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.