another Point To Ponder
The leaf surfaces of several weed species are
alkaline (high pH)
Causes dissociation of weak-acid herbicides
resulting in reduced uptake
The condition is generally limited to broadleaf
weeds
Surface pH of Weed Leaves
Dicotyledons
Velvetleaf
Redroot Pigweed
Catchweed Bedstraw
Tall Morningglory
Pale Smartweed
Common Groundsel
Teaweed (Prickly Sida)
Wild Mustard
Black Nightshade
Monocotyledons
8.5 - 8.75
8.1 - 8.2
7.6 - 7.75
8.0 - 8.2
7.5 - 7.6
7.75 - 7.8
8.3 - 8.6
8.4
8.2
(most are nearly
neutral at pH 7.0)
Management of Leaf Surface pH
High leaf surface pH can be managed by
utilizing spray solutions that have been
acidified. Acidified spray solutions can easily
and effectively neutralize or even lower the pH
level.
Elements of the Management
of Water / Spray Solution pH
•
Know the water pH
•
Know the susceptibility of pesticide
•
Use acidifiers or adjuvants with acidification
properties to adjust the pH level. Know that
some things (copper containing fungicides)
should NOT be acidified.
•
Sulfonyl ureas??
Hard Water
Ca++,
Mg++,
Zn++
NA+,
K +,
Mn +
Fe ++,
Fe +++,
Al +++
Antagonistic Effects of Hard Water
on Glyphosate Molecules
Glyphosate and the cations will form a strong complex which
is physically large. This can prevent or hinder uptake of the
herbicide into the plant, effectively reducing herbicide
activity.
O
Mg++,-
-
Ca++,
O
+
P - CH2 NH2 CH2 C -
O
O
O
Sodium and Potassium = (least effect)
Calcium, iron, magnesium and copper have most effect
The Effect of Calcium on
Herbicide uptake by Setaria faberi
Untreated
Glyphosate
(no Calcium)
Glyphosate Nicosulfuron Nicosulfuron
(no Calcium)
+ CaCl2
+ CaCl2
Dirt and other stuff
Dust:
Reduces the wettability and coverage of
applied spray.
Mineral composition of dust may antagonize
some herbicides.
Organic matter may bind to some herbicides.
Clay constituency can neutralize some
herbicides.
Exudates: Some weeds called Halophytes (Velvetleaf and
Lambsquarter) have salt or chalk glands which
exude calcium deposits onto the surface.
Water Conditioning Strategies to Prevent
Antagonism of Weak-Acid herbicides
Removal of antagonistic ions by Ammonium Sulfate
Conditioning
Acidification to reduce the number of negatively
charged herbicide ions, or to select for the ionic
charge with the least potential for antagonism.
The Pesticide Application Process
The Spray Solution
Atomization
Spray
Drift
Transport to Target
Impaction / Deposition
Post-Impact Drop Behavior
Spreading, Retention, Penetration, Translocation
Contact Action
Systemic Action
Biological Effect