PESTICIDE MODES OF ACTIVITY – THE IMPORTANCE OF ROTATION
Roger Bryan  and Raymond Cloyd

Today’s insecticides and fungicides are manufactured from literally hundreds of chemical compounds – both organic and inorganic.  These chemical compounds, in turn, can be grouped into several dozen chemical groups.  Each group of insecticides and fungicides has associated with it a particular mode of activity or mode of action (MOA). These MOAs serve to describe how a particular chemical or chemical group acts to kill or disable insects or fungi.  Table 1 summarizes the six major MOAs and associated chemical groups for insecticides, and Table 2 summarizes the six major MOAs and associated chemical groups for fungicides.

Table 1.  Chemical Group and Insecticide Modes of Activity

MOA

CHEMICAL GROUP

HOW THEY WORK

1

Organophosphates and Carbamates

Inhibit the enzyme acetylcholinesterase which serves to interrupt the transmission of nerve impulses.

2

Chlorinated Hydro- carbons and Pyrethroids

Act as axonic poisons by interfering with the sodium channels of both the peripheral and central nervous system thereby stimulating repetitive nervous discharges, leading to paralysis.

3

Macrocyclic Lactone and Carbazates

Interferes with the gamma-amino butyric acid (GABA) receptors of insect neurons leading to repetitive nervous discharges.

4

Chloronicotinyls

Bind to acetylcholine receptors thereby disrupting nerve impulse transmission.

5

Pyridine, Triazine, Insect Growth Regulators

Insect growth regulators are chitin synthesis inhibitors or juvenile hormone mimics.  Chitin synthesis inhibitors prevent the formation of chitin, an essential compound in an insect’s exoskeleton. Juvenile hormone mimics cause insects to remain in an early/young life stage.

6

Soaps and Oils

Soaps and oils work by either damaging the waxy layer of the exoskeleton of soft-bodied insects, resulting in dessication (drying), or by covering the breathing pores (spiracles) of insects, resulting in suffocation.

Here’s the important point for rosarians: insects and fungi can develop resistance to insecticides and fungicides if products having the same MOAs are used repeatedly. The organism targeted for control by a chemical, or chemicals, with a particular MOA will ultimately mutate and possibly become immune to that MOA.  The solution to this problem is to rotate the chemicals used among the different MOAs so that, for example, the fungus that is becoming resistant to MOA-x is now confronted with MOA-y, and so on.  Rotation among the MOAs can be done over time.  For example, spray a product with MOA-x one week and one with MOA-y the next.  For equal effectiveness, products with two different MOAs can be tank-mixed (if they are otherwise compatible) and applied together.  An example of the latter approach can be found in the over-the-counter insecticide Bayer Advanced Garden Rose & Flower Insect Killer manufactured by the Bayer Company.  This product combines Merit (MOA 4) and Tempo (MOA 2) in a ready to use (RTU) spray.


Table 2.  Chemical Group and Fungicide Modes of Action

MOA

CHEMICAL GROUP

HOW THEY WORK

1

Demethylation Inhibitor (DMI) fungicides such as triazoles and imidazoles,

Inhibits the biosynthesis of sterol in fungal membranes.  There are five classes of these sterol biosynthesis inhibitors (SBI) encompassing 44 different active ingredients – for example, propiconazole, the active ingredient in Banner MAXX.

2

Methyl Benzimidazole Carbamates – the MBC fungicides such as thiophanates, benzimidazoles, etc.

Interferes with fungal mitosis (normal cell division).  There are four groups of mitosis inhibiting chemicals encompassing nine different active ingredients – for example, thiophanate-methyl, the active ingredient in Cleary’s 3336F.

3

Dicarboximides and Carbamates

Inhibits membrane synthesis and promotes the oxidation of fungal lipid (“fat”) molecules.

4

Phenylamides (PA) fungicides

Inhibits the synthesis of nucleic acids, both DNA and RNA.  There are three other groups closely related to the PA group, and they and the PA group encompass 12 different active ingredients.

5

Carboxamides plus Quinone outside Inhibitors (QoI) and Quinone inside Inhibitors (QiI) fungicides

Interferes with fungal respiration.  There are seven classes of respiration inhibitors encompassing 28 different active ingredients – for example, trifloxystrobin, the active ingredient in the QoI fungicide, Compass.

6

Multi-Site Contact Activity

These fungicides simply act to kill the fungus by overwhelming it with materials that are poisonous to it.  These fungicides are primarily based on inorganics such as aluminum, copper, and sulfur, and upon the dithiocarbamates combined with zinc and manganese ions – this latter group includes the fungicides Manzate, Maneb, and Junction. Low risk of resistance buildup.

Now you know that there are basically six MOAs for insecticides and fungicides.  And you know not to repeatedly use insecticides or fungicides from the same MOA.  However, I recognize that it’s unlikely you can go to your local garden supply store and say to the salesperson, “I’d like 6 oz. of a methyl benzimidazole carbamate fungicide, please” – I don’t think that would work.  Therefore, in the next two tables I’ve identified popular-with-rosarians insecticides (Table 3) and fungicides (Table 4) by their trade names, and grouped them into their respective MOAs.  Also identified in the tables are the toxicities of each product, based on the signal word on the labels, i.e., C = Caution, W = Warning, and D = Danger, and the type of product, i.e., S = Systemic, T = Translaminar or Locally Systemic, and C = Contact.  Systemic means the product is absorbed by the plant and moves upward through the plant’s vascular system (but not into the bloom).  Translaminar means the product is absorbed by the plant but does not move through it.  And, contact means the product is active only on the plant’s surface where it was sprayed.

Based on my experience with the rotation of fungicides I use a combination of the two rotation approaches to successfully control (indeed, eliminate) black spot.  For example, I use Banner MAXX, Cleary’s 3336F, and Compass – each of which has a different MOA.  One time I spray with a tank mix of Banner MAXX and Compass, and the next with a mixture of Cleary’s 3336F and Compass.  This avoids the fungus being exposed to the same MOA for any length of time.

Table 3.  Insecticide Modes of Action

MOA

CHEMICAL CLASS

TRADE NAME

TOXICITY

TYPE

TYPICAL APPLICATIONS

C

W

D

S

T

C

1

Organophosphates

Orthene

 

 

 

 

Control of aphids, leafrollers, jap beetles, midge and thrips on roses

Acephate 75

 

 

 

 

Control of aphids, thrips and jap beetles on roses

Malathion

 

 

 

 

Control of aphids, jap beetles, leafhoppers, scale, thrips on roses

DuraGuard ME

 

 

 

 

Control of aphids and thrips on roses

Cygon

 

 

 

 

Control of aphids, leafhoppers and thrips on roses

Carbamates

Mesurol 75-W

 

 

 

 

Control of aphids and thrips on ornamental plants

Sevin

 

 

 

 

Control of aphids, jap beetles, leafrollers, scale, etc. on roses

2

Pyrethroids

Talstar

 

 

 

 

Control of aphids, jap beetles, thrips, spider mites, etc. on roses

Tempo*

 

 

 

 

Control of aphids, budworms, thrips, etc. on roses

Mavrik

 

 

 

 

Control of aphids, thrips and spider mites on roses

Scimitar

 

 

 

 

Control of aphids, budworms, jap beetles,thrips, etc. on roses

Astro

 

 

 

 

Control of aphids, jap beetles, whiteflies, etc. on roses

Chlorinated

Hydrocarbons

Kelthane

 

 

 

 

Control of spider mites on roses

Thiodan

 

 

 

 

Control of pests on field crops (eg., corn, tomatoes, etc.)

Lindane

 

 

 

 

Seed and seedling protection of field crops (eg., wheat, barley, etc.)

3

Glycoside

Avid

 

 

 

 

Control of spider mite adults and leafminers on roses

Carbazate

Floramite

 

 

 

 

Control of spider mites, at all life stages, on roses

Phenoxypyrazole

Akari 5SC

 

 

 

 

Control of spider mites, at all life stages, on roses

4

Chloronicotinyls

Marathon

 

 

 

 

Control of jap beetles, and thrips on roses

Merit*

 

 

 

 

Control of aphids, jap beetles, thrips, etc. on roses

Spinosin

Conserve

 

 

 

 

Control of thrips and spider mites on roses

5

Growth Regulators/

Inhibitors

Azatin

 

 

 

 

Control of caterpillars, jap beetles and leafrollers on roses

Hexygon

 

 

 

 

Control of spider mite eggs and larva on roses

TetraSan

 

 

 

 

Control of spider mite eggs and larva on roses

6

Tetronic Acid Derivatives

Soaps and Oils

Forbid 4F

 

 

 

 

Control of spider mites, at all life stages, on roses

Kontos

 

 

 

 

“Two-way” systemic for controlling insects  and spider mites

Safer’s Soap

 

 

 

 

Control of aphids, leafhoppers, spider mites and thrips on roses

*Merit and Tempo are combined in a single product: Bayer Advanced Garden Rose & Flower Insect Killer

Modes of Action (MOA)

1.         Inhibits cholinesterase which, in turn, prevents termination of nerve impulse transmission

2.         Destabilizes nerve cell membranes   

3.       Inhibits nerve impulse transmission in chloride ion channels and mitochondrial electron transport thereby causing paralysis.

4.         Overstimulates and blocks postsynaptic nicotine acetylcholine receptors

5.         Prevents formation of insect exoskeleton or causes insect to remain in a young life stage

6.         Causes dessication (drying) or suffocation of insect

Toxicity: C=Caution, W=Warning, D=Danger

Type: S=Systemic, T=Translaminar (Locally Systemic), C=Contact

Effective rotation of insecticides requires moving from one MOA to another. Moving from one chemical class, or trade name within a single MOA will not prevent  the buildup of insect resistance to insecticides.

Table 4.  Fungicide Modes of Action

MOA

CHEMICAL GROUP

TRADE NAME

TOXICITY

TYPE

TYPICAL APPLICATIONS FOR THE CONTROL OF

C

W

D

S

T

C

1

Dimethylation Inhibitors (DMI)

Banner MAXX

 

 

 

 

Blackspot, powdery mildew, and rust on roses

Bayleton

 

 

 

 

Powdery mildew on roses

Rubigan

 

 

 

 

Powdery mildew on roses

Eagle

 

 

    

 

 

Blackspot, powdery mildew, and rust on roses

Funginex*

 

 

 

Blackspot, powdery mildew, and rust on roses

Immunox

 

 

 

 

Blackspot and powdery mildew on roses

Fertilome Liquid

 

 

 

 

Blackspot, powdery mildew, and rust on roses

Systhane

 

 

 

 

Blackspot, powdery mildew, and rust on roses

Decree 50WDG  √          √ Botrytis on roses

2

Methyl Benzimi-

dazole Carbamates

Cleary’s 3336 **

 

 

 

 

Blackspot and powdery mildew on roses

Fungo  Flo

 

 

 

 

Blackspot and botrytis on roses

3

Dicarboximides

Chipco 26019

 

 

 

 

Botrytis on roses

4

Phenylamides

Subdue Maxx

 

 

 

 

Root and stem rot when applied as drench or surface spray

5

Quinone outside Inhibitors (QoI)

Compass

 

 

 

 

Blackspot, powdery and downy mildews, botrytis and rust on roses

Heritage

 

 

 

 

Downy and powdery mildew, rust and leaf spot on roses

6

Inorganics

(Metals)

Aliette

 

 

 

 

Downy mildew on roses

Phyton 27

 

 

 

 

Backspot, powdery and downy mildews and botrytis on roses

Dithiocarbamates

Fore

 

 

 

 

Blackspot, leaf spot and rust on roses

Manzate  **

 

 

 

 

Blackspot and botrytis on roses

Maneb

 

 

 

 

Blackspot, leafspot and rust on roses

Daconil Ultrex

 

 

 

 

Blackspot and botrytis on roses

*Now called Rose & Shrub Disease Control          **Combined in a single product:  Zyban

Modes of Action (MOA)        

1.       Inhibits sterol biosynthesis in membranes

2.       Inhibits mitosis and cell division

3.       Inhibits the production of lipids and membrane synthesis

4.       Inhibits nucleic acid synthesis

5.       Inhibits respiration

6.       Multi-mode/multi-site activity – inherently low (or no) risk for buildup of resistance

Toxicity: C=Caution, W=Warning, D=Danger

Type: S=Systemic, T=Translaminar (Locally Systemic), C=Contact

Effective rotation of fungicides requires moving from one MOA to another. Moving from one chemical group, or trade name within a single MOA will not prevent the fungus’ buildup of resistance to fungicides.