# GTBOP Webinar Archives — Archive Summary
Getting the Best of Pests — Processed webinar archives for the UGA Center for Urban Agriculture

Skip to content
Skip to main content

# GTBOP Webinar Archive Summary

## Principles of Insecticide Mode of Action

Webinar Date: October 18, 2017 Speaker: Dr. Michael Scharf, O.W. Rawlins Orkin Endowed Chair in Urban Entomology and Molecular Physiology, Department of Entomology, Purdue University Moderator: Dr. Dan Suiter, Extension Entomologist, University of Georgia Duration: 1:07:06 Series: Structural Pest Control CEU Categories: GA — 2 HPC (Cat 35: Industrial, Institutional, Structural and Health Related)

## NARRATIVE SUMMARY

Dr. Michael Scharf of Purdue University presented a comprehensive overview of insecticide classification and mode of action designed to strengthen pest management professionals' understanding of how their chemical tools work. Scharf framed the practical importance of this knowledge around six themes: applicator and customer safety, accurate interpretation of trade literature, pollinator protection, resistance management, product sustainability, and the ability to design customized applications through situational pest management.

Scharf began with a condensed review of insect physiology, covering the five systems most relevant to insecticide activity: the nervous system, the cuticle, the digestive tract, the tracheal system, and the musculature. He explained that insecticides interact with specific protein target sites through a key-and-lock relationship, and that all insecticide effects can be reduced to just four modes of action — stimulation, blockage, modulation, and inhibition. He introduced the LD50 concept and emphasized that modern insecticides are dramatically more toxic to insects than to mammals, with some classes like the diamides carrying such low mammalian toxicity that the EPA initially required no signal word.

The presentation then systematically covered five neurotoxic insecticide classifications: sodium channel agents (pyrethroids, indoxacarb, metaflumizone), chloride channel agents (fipronil, isoxazolines, abamectin), acetylcholine receptor agents (nicotinoids, sulfoximines, spinosyns), acetylcholinesterase inhibitors (organophosphates, carbamates), and combination products pairing nicotinoids with pyrethroids for potentiation effects. Scharf followed with four non-neurotoxic classifications: muscular calcium channel agents (diamides such as chlorantraniliprole and cyantraniliprole), insect growth regulators (juvenile hormone analogs like pyriproxyfen and chitin synthesis inhibitors), inhibitors of energy production (hydramethylnon, chlorfenapyr, sulfuryl fluoride, boric acid), and cuticle dehydrating dusts (silica gel, diatomaceous earth).

Scharf concluded by discussing practical factors that affect insecticide performance, including formulation types, pest behavior that can amplify efficacy through secondary and tertiary kill, the role of sanitation in an IPM framework, and resistance management. He identified resistance as likely the number one cause of callbacks in cockroach accounts and recommended rotating active ingredients every three months or even monthly. A Q&A session moderated by Dr. Suiter addressed combination product resistance risks, the flow of new active ingredients to market, the IRAC classification system, the distinction between nicotinoids and neonicotinoids, oral versus dermal toxicity, repellent versus non-repellent insecticides, and the growing consumer demand for essential oil-based products.

## YOUTUBE TIMESTAMPS

0:00 Introduction and Speaker Credentials 1:45 Why Understanding Mode of Action Matters 6:03 Presentation Outline 7:13 Additional Resources — PCT Article and UGA Publication 8:26 Insect Physiology Overview — Nervous System, Cuticle, Gut, Trachea, Muscles 14:02 Insecticide Classification Basics — Chemical Structure 16:01 Target Site and Mode of Action — Key and Lock Analogy 17:51 Four Basic Modes of Action — Stimulation, Blockage, Modulation, Inhibition 19:41 The LD50 Concept and Mammalian Safety 22:46 Overview of Neurotoxic and Non-Neurotoxic Classifications 23:36 The Insect Nervous System — Neurons, Synapses, Neurotransmitters 26:45 Neurophysiology Demonstration — Fipronil and Nerve Excitation 28:40 Nervous System Target Sites — Roadmap of Ion Channels and Receptors 32:24 Sodium Channel Insecticides — Pyrethroids, Indoxacarb, Metaflumizone 34:12 Chloride Channel Insecticides — Fipronil, Isoxazolines, Abamectin 36:20 Acetylcholine Receptor Insecticides — Nicotinoids, Sulfoximines, Spinosyns 37:28 Acetylcholinesterase Inhibitors — Organophosphates and Carbamates 38:04 Combination Products — Nicotinoid-Pyrethroid Potentiation 39:07 Non-Neurotoxic Insecticides Overview 40:02 Muscular Calcium Channel Agents — Diamides 41:42 Insect Growth Regulators — JH Analogs and Chitin Synthesis Inhibitors 45:18 Inhibitors of Energy Production — Hydramethylnon, Chlorfenapyr, Fumigants 46:56 Cuticle Dehydrating Dusts — Silica Gel and Diatomaceous Earth 48:15 Factors Affecting Insecticide Efficacy 48:46 Stability, Persistence, and Formulations 51:02 Pest Behavior — Secondary and Tertiary Kill, Trophallaxis 53:04 Sanitation and IPM 54:19 Resistance Management — Rotation Strategies 56:30 Summary Points 57:44 Additional Resources 57:53 Q&A — Combination Product Resistance 59:53 Q&A — Flow of New Active Ingredients to Market 1:01:08 Q&A — Chlorantraniliprole and Non-Target Invertebrates 1:01:40 Q&A — IRAC Classification System 1:02:38 Q&A — Nicotinoids vs. Neonicotinoids 1:03:43 Q&A — Oral vs. Dermal Toxicity Routes 1:04:43 Q&A — Repellent vs. Non-Repellent Insecticides 1:05:32 Q&A — Essential Oils and 25B Exempt Products

## QUESTIONS & ANSWERS

Q: What are the four basic modes of action that all insecticides fall into? A: According to Dr. Scharf, all insecticides disrupt target sites through one of just four mechanisms: stimulation (causing a target to fire more rapidly), blockage (shutting a target off), modulation (subtly changing the shape and function of a target, as pyrethroids do to sodium channels), and inhibition (preventing an enzyme from functioning, as organophosphates do to acetylcholinesterase). Understanding these four categories provides a framework for classifying any insecticide a professional might encounter.

Q: Why are diamide insecticides like chlorantraniliprole considered especially safe for mammals? A: Diamides target muscular calcium channels that are highly specific to insects. They stimulate these channels, causing uncontrolled muscle contraction that burns up the insect's energy and leads to paralysis and death over several days. Their mammalian toxicity is so low that the EPA initially did not require a signal word, though manufacturers voluntarily adopted a "caution" label. Despite this safety profile, Scharf emphasized that applicators should still follow all safety guidelines when using them.

Q: How does fipronil work at the neurological level? A: Fipronil is a phenylpyrazole that targets chloride channels in the insect nervous system. Under normal conditions, chloride channels allow negatively charged chloride ions into neurons, which has a calming or "mellowing" effect on nerve activity. Fipronil blocks these channels, removing that calming influence and causing excitation — the insect's nervous system essentially fires uncontrollably. Scharf demonstrated this visually using nerve recordings from dissected American cockroaches, showing a dramatic increase in firing rate and intensity after fipronil application.

Q: What is the difference between repellent and non-repellent insecticides? A: Scharf explained that the distinction largely comes down to pyrethroids versus everything else. Pyrethroids are highly detectable to insects — he compared them to "pepper spray" — making them strongly repellent. Most other active ingredients, including fipronil and nicotinoids, are not nearly as detectable, which is why they are classified as non-repellent. This distinction became particularly prominent when non-repellent termiticides entered the market approximately 15 years before this presentation.

Q: Why is resistance considered a major concern for cockroach management? A: Scharf identified resistance as likely the number one cause of callbacks in cockroach accounts. His research has documented cockroaches that can eat bait as their sole food source for a full month and survive. He noted that resistance is not limited to older chemistries — even combination products containing two active ingredients can face dual resistance when cockroach populations develop tolerance to both nicotinoids and pyrethroids simultaneously. He recommended rotating active ingredients every three months or even monthly to help manage resistance.

Q: How do pest behaviors like trophallaxis and secondary kill affect insecticide efficacy? A: Scharf described three examples of behavior-mediated insecticide transfer. In cockroaches, secondary and tertiary kill occurs when one cockroach eats bait, excretes the insecticide, and other cockroaches consume the feces — research has shown the toxin can pass through two digestive tracts and still affect a third cockroach. Flea larvae can be exposed when adult fleas treated by veterinary products defecate insecticide-laden feces that larvae consume as nutrition. Social insects like termites and ants spread insecticides through trophallaxis (food sharing from both mouth and anus) and allogrooming, which is why slow-acting insecticides are preferred for these pests.

Q: What role does sanitation play in insecticide effectiveness? A: Scharf emphasized that poor sanitation always makes insecticides less effective, regardless of how pest-specific modern products have become. Excess food in an account competes directly with bait placements, reducing consumption. Clutter creates untreatable harborage areas where pests can avoid contact with residual treatments. Dirt and grease on surfaces can physically bind and inactivate insecticides. He framed sanitation as a core component of the IPM mindset that directly enhances chemical efficacy.

Q: What is the IRAC and how can it help pest management professionals? A: IRAC stands for the Insecticide Resistance Action Committee, a global organization with representatives from all major insecticide manufacturers. IRAC develops mode of action classifications that help professionals understand which products share the same target sites. Their classification chart, updated once or twice a year, shows the full landscape of available active ingredients organized by mode of action. Professionals can use this resource to plan effective product rotations by ensuring they alternate between different mode of action groups rather than simply switching trade names.

Q: How do insect growth regulators work differently from neurotoxic insecticides? A: Unlike neurotoxins that target the nervous system for rapid effects, insect growth regulators disrupt the hormones and enzymes that control development and molting. Juvenile hormone analogs like pyriproxyfen mimic the insect's own juvenile hormone, leading to cuticle deformation and extra juvenile stages that cannot reproduce — causing the population to crash over time. Chitin synthesis inhibitors block the enzyme responsible for forming the exoskeleton during molting, leading to death during the molt or producing malformed cuticles that cause a characteristic "jackknife" effect in treated termites. Scharf noted that visible wing twist in cockroach populations is a reliable indicator that IGRs are already affecting that population.

Q: Why are insecticides generally more toxic through ingestion than through contact exposure? A: Scharf explained that both the insect cuticle and mammalian skin serve as highly effective barriers to insecticide penetration. The insect cuticle is a complex, multi-layered, waterproof structure that contact insecticides must traverse to reach internal target sites. In contrast, the gut lining is a much thinner layer of cells, allowing ingested insecticides to penetrate far more readily. The same principle applies to mammals — human skin is an exceptionally resistant barrier compared to the gut, which is why oral exposure routes are almost always more toxic than dermal exposure for any given active ingredient.

Q: What is the outlook for new active ingredients entering the urban pest management market? A: Scharf acknowledged that the flow of new active ingredients has slowed and the market has become heavily generic. While all major manufacturers maintain product pipelines, bringing a new active ingredient to market costs hundreds of millions to billions of dollars, and the economics must justify the investment. He noted that the urban pest management market represents a smaller slice of the overall pie compared to agriculture, which affects manufacturer incentives. Scharf encouraged the industry to advocate vocally to manufacturers about the need for new tools, particularly given growing resistance pressures.

## ADDITIONAL RESOURCES

The following resources were referenced by the speaker during the presentation:

- Scharf, M.E. and D.L. Suiter. "Insecticide Primer and Insecticide Mode of Action." PCT Magazine, 2011.
- Scharf, M.E. and D.L. Suiter. Insecticide Basics for the Pest Management Professional. University of Georgia publication (available free of charge; URL referenced in presentation slides).
- IRAC (Insecticide Resistance Action Committee) — Mode of action classification chart, updated annually.

## CHAIN OF CUSTODY

- Source document: Corrected SRT from Stage 1 (GTBOP_Transcript_2017-10-18_InsecticideMOA.srt)
- Source verified: 742 blocks, 2,968 lines, full read confirmed with coverage proof during Stage 1
- Webinar date confirmed: Via original program announcement email from Tami Adams Boyle

Processed for UGA Center for Urban Agriculture / GTBOP Archives