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

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!

Insecticide LD50s

Acephate (Orthene) 980Chlorpyrifos (Dursban) 270Diazinon 400Ethoprop (Mocap) 62Fonofos (Crusade) 18Isofenphos (Oftanol) 20Isazofos (Triumph) 40-60Malathion 1000Trichlorfon (Dylox/Proxol) 250

Organophosphates (acetylcholinesterase inhibitors)

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/kgsignal words: danger, poison, skull & crossbones

II = medium toxicity – oral LD50 >50 to 500 mg/kgsignal words: warning

III = low toxicity – oral LD50 >500 to 2000 mg/kgsignal words: caution

IV = practically non-toxic – oral LD50 >2000 mg/kgsignal words: caution

Bendiocarb (Turcam) 156Carbaryl (Sevin) 246

Bifenthrin (Talstar) 375Cyfluthrin (Tempo) 826Fluvalinate (Mavrik) 282L-cyhalothrin (Scimitar) 79Permethrin (Astro) 430

Pyrethroids (disrupt nerve sodium pump)

Carbamates (acetylcholinesterase inhibitors)

Insecticide LD50s

Halofenozide (MACH2) 2850Diacylhydrazine (molt accelerating compound, induces molt)

Spinosads (Conserve) 3783-5000Spinosad (synaptic stimulation nicotinic acetycholine sites)

Fipronil (Chipco Choice) 97Phenylpyrazoles (GABA receptor disruption)

New Insecticide LD50s

Azadirachtin A & B (Azatrol, Neem, etc.)>3540

Tetranortriterpenoid (ecdysone blocker; antifeedant)

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 0.003 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!

Imidacloprid (Merit) 450Nitroguanidine (post-synaptic block, nicotinic ACH sites)

Thiamethoxam (Meridian) 1563Nitroguanidine (post-synaptic block, nicotinic ACH sites)

New Insecticide LD50s

Clothianidin (Arena) >5000Nitroguanidine (post-synaptic block, nicotinic ACH sites)

The Neonicotinoids

Acetamiprid (TriStar) 217Pyridylmethylamine (post-synaptic block, nicotinic ACH sites)

Dinotefuran (Safari) >2000Nitroguanidine (post-synaptic block, nicotinic ACH sites)

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!

Chemical Controls(groups)

PesticidesRepellents

Attractants/PheromonesDesiccants

Not appropriate for turf & ornamentals!

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!

Insecticide Groups

Synthetic OrganicsInorganicsBotanicalsMicrobials

Fatty Acid Salts (soaps)Oils

Not appropriate for turf but okay for ornamentals!

OrganochlorinesOrganophosphates

CarbamatesPyrethroids

Insect Growth Regulators (IGR)Neonicotinoids

Insecticide GroupsSynthetic Organics

Boric AcidSulfur

Mercury, Lead, ArsenateFluoride

Cryolite (sodium fluoaluminate)Silica (diatomaceous earth)

Insecticide GroupsInorganics

Not appropriate for turf & ornamentals!

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.

Pyrethrins RyaniaRotenone SabadillaNicotine Citronella Azadirachtins (neem oil)

Insecticide GroupsBotanicals

No longer available!

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.

Bacillus thuringiensis (Bt)(=delta-endotoxin)

Thuringiensin (Bt exotoxin)Avermectins

ChitinSpinosyns

Insecticide GroupsMicrobials

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.

Microbial InsecticidesBt δ-endotoxins

Variety Target Examples

Bt. kurstakiBt. aizawaiBt. israelensis

Bt. tenebrionis(=san diego)

Bt. japonensis(strain 'Buibui')

caterpillarscaterpillarsmosquitoes

leaf beetles

scarab grubs

Dipel®, MVP®Mattch®Vectobac®

M-one®

How Insecticides & Miticides Work

FATTY ACID SALTS (SOAP)

Insecticidal/miticidal Soaps

[Detergents]

PESTICIDE GROUPS

OILS

Petroleum Oils (mineral oil)Dormant oilHorticultural oil

Citrus Oils(d-Limonene)

Soybean Oil, etc.

PESTICIDE GROUPS

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.

Modes of ActionAffect on Target – System

NeuralCellular (metabolism)

RespiratoryInsect & Mite Cuticle

Growth Regulators - HormonalDesiccants

Most not appropriate for turf or ornamentals!

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.

DJS

Affects on Nerves

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.

DJS

Affects on Nerves

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.

Modes of ActionNervous Systems

Neural membrane disruption(ion transport disruption)

sodium/potassium ions chloride ions(ion pumps)

(organochlorines & pyrethroids)(fiproles = phenylpyrazoles)

Modes of ActionNervous System

Neural synapse disruption

acetylcholine (ACh) cholinesterase (ChE)(neural transmitter) (neural transmitter eraser)

(organophosphates & carbamates - inhibit ChE)(alkaloids, like nicotine, mimic ACh)

Modes of ActionNervous Systems

Neural post-synapse disruption

acetylcholine (ACh) ACh receptor(neural transmitter) (neural transmitter receptor)

(neonicotinoids and spinosyns)

(blocks) (stimulates)

Modes of ActionCellular Systems

Cell membrane disruptionlipo-protein complexes

all cells(soaps & oils)

midgut cells(Bt. δ-endotoxins)

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.

Modes of ActionCellular Systems

Metabolic disruption

(rotenone, dinitrophenols, pyrroles, hydramethylnon, arsenicals & fluorine)

ADP ATP(energy cycle)

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!

Modes of ActionGrowth Regulators (IGRs)

Juvenile hormone mimics(hydramethylnon)

Cuticle formation disruption(azadirachtin, diflubenzuron)

(hexythiazox?)

Molting hormone agonist(halofenozide, methoprene, fenoxycarb)

MODES OF ACTIONRESPIRATORY SYSTEMS

Suffocation

(soaps & oils, ??)

MODES OF ACTIONINSECT & MITE CUTICLE

(diatomaceous earth)

Physical damage

abrasion of cuticle surface

MODES OF ACTIONDESICCANTS

Drying agents

(silica aerogel, salts)

Modes of Transmission(Reaching the Target)

ContactStomachInhalationSystemic

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.

Control Approaches

ToleranceCurative

PreventiveRescue

Multiple TargetChemical-Cultural-BiologicalIntegrated Pest Management

Plant Health Care

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.

JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC

Japanese Beetle Life Cycle in Ohio

Curative StrategyPreventive Strategy

RESCUE

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).

Northern Masked Chafer Life Stages

1 26 120!Differential susceptibility of masked chafer grubs to an insecticide using the first instar amount as a unit of 1.

Application Timing(timing strategies)

Calendar Dates (& Rounds)Degree-days or PhenologyIPM - sample & thresholds

IPM - risk assessment

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.

Sprays: Important Factors

FormulationsVolume & PressureDrift & Nozzle Size

Adsorption & PhotodegradationMixes - Compatibility

Area versus Cover Sprays

Why Insecticides &Miticides Don‘t Work

TimingCoverage & Residual

Mix IncompatibilityPest Resurgence

Degradation (photo, microbial, chemical)

Pest Resistance

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.