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Getting the Best of Pests — Processed webinar archives for the UGA Center for Urban Agriculture

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# Insecticide Mode of Action — Quick Reference Compendium

## 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.