1. Groups, Modes of Action &
Insecticides 101 –
Groups, Modes of Action &
Usage Strategies
David J. Shetlar, Ph.D.
The “BugDoc”
The Ohio State University,
OARDC & OSU Extension
Columbus, OH
© January, 2005, D.J. Shetlar, all rights reserved

2. Notes:
Before we proceed, we must look at where we have been and where we are headed with insecticides and miticides.
Rachael Carson in Silent Spring pointed out that insecticides were getting more and more toxic to deal with insects that were becoming resistant! During her time, we had organochlorine insecticides (e.g., DDT, Chlordane, Lindane, Heptachlor, etc.) that persisted for years, even decades, and they often ended up in the food chain only to be biomagnified in the food chain, especially in predators.
Organochlorine insecticides were quickly followed with organophosphates and carbamate insecticides. Indeed, these compounds appeared to become more and more toxic with each new compound developed. While most organophosphates and carbamates had shorter environmental residuals, they were still quite toxic and insects and mites continued to develop resistance.
With EPA’s FQPA review, most organophosphates and carbamates have been restricted from residential usage and new pesticide categories have been developed. We need to look at these new groups!

3. Insecticide LD50s Acephate (Orthene) 980 Chlorpyrifos (Dursban) 270
Organophosphates (acetylcholinesterase inhibitors)
Acephate (Orthene)
Chlorpyrifos (Dursban)
Diazinon
Ethoprop (Mocap)
Fonofos (Crusade)
Isofenphos (Oftanol)
Isazofos (Triumph)
Malathion
Trichlorfon (Dylox/Proxol)

4. Notes:
Do you remember your pesticide categories? Remember that they are based primarily on oral and dermal LD50s.
What’s the LD50? It’s the Lethal Dose of a chemical compound, measured as milligrams of chemical per kilogram of body weight (mg/kg), which would theoretically kill 50% of an exposed population. In reality, this is determined by placing a range of doses into the stomachs of rats (sometimes mice) and determining how many die. A graph of mortality versus dose is constructed and the 50% mortality level is calculated.
How are LD50 used by EPA? EPA has established four pesticide categories:
I = high toxicity – oral LD50 <50 mg/kg
signal words: danger, poison, skull & crossbones
II = medium toxicity – oral LD50 >50 to 500 mg/kg
signal words: warning
III = low toxicity – oral LD50 >500 to 2000 mg/kg
signal words: caution
IV = practically non-toxic – oral LD50 >2000 mg/kg

5. Insecticide LD50s Bendiocarb (Turcam) 156 Carbaryl (Sevin) 246
Carbamates (acetylcholinesterase inhibitors)
Bendiocarb (Turcam) 156
Carbaryl (Sevin) 246
Pyrethroids (disrupt nerve sodium pump)
Bifenthrin (Talstar) 375
Cyfluthrin (Tempo)
Fluvalinate (Mavrik) 282
L-cyhalothrin (Scimitar) 79
Permethrin (Astro)

6. New Insecticide LD50s Azadirachtin A & B (Azatrol, Neem, etc.)
>3540
Tetranortriterpenoid (ecdysone blocker; antifeedant)
Spinosads (Conserve)
Spinosad (synaptic stimulation nicotinic acetycholine sites)
Halofenozide (MACH2)
2850
Diacylhydrazine (molt accelerating compound, induces molt)
Fipronil (Chipco Choice) 97
Phenylpyrazoles (GABA receptor disruption)

7. Notes:
While you will see that azadirachtin, spinosads and halofenozide are definitely Category IV insecticides (practically non-toxic), how can fipronil be considered as a “new, lower risk insecticide” with such a low LD50??
Fipronil, while quite toxic, is used at very, very low rates, often less than pounds of active ingredient per acre! Potential exposure to such low rates are considered to be insignificant to people, pets and other animals, but is still quite toxic to insects. In fact, one of the most popular topical flea and tick products used for application to cats and dogs is based on Fipronil!
This brings about another dictum commonly stated in toxicology: “The dose makes the poison!”
Organochlorine, organophosphate and carbamate insecticides were usually used in pounds of active ingredient per acre while many of the newer insecticides, including pyrethroids, are used at tenths and hundredths of pounds of active ingredient per acre, at least one order of magnitude less!

8. New Insecticide LD50s Imidacloprid (Merit) 450 Acetamiprid (TriStar)
The Neonicotinoids
Imidacloprid (Merit)
450
Nitroguanidine (post-synaptic block, nicotinic ACH sites)
Acetamiprid (TriStar)
217
Pyridylmethylamine (post-synaptic block, nicotinic ACH sites)
Clothianidin (Arena)
>5000
Nitroguanidine (post-synaptic block, nicotinic ACH sites)
Thiamethoxam (Meridian)
1563
Nitroguanidine (post-synaptic block, nicotinic ACH sites)
Dinotefuran (Safari)
>2000
Nitroguanidine (post-synaptic block, nicotinic ACH sites)

9. Notes:
The neonicotinoids is one of the newest categories of insecticides. Imidacloprid was the first in this chemical category to obtain registration, both in the United States and internationally. Note that the pure active ingredient is technically a category II (between oral LD50 of 50 and 500), but the dermal LD50 is very high and formulated products are well above category II levels. Other neonicotinoids have differing toxicological profiles with Clothianidin and Dinotefuran reaching category IV (practically non-toxic) levels!

10. Attractants/Pheromones
Chemical Controls
(groups)
Pesticides
Repellents
Attractants/Pheromones
Desiccants
Not appropriate for turf & ornamentals!

11. Notes:
Remember that in the IPM principles, we talk about cultural controls, biological controls and chemical controls.
While most people think of pesticides when the term “chemical control” is used, entomologists included more than just these acute toxicants. Repellents are not commonly used on plants, but we often use them on our bodies or applied to pest to repel mosquitoes, fleas and ticks. There are repellent materials being developed for ornamentals usage. Pheromones (attractants) are chemicals that insects use to communicate, often sexual readiness or alarm. We often use sex pheromones to monitor insect activity so that pesticide applications can be properly timed.
Desiccants are useful in dry environments, such as in our homes and buildings for control of cockroaches and other creeping and crawling pests. However, outside environments often deactivate such desiccants or the desiccants can be phytotoxic to the plants!

12. Fatty Acid Salts (soaps)
Insecticide Groups
Synthetic Organics
Inorganics
Botanicals
Microbials
Fatty Acid Salts (soaps)
Oils
Not appropriate for turf but okay for ornamentals!

13. Insect Growth Regulators (IGR)
Insecticide Groups
Synthetic Organics
Organochlorines
Organophosphates
Carbamates
Pyrethroids
Insect Growth Regulators (IGR)
Neonicotinoids

14. Inorganics Insecticide Groups Boric Acid Sulfur
Mercury, Lead, Arsenate
Fluoride
Cryolite (sodium fluoaluminate)
Silica (diatomaceous earth)
Not appropriate for turf & ornamentals!

15. Notes:
“Inorganic” means that these are compounds based on earth-elemental materials and the traditional organic atoms of carbon, hydrogen and oxygen are not present.
Many inorganic compounds are relatively harmless to animals (e.g., sulfur, boric acid, cryolite) while others, especially the heavy metal compunds are highly toxic and persistent in the environment (e.g., lead, mercury, fluorides, and arsenicals).
In the 1920s until the 1950s, we commonly used lead-arsenate as a general garden insecticide! Even mercury-based fungicides were common into the 1980s!
Sulfur is still used as an “organic” fungicide and miticide.

16. Botanicals Insecticide Groups Pyrethrins Ryania Rotenone Sabadilla
Nicotine Citronella
Azadirachtins (neem oil)
No longer available!

17. Notes:
While most botanicals are “natural, organic” materials, this doesn’t mean that they are naturally safe! Nicotine sulfate was a very commonly used insecticide in the 1930s into the 1960s, but we now know that it is a carcinogen and mutagen! Many of the rest of these botanical insecticides are not being supported as EPA calls for current toxicological data on each of them. This is most likely because companies don’t want to invest millions of dollars in compounds that may end up like nicotine – being a carcinogen and/or mutagen! The only ones with more complete data packages are pyrethrum and citronella.

18. Microbials Insecticide Groups Bacillus thuringiensis (Bt)
(=delta-endotoxin)
Thuringiensin (Bt exotoxin)
Avermectins
Chitin
Spinosyns

19. Notes:
Microbial pesticides are pesticides generally derived from microbes or microbial by-products. There are very few true biological microbial pesticides. This would be using something like the milky disease of white grubs which uses the actual bacteria that cause a lethal infection in white grubs that ingest the spores. Most microbial pesticides use chemicals produced by microbes.
Bacillus thuringiensis is a soil-dwelling bacterium that has over 30K identified strains! Many strains produce a protein granule, many of which are toxic to insects. The proteins that are active bind to insect gut cells and punch holes in them, thereby causing a secondary infection by bacteria leaking from the gut into the body cavity.
Avermectins are chemcials produced by single-celled fungi that have insecticidal and miticidal properties. Spinosyns are similar in that these are chemicals excreted by a soil-dwelling bacterium that over-stimulates insect nervous systems.

20. Microbial Insecticides
Bt δ-endotoxins
Variety
Target
Examples
Bt. kurstaki
caterpillars
Dipel®, MVP®
Bt. aizawai
caterpillars
Mattch®
Bt. israelensis
mosquitoes
Vectobac®
Bt. tenebrionis
leaf beetles
M-one®
(=san diego)
Bt. japonensis
scarab grubs
(strain 'Buibui')

21. FATTY ACID SALTS (SOAP)
How Insecticides & Miticides Work
PESTICIDE GROUPS
FATTY ACID SALTS (SOAP)
Insecticidal/miticidal Soaps
[Detergents]

22. PESTICIDE GROUPS OILS Petroleum Oils (mineral oil) Citrus Oils
Dormant oil
Horticultural oil
Citrus Oils
(d-Limonene)
Soybean Oil, etc.

23. Notes:
Soaps and oils appear to work much the same way on living organisms. They are membrane disruptors, causing cell membranes to be broken open, thereby spilling out the cell contents!
Soaps are fatty-acid-salts, originally made by boiling fats with lye. Depending on the length of the fatty acids used, you can have insecticidal properties or herbicidal properties. Of course most soaps also have antibiotic properties. Soaps emulsify (not dissolve!) fats and oils allowing them to be suspended and washed away with water. Detergents are very different chemically from soaps, but they do the same thing, emulsify. While detergents can be great insecticides, most detergent manufacturers do not want to register their detergents as pesticides!
Oils, whether petroleum based or plant based, can dissolve fats and oils. When coming into contact with a cell membrane, a lipo-protein matrix, oils break the bonds and destroy the matrix.

24. Affect on Target – System
Modes of Action
Affect on Target – System
Neural
Cellular (metabolism)
Respiratory
Insect & Mite Cuticle
Growth Regulators - Hormonal
Desiccants
Most not appropriate for turf or ornamentals!

25. Notes:
Insecticides and miticides can adversely affect their targets in a multitude of ways. Most commonly, disrupting the nervous system has been the major mode of action of insecticides. The effects are usually rapid, providing immediate “satisfaction” to the user.
Pesticides can also interfere with cell processes, such as energy transfer (metabolism), respiration (movement of oxygen and carbon dioxide), cell membrane maintenance, or ability to retain water.
Many pesticides modify the growth and development of their targets, basically acting like or modifying hormone activity. Most entomologist shy away from calling these Insect Growth Regulators, “hormones” because many people may falsely conclude that these are the same hormones that regulate their growth! But, insect growth regulators and plant growth regulators influence different hormonal systems than those possessed by humans and other vertebrates.

26. Affects on Nerves
DJS

27. Notes:
Neural transmission is a complicated process that includes an actual electrical impulse that travels up and down nerve cords (axons) and chemical transmission between nerve cells (synapse).
The electrical transmission is accomplished by nerve cells maintaining an ion imbalance inside and outside the cell membrane. This is done through movement of sodium (Na+), potassium (K+) and chloride (Cl-) ions to opposite sides of the cell wall (sodium channel and GABA channels). When the electrical impulse passes, this electrical charge is switched, but almost immediately it’s restored.
At the synapse, when the electrical impulse arrives, small packets (vacuoles) of acetylcholine are released within the synapse gap and almost instantaneously move to the receiving nerve (acetylcholine receptors) which starts another electrical impulse. To keep the acetylcholine from continually firing the receiving nerve, there is an enzyme (acetylcholine-esterase) that quickly destroys the acetylcholine, thereby resetting the synapse for the next nerve impulse.

28. Affects on Nerves
DJS

29. Notes:
Organochlorine insecticides, pyrethoids as well as the new fiproles disrupt the sodium/potassium/chloride channel systems that maintains the electrical charge of nerve cell membranes. When this is disrupted, nerves can not properly transmit the electrical impulse.
Organophosphate and carbamate insecticides block the acetylchline-esterase enzyme which causes the receiving nerve to keep firing. This causes the affected animals to virtually twitch to death!
Neonicotinoids fill up the acetylcholine receptors, actually the nicotinic-adetylcholine receptors (which insects have), thereby blocking neural transmission. Affected insects simply stop activity especially feeding, grooming and protective behaviors.
Spinosyns appear to act like acetylcholine, stimulating the receptors of the receiving nerve. The result is much the same as organophosphate or carbamate activity, but insects, again, are differentially affected by spinosyns.

30. Nervous Systems Modes of Action Neural membrane disruption
(ion transport disruption)
sodium/potassium ions
chloride ions
(ion pumps)
(organochlorines & pyrethroids)
(fiproles = phenylpyrazoles)

31. Nervous System Modes of Action Neural synapse disruption
acetylcholine (ACh)
cholinesterase (ChE)
(neural transmitter)
(neural transmitter eraser)
(organophosphates & carbamates - inhibit ChE)
(alkaloids, like nicotine, mimic ACh)

32. Neural post-synapse disruption
Modes of Action
Nervous Systems
Neural post-synapse disruption
acetylcholine (ACh)
ACh receptor
(neural transmitter)
(neural transmitter receptor)
(blocks)
(stimulates)
(neonicotinoids and spinosyns)

33. Cellular Systems Modes of Action Cell membrane disruption
lipo-protein complexes
all cells
(soaps & oils)
midgut cells
(Bt. δ-endotoxins)

34. Notes:
Soaps and oils appear to break apart the lipo-protein matrix used in cell membranes. This destroys the cells, causing their contents to leak out or disruptive ions are allowed in.
The delta-endotoxin of Bt strains appear to attach to the cell walls of the cells lining an insect’s mid-gut. These proteins eventually punch through the cell walls, again destroying the cells and allowing gut contents to leak into the body of the insect, thereby causing a secondary, and usually lethal, infection.

35. Cellular Systems Modes of Action Metabolic disruption ADP ATP
(energy cycle)
(rotenone, dinitrophenols, pyrroles, hydramethylnon, arsenicals & fluorine)

36. Notes:
All living things seem to use the same basic metabolic pathways essential in transforming energy into useful forms that can allow the cell to construct new molecules and modify other ones. Most of this energy transfer is located in the cell’s mitochondria and the Kreb’s cycle is the one most studied in basic biology. Within this cycle, adenosine diphosphate (ADP) and adenosine triphosphate (ATP) are the major energy transfer molecules. Various pesticides can interfere with this energy transfer, thereby shutting down cellular functions.
Unfortunately, since these pathways are fairly universal, pesticides affecting these pathways often have adverse affects on most living organisms!

37. Growth Regulators (IGRs)
Modes of Action
Growth Regulators (IGRs)
Cuticle formation disruption
(azadirachtin, diflubenzuron)
(hexythiazox?)
Molting hormone agonist
(halofenozide, methoprene, fenoxycarb)
Juvenile hormone mimics
(hydramethylnon)

38. MODES OF ACTION
RESPIRATORY SYSTEMS
Suffocation
(soaps & oils, ??)

39. MODES OF ACTION INSECT & MITE CUTICLE Physical damage
abrasion of cuticle surface
(diatomaceous earth)

40. MODES OF ACTION
DESICCANTS
Drying agents
(silica aerogel, salts)

41. Modes of Transmission Contact Stomach Inhalation Systemic
(Reaching the Target)
Contact
Stomach
Inhalation
Systemic

42. Notes:
Most insecticides get into insects through the cuticle (by contact) to through the gut (by ingestion). Contact can occur by actually hitting the insect with droplets of the insecticide mix or the residue of the insecticide can be left on surfaces. When the insect walks across these surfaces, sufficient material is picked up by their tarsi (feet) or other parts of the body, absorbed through the cuticle to kill the insect.
Inhalation is not a common mode of insecticide intake, however, some greenhouse fumigants and termite fumigation materials are still applied in this manner.
Systemic normally means that the plant picks up the insecticide so when the insect feeds on the plant, it ingests the insecticide at the same time. Systemic can be further subdivided into translaminar systemic meaning that the insecticide is just absorbed into the underlying plant tissues, or translocated systemic, meaning that the insecticide is moved through the plant parts within the vascular system. Orthene is translaminar when applied to plant leaves and translocated if injected into the soil where the roots can pick it up.

43. Control Approaches Tolerance Curative Preventive Rescue
Multiple Target
Chemical-Cultural-Biological
Integrated Pest Management
Plant Health Care

44. Notes:
Every pest situation comes with its own unique set of variables. In many cases, the pest doesn’t cause extensive damage, so the pest can be tolerated. Remember, just because you see a pest doesn’t mean you have to do something! Some pests are more easily controlled in a preventive mode and some pesticides work better as preventive applications (especially fungicides). Other pests can be monitored and controlled after they appear to reach levels that can not be tolerated. This is using the curative strategy. When pests are not detected and they cause extensive damage, then we often have to “rescue” the plants with more toxic and quicker acting pesticides.
Pests rarely occur one-at-a-time on plants and applications of certain pesticides can knock out more than one pest with a single application, if the application is made at a specific time! This is the multiple target principle.
We also should keep in mind overarching approaches when using pesticides, such as IPM (using biological, cultural and chemical tactics together) and Plant Health Care.

45. Japanese Beetle Life Cycle in Ohio
JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC
Preventive Strategy
Curative Strategy
RESCUE

46. Application Targeting
Where is pest located?
(upper or lower leaf surface, thatch, soil,
borer, leafminer, etc.?)
When is pest MOST susceptible?
(know life cycle and most susceptible stage(s).

47. Northern Masked Chafer Life Stages1 26
120!
Differential susceptibility of masked chafer grubs to an insecticide using the first instar amount as a unit of 1.

48. Application Timing (timing strategies) Calendar Dates (& Rounds)
Degree-days or Phenology
IPM - sample & thresholds
IPM - risk assessment

49. Notes:
There are dozens of ways to time applications of pesticides. The simplest method is to look for the pests and make an application when their levels warrant control. However, some pests are difficult to detect until they are in a stage that may be more difficult to control. In these cases, pest managers often resort to calendar date applications. The problem with calendar date applications is that each year is often unique, sometimes being cooler or warmer than “normal.” In this case using degree-day models or phenological sequences can help account for the unique weather conditions being encountered. Degree-days are simply a measure of heat units above a certain threshold (usually 50 degrees F for insects). Phenology is keeping track of what plants and pests are doing (e.g., flowering of dogwood is the time that annual bluegrass weevils lay eggs).
Risk rating is simply keeping records of what occurred in pervious seasons. If you had a plant affected by spider mites or turf infested with white grubs, these pests are more likely to occur again next season than if they had not been present this season.

50. Sprays: Important Factors
Formulations
Volume & Pressure
Drift & Nozzle Size
Adsorption & Photodegradation
Mixes - Compatibility
Area versus Cover Sprays

51. (photo, microbial, chemical)
Why Insecticides &
Miticides Don‘t Work
Timing
Coverage & Residual
Mix Incompatibility
Pest Resurgence
Degradation
(photo, microbial, chemical)
Pest Resistance

52. Notes:
When an insecticide or miticide fails, most applicators suspect that the pests have suddenly developed resistance. While resistance is always a potential issue, it is actually a fairly rare occurrence with pests of turfgrasses and ornamental plants! This is when I begin my ten questions about how and when the application was made.
For miticides, thorough coverage is essential for success since the tiny mites can literally walk around the droplets of miticides! For white grubs, was the insecticide irrigated in sufficiently to move the insecticide to the soil-thatch interface? How much thatch was present? If more than a half inch, most of the insecticide likely adsorbed to the organic matter!
When did you last calibrate your sprayer or spreader? Many insecticides are used at very precise rates and any rates less than what is recommended can result in failure.
New insecticides can be susceptible to photodegradation, so residues left on leaf surfaces often disappear in short order.