1. Effective Pesticide Application
Reeves Petroff
Pesticide Education Specialist
MSU Extension

2. A Basic Understanding of Effective Pesticide Application
Should not be Confusing !

3. Or Painful !
but it can be!!

4. The Pesticide Application Process Impaction / Deposition
The Spray Solution
Atomization
Transport to Target
Impaction / Deposition
Post-Impact Drop Behavior Spreading, Retention, Penetration, Translocation
Contact Action
Systemic Action
Biological Effect

5. Spray Solution - Water Primary diluent
Large percentage of spray solution volume
Can have a negative influence on pesticide performance. pH
Minerals

6. 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

7. 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”.

8. 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.

9. Examples of Stability relative to pH
Insecticides
Bendiocarb
Carbaryl
Dimethoate
Trichlorfon
Half-Life
45 min pH 9
3.2 hrs pH 9
48 min pH 9
63 min pH 8
Fungicides
Captan
Iprodione
8.3 hrs pH 7
2 min pH 10
< 1 hr pH 9

10. Example of Instability Alkaline Hydrolysis produced by high pH
Dimethoate S
(CH3O)2 - P - S - CH2CONHCH3
(CH3O)2 - P - OH H - S - CH2CONHCH3
At high (alkaline) pH, OH_ ions attack the molecule here
Producing
Molecules that are inactive as insecticides

11. Chlorpyrifos At high (alkaline) pH, OH_ ions attack the molecule here
S N
(CH3O)2 - P - O Cl
Cl Cl N
S OH Cl
(CH3O)2 - P - OH Cl Cl
At high (alkaline) pH, OH_ ions attack the molecule here

12. Examples of pH and Stability
Herbicides
Clodinafop
Diclofop
Flumiclorac
Half-Life
2.5 hrs pH 9
12 hrs pH 9
6 min pH 9

13. pH also affects the Ohio group of herbicides
Weak Acids - OH

14. 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

15. Dissociation of a Herbicide Molecule to an ionic form at Alkaline pH
Picloram (Tordon)
At low pH At high pH
Neutral Molecule Ionic Molecule O
C-OH O
C-O_ N N CL CL CL CL CL CL
NH2
NH2

16. Dissociation of a Herbicide Molecule to an ionic form at Alkaline pH
At low pH At high pH
Neutral Molecule Ionic Molecule CL CL CL O
OCH2C-OH O
OCH2C-O_ CL CL

17. Dissociation of a Herbicide Molecule to an ionic form at Alkaline pHCL

18. The effect of pH on Weak-Acid herbicides
Want: Neutral or uncharged whole molecule
Don’t want: Charged or Ionic form

19. 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.
Vascular Bundle
Non-ionized pesticides freely
cross plant membranes
Leaves
pH Xylem 150 cm / hr
Phloem – 90 cm / hr pH 8
Roots
Non-ionized pesticides freely
cross plant membranes

20. 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.
Vascular Bundle
Leaves
Xylem 150 cm / hr
Phloem – 90 cm / hr
Roots
Non-ionized pesticides freely
cross plant membranes
Start
Source: Nufarm

21. 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

22. Surface pH of Weed Leaves
Dicotyledons
Velvetleaf
Redroot Pigweed
Catchweed Bedstraw
Tall Morningglory
Pale Smartweed
Common Groundsel
Teaweed (Prickly Sida)
Wild Mustard
Black Nightshade
8.4
8.2
Monocotyledons
(most are nearly neutral at pH 7.0)

23. 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.

24. 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??

25. Hard Water
Ca++, Mg++, Zn++
NA+, K +, Mn +
Fe ++, Fe +++, Al +++

26. 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 O O + -
Mg++,
O P - CH2 NH2 CH2 C -
O P - CH2 NH2 CH2 C -
Ca++, O O
Sodium and Potassium = (least effect)
Calcium, iron, magnesium and copper have most effect

27. The Effect of Calcium on Herbicide uptake by Setaria faberi
Untreated
Glyphosate
(no Calcium)
Glyphosate
+ CaCl2
Nicosulfuron
(no Calcium)
Nicosulfuron
+ CaCl2

28. 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.

29. 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.

30. The Pesticide Application Process Impaction / Deposition
The Spray Solution
Atomization
Transport to Target
Impaction / Deposition
Post-Impact Drop Behavior Spreading, Retention, Penetration, Translocation
Spray Drift
Contact Action
Systemic Action
Biological Effect