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 Activity
|
MOA |
CHEMICAL
CLASS |
TRADE
NAME |
TOXICITY |
TYPE |
TYPICAL
APPLICATIONS FOR THE CONTROL OF |
||||
|
C |
W |
D |
S |
T |
C |
||||
|
1 |
Organophosphates |
Orthene |
|
√ |
|
|
√ |
|
Aphids,
leafrollers, jap beetles, midge and thrips on roses |
|
Acephate
75 |
√ |
|
|
|
√ |
|
Aphids,
jap beetles, thrips on roses |
||
|
Malathion |
√ |
|
|
|
|
√ |
Aphids,
jap beetles, leafhoppers, scale, thrips on roses |
||
|
DuraGuard
ME |
√ |
|
|
|
|
√ |
Aphids,
thrips on roses |
||
|
Cygon |
|
√ |
|
√ |
|
|
Aphids,
leafhoppers and thrips on roses |
||
|
Carbamates |
Mesurol
75-W |
|
|
√ |
|
|
√ |
Aphids,
thrips on ornamental plants |
|
|
Sevin |
√ |
|
|
|
|
√ |
Aphids,
jap beetles, leafrollers, scale, on roses |
||
|
2 |
Pyrethroids |
Talstar |
√ |
|
|
|
|
√ |
Aphids,
jap beetles, thrips, spider mites on roses |
|
Tempo* |
√ |
|
|
|
|
√ |
Aphids,
budworms, thrips, on roses |
||
|
Mavrik |
√ |
|
|
|
|
√ |
Aphids,
thrips and spider mites on roses |
||
|
Scimitar |
√ |
|
|
|
|
√ |
Aphids,
budworms, jap beetles, thrips on roses |
||
|
Astro |
√ |
|
|
|
|
√ |
Aphids,
jap beetles, whiteflies on roses |
||
|
Chlorinated Hydrocarbons |
Kelthane |
|
|
√ |
|
|
√ |
Spider
mites on roses |
|
|
Thiodan |
|
√ |
|
|
|
√ |
Aphids
and eriophyid mites on roses |
||
|
Lindane |
√ |
|
|
|
|
√ |
Seed
and seedling protection of grain crops |
||
|
3 |
Macrocyclic
Lactone |
Avid |
|
√ |
|
|
√ |
|
Spider
mites and leafminers on roses |
|
Carbazate |
Floramite |
√ |
|
|
|
|
√ |
Spider
mites (all life stages) on roses |
|
|
4 |
Chloronicotinyls |
Marathon |
√ |
|
|
√ |
|
|
Jap
beetles, and thrips on roses |
|
Merit* |
√ |
|
|
√ |
|
|
Aphids,
jap beetles, thrips, on roses |
||
|
Spinosyn |
Conserve |
√ |
|
|
|
√ |
|
Thrips
and spider mites on roses |
|
|
5 |
Growth
Regulators/ Inhibitors |
Azatin |
√ |
|
|
|
|
√ |
Caterpillars,
jap beetles and leafrollers on roses |
|
Hexygon |
√ |
|
|
|
|
√ |
Spider
mite eggs and larva on roses |
||
|
6 |
Tetronic
Acid Derivatives |
Safer’s
Soap |
|
√ |
|
|
|
√ |
Aphids,
leafhoppers, spider mites and thrips on roses |
|
Sunspray
Oil |
√ |
|
|
|
|
√ |
Aphids,
leafminers, scale, on roses |
||
| Forbid 4F | √ |
√ |
Control of spider mites, at all life stages, on roses | ||||||
*Merit
and Tempo are combined in a single product: Bayer Advanced Garden Rose &
Flower Insect Killer
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 |
√ |
|
|
√ |
|
|
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 |
|
|
√ |
|
|
√ |
Blackspot
and botrytis on roses |
||
*Now
called Rose & Shrub Disease Control