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 CHEMICAL GROUP HOW THEY WORK Organophosphates and
Inhibit the enzyme acetylcholinesterase which serves to interrupt the transmission of
Carbamates Chlorinated Hydro-
Act as axonic poisons by interfering with the sodium channels of both the peripheral
carbons and Pyrethroids
and central nervous system thereby stimulating repetitive nervous discharges, leading to paralysis.
Macrocyclic Lactone and
Interferes with the gamma-amino butyric acid (GABA) receptors of insect neurons
Carbazates
leading to repetitive nervous discharges.
Chloronicotinyls
Bind to acetylcholine receptors thereby disrupting nerve impulse transmission.
Pyridine, Triazine, Insect
Insect growth regulators are chitin synthesis inhibitors or juvenile hormone mimics.
Growth Regulators
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.
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 CHEMICAL GROUP HOW THEY WORK Demethylation Inhibitor
Inhibits the biosynthesis of sterol in fungal membranes. There are five
(DMI) fungicides such as
classes of these sterol biosynthesis inhibitors (SBI) encompassing 44
triazoles and imidazoles,
different active ingredients – for example, propiconazole, the active ingredient in Banner MAXX.
Methyl Benzimidazole
Interferes with fungal mitosis (normal cell division). There are four
Carbamates – the MBC
groups of mitosis inhibiting chemicals encompassing nine different
fungicides such as
active ingredients – for example, thiophanate-methyl, the active
thiophanates, benzimidazoles, Dicarboximides and
Inhibits membrane synthesis and promotes the oxidation of fungal lipid
Carbamates 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.
Carboxamides plus Quinone
Interferes with fungal respiration. There are seven classes of respiration
outside Inhibitors (QoI) and
inhibitors encompassing 28 different active ingredients – for example,
Quinone inside Inhibitors (QiI) trifloxystrobin, the active ingredient in the QoI fungicide, Compass. fungicides 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 TOXICITY
MOA CHEMICAL TRADE TYPICAL APPLICATIONS FOR THE CONTROL OF √ Aphids, leafrollers, jap beetles, midge and Acephate 75 √ √ Aphids, jap beetles, thrips on roses phosphates Malathion √ √ Aphids, jap beetles, leafhoppers, scale, DuraGuard √ √ Aphids, thrips on roses √ √ Aphids, leafhoppers and thrips on roses √ √ Aphids, thrips on ornamental plants Carbamates 75-W √ √ Aphids, jap beetles, leafrollers, scale on roses √ √ Aphids, jap beetles, thrips, spider mites on roses √ √ Aphids, budworms, thrips on roses Pyrethroids Mavrik √ √ Aphids, thrips and spider mites on roses Scimitar √ √ Aphids, budworms, jap beetles, thrips on roses √ √ Aphids, jap beetles, whiteflies on roses Kelthane √ √ Spider mites on roses Chlorinated Thiodan √ √ Aphids and eriophyid mites on roses Hydro- carbons √ √ Seed and seedling protection of grain crops Macrocyclic Avid √ √ Spider mites and leafminers on roses Carbazate Floramite √ √ Spider mites (all life stages) on roses Marathon √ √ Jap beetles, and thrips on roses nicotinyls √ √ Aphids, jap beetles, thrips on roses Spinosyn Conserve √ √ Thrips and spider mites on roses √ √ Caterpillars, jap beetles and leafrollers on roses Growth Regulators/ √ √ Spider mite eggs and larva on roses Inhibitors Safer’s √ √ Aphids, leafhoppers, spider mites and Tetronic Acid Derivatives Sunspray √ √ Aphids, leafminers, scale on roses Soaps and 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 CHEMICAL TYPICAL APPLICATIONS FOR THE CONTROL OF √ √ Blackspot, powdery mildew, and rust on roses Bayleton √ √ Powdery mildew on roses Dimethylation 1 Inhibitors √ √ Powdery mildew on roses
Blackspot, powdery mildew, and rust on roses
Funginex* √ √ √ Blackspot, powdery mildew, and rust on roses √ √ Blackspot and powdery mildew on roses Fertilome √ √ Blackspot, powdery mildew, and rust on roses Systhane √ √ Blackspot, powdery mildew, and rust on roses √ Botrytis on roses Cleary’s √ √ Blackspot and powdery mildew on roses 2 Benzimi- Fungo Flo √ √ Blackspot and botrytis on roses Carbamates 3 Dicarbox- √ √ Botrytis on roses 4 Phenylamides Subdue √ √ Root and stem rot when applied as drench or √ √ Blackspot, powdery and downy mildews, 5 outside Inhibitors Heritage √ √ Downy and powdery mildew, rust and leaf spot on roses Inorganics √ √ Downy mildew on roses Phyton 27 √ √ Backspot, powdery and downy mildews and 6 (Metals) √ √ Blackspot, leaf spot and rust on roses Dithiocarbam Manzate ** √ √ Blackspot and botrytis on roses ates √ √ Blackspot, leafspot and rust on roses √ √ Blackspot and botrytis on roses
*Now called Rose & Shrub Disease Control **Combined in a single product: Zyban
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