Extracted from Dr. Michael Scharf’s GTBOP Presentation (October 18, 2017)
Prepared by: Rich Braman, UGA Cooperative Extension / Center for Urban Agriculture
For: Dr. Dan Suiter & Dr. Michael Scharf — Bulletin revision reference
Source: GTBOP_ProseTranscript_2017-10-18_InsecticideMOA.md
Purpose
This document consolidates every insecticide class, target site, product name, and relationship mentioned in Dr. Scharf’s presentation into reference tables. These can serve as:
- Quick-check references during the writing process
- Source material for bulletin tables and figures
- Verification that all content is accounted for in the revised bulletin
All content below is derived exclusively from the presentation transcript. Items marked with ⚠️ may benefit from updating with current information.
TABLE 1: Master Classification — All Nine Insecticide Classes
Neurotoxic Insecticides (5 classes)
| # | Class | Target Site | Location on Neuron | Mode of Action | Effect on Insect | Representative Products |
|---|---|---|---|---|---|---|
| 1 | Pyrethroids / Pyrethrins / DDT | Sodium channels | Axon | Stimulation (modulation) | Excitation → knockdown, incoordination | Pyrethroids (various), pyrethrins |
| 2a | Oxadiazines | Sodium channels | Axon | Blockage | Inhibition → paralysis (“on switch stuck off”) | Indoxacarb |
| 2b | Semicarbazones | Sodium channels | Axon | Blockage | Inhibition → paralysis | Metaflumizone |
| 3a | Phenylpyrazoles | Chloride channels (GABA receptor) | Post-synaptic | Blockage | Excitation (blocks mellowing effect) | Fipronil |
| 3b | Isoxazolines | Chloride channels | Post-synaptic | Blockage | Excitation | Fluralaner, sarolaner |
| 3c | Avermectins | Chloride channels (glutamate receptor) | Post-synaptic | Stimulation | Inhibition → paralysis (opposite of fipronil) | Abamectin |
| 4a | Neonicotinoids / Nicotinoids | Acetylcholine receptor | Post-synaptic (synapse) | Stimulation | Excitation | Imidacloprid (nicotinoid), clothianidin (neonicotinoid) |
| 4b | Sulfoximines | Acetylcholine receptor | Post-synaptic (synapse) | Stimulation | Excitation | Sulfoxaflor |
| 4c | Spinosyns | Acetylcholine receptor | Post-synaptic (synapse) | Stimulation | Excitation | Spinosad |
| 5 | Organophosphates / Carbamates | Acetylcholinesterase enzyme | Synapse | Inhibition | Excitation (ACh accumulates) | Various ⚠️ |
Non-Neurotoxic Insecticides (4 classes)
| # | Class | Target Site | Mode of Action | Effect on Insect | Representative Products |
|---|---|---|---|---|---|
| 6 | Diamides | Neuromuscular calcium channels | Stimulation | Contraction → energy depletion → paralysis → death | Chlorantraniliprole, cyantraniliprole |
| 7a | Juvenile hormone analogs (IGR) | Hormonal regulation of molting | Mimicry | Cuticle deformation, extra juvenile stages, population crash | Pyriproxyfen |
| 7b | Chitin synthesis inhibitors (IGR) | Chitin synthesis enzyme | Inhibition | Death during molting; “jackknife effect” in termites | Various ⚠️ |
| 8 | Mitochondrial respiration inhibitors | Mitochondria (respiratory chain) | Inhibition | Energy production failure → death | Hydramethylnon, chlorfenapyr, sulfuryl fluoride, methyl bromide ⚠️, DSOBTH, boric acid |
| 9 | Cuticle dehydrating dusts | Epicuticular wax layer | Physical abrasion | Water loss → dehydration → death | Silica gel, diatomaceous earth |
TABLE 2: Four Basic Modes of Action
| Mode of Action | What It Does | Example Target | Example Insecticide Class |
|---|---|---|---|
| Stimulation | Causes target to become more active | Sodium channels → fire more | Pyrethroids |
| Blockage | Shuts target off | Sodium channels → can’t fire | Indoxacarb |
| Modulation | Subtly changes target shape/function | Sodium channel conformation | Pyrethroids (also modulators) |
| Inhibition | Prevents an enzyme from functioning | Acetylcholinesterase → can’t degrade ACh | Organophosphates, carbamates |
Note: Scharf emphasized that ALL insecticide effects can be categorized into just these four types.
TABLE 3: Target Sites on the Neuron — Spatial Relationships
| Location | Structure | Natural Function | Insecticides Targeting It |
|---|---|---|---|
| Axon (long body of nerve) | Sodium channels | ”On switch” — opening initiates nerve impulse | Pyrethroids, pyrethrins, DDT (stimulate); Indoxacarb, metaflumizone (block) |
| Post-synaptic membrane | GABA-gated chloride channels | ”Mellowing” — negative chloride dampens activity | Fipronil, isoxazolines (block → excitation) |
| Post-synaptic membrane | Glutamate-gated chloride channels | ”Mellowing” — inhibitory | Avermectins/abamectin (stimulate → paralysis) |
| Post-synaptic membrane | Acetylcholine receptors (nAChR) | Carry signal across synapse (sodium channel) | Neonicotinoids, sulfoximines, spinosyns (stimulate) |
| Synapse | Acetylcholinesterase enzyme | Breaks down ACh after signal transmission | Organophosphates, carbamates (inhibit) |
| Neuromuscular junction | Calcium channels | Trigger muscle contraction | Diamides (stimulate → sustained contraction) |
TABLE 4: Products and Active Ingredients Mentioned
| Active Ingredient / Product | Chemical Class | Target Site | Primary Use Mentioned | Notes |
|---|---|---|---|---|
| Pyrethrins | Natural pyrethroid | Sodium channels | General knockdown | Rapid knockdown; repellent |
| Pyrethroids (various) | Synthetic pyrethroids | Sodium channels | General pest control | ”Like pepper spray” — highly repellent; widespread bedbug resistance |
| DDT | Organochlorine | Sodium channels | Historical reference | Same target site as pyrethroids |
| Indoxacarb | Oxadiazine | Sodium channels (blocker) | Urban pest control | ”Really big urban insecticide” |
| Metaflumizone | Semicarbazone | Sodium channels (blocker) | Ectoparasites; possible urban | Newer product at time of presentation |
| Fipronil | Phenylpyrazole | Chloride channels (blocker) | Urban pest control | Off-patent; consumer products available; “one of our biggest” |
| Fluralaner | Isoxazoline | Chloride channels | Veterinary/pet (fleas) | Cross-resistance potential with fipronil |
| Sarolaner | Isoxazoline | Chloride channels | Veterinary/pet (fleas) | Cross-resistance potential with fipronil |
| Abamectin | Avermectin | Chloride channels (stimulator) | Gel baits | Opposite effect from fipronil despite similar target |
| Imidacloprid | Nicotinoid | Acetylcholine receptor | Various | Example of “nicotinoid” (looks more like nicotine) |
| Clothianidin | Neonicotinoid | Acetylcholine receptor | Various | Example of “neonicotinoid” (structurally evolved from nicotine) |
| Sulfoxaflor | Sulfoximine | Acetylcholine receptor | Newer product | New class at same target site as neonics |
| Spinosad | Spinosyn | Acetylcholine receptor | Landscape market | Same target site as neonics |
| Nicotine | Natural alkaloid | Acetylcholine receptor | Historical (tobacco) | The original — toxic to insects and mammals |
| Organophosphates (various) | Organophosphate | Acetylcholinesterase | Declining urban use | Not insect-specific; heavy restrictions |
| Carbamates (various) | Carbamate | Acetylcholinesterase | Declining urban use | Not insect-specific; heavy restrictions |
| Chlorantraniliprole | Diamide | Calcium channels (muscle) | Various | No signal word required by EPA; manufacturers added “Caution” |
| Cyantraniliprole | Diamide | Calcium channels (muscle) | Various | Newer diamide |
| Pyriproxyfen | Juvenile hormone analog | Hormonal (IGR) | Cockroach control | Wing twist indicator in cockroaches |
| Hydramethylnon | Amidinohydrazone | Mitochondria | Cockroach bait | Energy production inhibitor |
| Chlorfenapyr | Pyrrole | Mitochondria | Various (has food label) | Relatively safe; resistance potential noted |
| Sulfuryl fluoride | Inorganic fluoride | Mitochondria | Fumigation | |
| Methyl bromide | Halogenated hydrocarbon | Mitochondria | Fumigation | ⚠️ Largely phased out |
| DSOBTH | Borate | Mitochondria/respiration | Wood treatment | Disodium octaborate tetrahydrate |
| Boric acid | Borate | Mitochondria + gut lining | Various | Dual mode: chemical (respiration) + physical (abrasive/desiccant) |
| Silica gel | Inorganic dust | Epicuticular wax | Dust application | Physical mode — abrades waxy layer |
| Diatomaceous earth | Inorganic dust (biogenic) | Epicuticular wax | Dust application | Silicon from ground diatom exoskeletons |
TABLE 5: Combination Products
| Component 1 | Component 2 | Mechanism | Notes |
|---|---|---|---|
| Neonicotinoid (acetylcholine receptor) | Pyrethroid (sodium channels) | Potentiation — two target sites simultaneously; “1+1=3” synergy | ”All start with tea”; dual resistance observed in roach populations; still require rotation |
TABLE 6: Insect-Specificity Spectrum
| Insecticide Class | Mammalian Safety | Notes |
|---|---|---|
| Diamides | Extremely high | No signal word required by EPA; 10,000+ x selectivity |
| Avermectins | High | Insect-specific target |
| Isoxazolines | High | Primarily vet/pet products |
| IGRs (JH analogs, CSIs) | High | Target insect-specific developmental processes |
| Neonicotinoids | Moderate-High | Insect-specific receptor but systemic/pollinator concerns |
| Fipronil | Moderate-High | GABA receptor differences provide selectivity |
| Pyrethroids | Moderate | Generally safe for mammals but repellent to insects |
| Organophosphates / Carbamates | Low | Not insect-specific; work against mammals equally |
Spectrum based on Scharf’s characterizations in the presentation. Not a quantitative ranking.
TABLE 7: Practical Field Indicators Mentioned
| What You See | What It Means | Relevant Product/Class |
|---|---|---|
| Immediate knockdown/incoordination | Sodium channel excitation | Pyrethroids, pyrethrins |
| Paralysis (insect immobile but alive) | Sodium channel blockage OR chloride stimulation | Indoxacarb, abamectin |
| Wing twist in cockroach nymphs/adults | Juvenile hormone disruption | Pyriproxyfen (JH analog IGR) |
| “Jackknife” body curl in termites | Malformed cuticle from chitin synthesis disruption | Chitin synthesis inhibitors |
| Lethargy and desiccation | Epicuticular wax loss | Silica gel, diatomaceous earth |
| Sustained muscle contraction → stillness | Calcium channel stimulation → energy depletion | Diamides (chlorantraniliprole, cyantraniliprole) |
TABLE 8: Key Physiological Barriers to Insecticide Penetration
| Barrier | Location | Challenge for Insecticide | Relevant Formulation Strategy |
|---|---|---|---|
| Cuticle | External body surface | Multi-layered; waterproof; waxy epicuticle | Contact formulations must penetrate all layers |
| Gut lining | Digestive tract interior | The “tube” inside is technically external to the body | Baits must cross gut wall to reach internal targets |
| Tracheal system | Throughout body | Physical tubes, not blood-carried oxygen | Fumigants exploit this unique insect anatomy |
TABLE 9: Key Terminology and Definitions from Presentation
| Term | Definition (as explained by Scharf) |
|---|---|
| LD50 | Lethal dose that kills 50% of test population; inverse relationship with toxicity (lower LD50 = more toxic) |
| Mode of action | The action of an insecticide at its target site (stimulation, blockage, modulation, or inhibition) |
| Target site | The specific protein or physiological location within the insect where an insecticide acts |
| Signal word | EPA-required label indicator of acute toxicity (Danger, Warning, Caution); diamides so safe none was required |
| Potentiation | Synergistic effect from hitting two target sites simultaneously; “one plus one equals three” |
| Trophallaxis | Food sharing among social insects (from mouth and anus); insecticide transfer mechanism |
| Allogrooming | Mutual grooming among social insects; insecticide transfer mechanism |
| Secondary kill | Death of an individual from consuming insecticide-contaminated feces or carcass of a treated individual |
| Tertiary kill | Death of a third individual from insecticide passed through two prior digestive tracts |
| IRAC | Insecticide Resistance Action Committee; industry body that classifies MOAs and publishes rotation guidance |
| Wing twist | Visible cuticle deformation in cockroaches exposed to juvenile hormone analog IGRs; field diagnostic indicator |
| Jackknife effect | Body curling in termites with malformed cuticle from chitin synthesis inhibitor exposure |
| Epicuticle | Outermost waxy/oily layer of insect cuticle; target of dehydrating dusts |
| Synapse | Gap between neurons where electrical signals convert to chemical (neurotransmitter) signals |
| Acetylcholine | Primary neurotransmitter that crosses synapses in the insect nervous system |
| GABA receptor | Chloride channel type at post-synaptic membrane; target of fipronil |
| Glutamate receptor | Chloride channel type; target of avermectins |
Cross-Reference: Same Target Site, Different Effects
One of the presentation’s key teaching points was that different insecticide classes can target the same site but have opposite effects:
| Target Site | Insecticide A | Effect A | Insecticide B | Effect B |
|---|---|---|---|---|
| Sodium channels | Pyrethroids | Stimulation → excitation | Indoxacarb | Blockage → paralysis |
| Chloride channels | Fipronil | Blockage → excitation | Abamectin | Stimulation → paralysis |
This contrast is valuable for teaching and for resistance management — switching between classes at the same site may still provide different selection pressures.
All data extracted exclusively from the October 18, 2017 GTBOP presentation by Dr. Michael Scharf as transcribed and corrected through the GTBOP archive pipeline. Items marked ⚠️ may have changed since the presentation date.