[Insecticide Primer] Insecticide Mode of Action

Editor's note: While many pest management professionals know exactly which product to use in which situation, they may not always know exactly how those products work. Does the product target the insect's nervous system? If so, in what way? Or does the product work in another way, like an insect growth regulator? What follows is a guide for PMPs to find the mode of action for most of the insecticides they use. Our hope is that this mode-of-action article becomes a "go to" resource for the pest management industry.

This article was excerpted from "Insecticide Basics for the Pest Management Professional" by Michael Scharf and Dan Suiter. Printable versions of the entire bulletin are available at www.caes.uga.edu/publications.

Insecticides have chemical structures that allow them to be classified based on the commonality of the active ingredient's chemistry. Thus, all members of an insecticide class have similar characteristics. The chemical structure of an insecticide generally defines its target site and its mode of action at that target site. Target site is defined as the physical location within an organism where the insecticide acts. Mode of action, alternatively, is defined as the action of an insecticide at its target site. In other words, the mode of action of an insecticide is the way in which it causes physiological disruption at its target site. Therefore, insecticide class, target site and mode of action are highly inter-connected concepts.
 

INSECTICIDES THAT TARGET THE INSECT NERVOUS SYSTEM

Most of the insecticides commonly used by PMPs can be technically classified as neurotoxins — i.e., their target site within the target organism is some aspect of the nervous system. Only a few of the commonly used insecticides, such as insect growth regulators (juvenile hormone analogs and chitin synthesis inhibitors) and a few miscellaneous active ingredients (borates, energy inhibitors and dehydrating dusts), do not target the nervous system.

To understand the mode of action of insecticides that target the insect nervous system, it is important to have a basic understanding of how the nervous system operates. In insects, the nervous system is composed of a series of highly specialized, interconnected cells, along which travel electrical charges called impulses (see Figure 1). Impulses are driven by the movement of electrically charged sodium, potassium and chloride ions into and out of nerve cells. The uninterrupted transmission of impulses along this series of cells is required for a nervous system to function properly. In insects, prolonged or irreversible disruption of a normal-functioning nervous system will result in death.

The area, or gap, between the end of one nerve cell and the beginning of the next nerve cell is referred to as the synapse (see Figure 2A ). When a nerve impulse terminates at the end of its nerve cell (the pre-synapse region), it must be transmitted across the synapse to the beginning of the next nerve cell (the post-synapse region). The transmission of impulses across the synapse is achieved by any one of a number of specific chemical messengers called neurotransmitters — "neuro" meaning nerve, and "transmitter" meaning to transmit, or carry. Important neurotransmitters discussed later include acetylcholine (Ach), gamma amino butyric acid (GABA) and glutamate. Neurotransmitters are released from the pre-synapse region, migrate across the synapse and are received by the post-synapse region at neurotransmitter-specific sites called receptor sites. When the neurotransmitter successfully binds to its receptor site at the post-synapse region, this triggers an impulse in the next nerve cell. In order to achieve the uninterrupted movement of impulses through the entire nervous system, this alternating system of electrical impulse to chemical transmitter and back to electrical impulse must function perfectly.

Insecticides that target the insect nervous system can be subdivided based on their specific target site within the nervous system. Specific neurological target sites include sodium and chloride channels and various components of the acetylcholine system. Insecticides that do not target the nervous system also can be subdivided by target site and mode of action, and include muscular calcium channel disruptors, insect growth regulators, inhibitors of energy production and non-specific cellular disruptors, as well as insecticides that act via desiccation (exoskeleton). Based on these subdivisions, in the following sections we present the mode of action of important insecticides used in urban and structural pest management.
 

I. Neurological Target Site: Sodium Channels

Chemical Class: Pyrethrins and Pyrethroids. This includes the active ingredients pyrethrins, bifenthrin, permethrin, cyfluthrin, beta-cyfluthrin, deltamethrin, cypermethrin, and lambda-cyhalothrin. Pyrethrins, known for 100+ years to have insecticidal properties, are the individual insecticidal components of pyrethrum, an extract of chrysanthemum flowers. Pyrethrins are fast acting, contact toxicants commonly found in products where quick knockdown is an important consideration. Pyrethrins alone are not very stable. They decompose rapidly at high temperatures and when exposed directly to sunlight or ultraviolet light. As a result, pyrethrins lose their insecticidal properties quickly.

The insecticidal activity of pyrethrins can be enhanced by applying them in the presence of an otherwise non-toxic chemical called a synergist. Within the insect body, pyrethrin molecules are inactivated by several types of enzymes, thus reducing the quantity of pyrethrin molecules available to affect nerve cells. This phenomenon, called detoxification, reduces the insecticide's effectiveness at the target site because less is available. To alleviate this problem, and allow more of the pyrethrin molecules to act against insect nerve cells, they are often applied along with a synergist. Synergists block the pyrethrin-inactivating enzymes, allowing more of the insecticide to reach its target site. Synergists, including MGK-264 and piperonyl butoxide (PBO), are often formulated with pyrethrin-based products.

Because pyrethrins have such limited residual activity, manufacturers modified their original molecular structure and synthesized an entire new class of more stable, pyrethrin-like insecticides called pyrethroids — pyrethr- referring to the pyrethrins, and -oid meaning like. Examples of pyrethroid insecticides commonly used by the pest management industry include bifenthrin, permethrin, cyfluthrin, beta-cyfluthrin, deltamethrin, cypermethrin and lambda-cyhalothrin.

Both pyrethrins and pyrethroids disrupt normal nerve function in a region of the nerve cell known as the axon (i.e., the target site). Their mode of action is to inhibit the on/off switch of nerve cells, called sodium channels, by delaying the rate at which they close, or turn off (see Figures 2A and B above). This mode of action results in uncontrolled, uninterrupted nerve firing seen as a convulsing insect (tremors and shaking) that quickly dies.

Pyrethroids are toxic to many Hymenoptera (ants, bees and wasps) and most aquatic animals, especially fish. Generally, pyrethroids are easily hydrolyzed (broken down in the presence of moisture) and are, thus, not very persistent. They are not very water soluble, a trait considered beneficial because this limits their movement in water (runoff) and soil. Although more photo-stable than pyrethrins, pyrethroids still have limited stability in sunlight. Interestingly, pyrethrins and some of the first pyrethroids have a negative temperature coefficient of toxicity — i.e., unlike most other insecticides, they exhibit greater toxicity at lower temperatures. Newer pyrethroids exhibit a positive temperature coefficient, meaning that they exhibit greater toxicity at higher temperatures.
 

Chemical Class: Oxadiazines. This includes the active ingredient indoxacarb. When indoxacarb enters the insect, it is broken down into a new molecule with insecticidal properties. This process, mediated by enzymes within the insect, is referred to as activation. After activation, the newly formed molecule (called a metabolite) targets sodium channels along the nerve axon (remember from above that sodium channels are the on-off switches of nerve cells). The active metabolite tightly binds to the sodium channel, and completely blocks sodium ion flow into nerve cells (see Figures 2A and B above). In a manner completely opposite to pyrethrins and pyrethroids, insects poisoned with indoxacarb appear paralyzed and limp, and are incapable of movement.
 

Chemical Class: Semicarbazones. This includes the active ingredient metaflumizone. The semicarbazones are a very new insecticide class for which our understanding is still developing. Early indications are that metaflumizone acts similar to the indoxacarb metabolite, in that it blocks sodium channels and prevents sodium ion movement into nerve cells. The result of this blockage is a loss of neurological function that is similar to that described for indoxacarb.
 

II. Neurological Target Site: Acetylcholine System

Chemical Class: Organophosphates (OPs) and Carbamates. These active ingredients were once used widely by the pest management industry, but are no longer. Human safety and health concerns, as well as their relatively high degree of mammalian toxicity, led to cancellation of many registrations in the U.S. However, because there are a few registrations remaining, we describe their mode of action here.

OPs and carbamates act by inhibiting the acetylcholinesterase (AchE) enzyme in the nervous system (see Figures 2A and B above). AchE performs a critical job in the nervous system by removing the neurotransmitter acetylcholine (Ach) from its receptor on the post-synapse nerve. Under natural conditions, AchE prevents overstimulation of the nervous system because it removes Ach. Without AchE, a stimulated nerve cannot return to its resting state. OPs and carbamates tie-up (inhibit) AchE, preventing it from removing Ach from its receptor site. The result is overstimulation of the nerve cell, and death of the insect. Because the AchE enzyme is very similar between insects and mammals, the OPs and carbamates are not very selective.

The OPs were initially developed in the 1930s and '40s by Germany as part of their war effort. Thereafter, the class evolved into a number of newer chemistries designed for the control of agricultural and urban pests. At the height of use in the 1960s-1970s, there were >50 OP insecticides in use worldwide. Examples of OPs commonly used by PMPs included chlorpyrifos (Dursban), dichlorvos (DDVP), malathion, diazinon, acephate (Orthene), propetamphos (Safrotin) and naled (Dibrom for mosquitoes). Some of these active ingredients have retained some of their use patterns.

The carbamates are synthetic insecticides modeled after a natural plant toxin (called physostigmine) from the Calabar bean. Carbamates were developed in the U.S. in the 1950s. At their height of usage, there were about 20 to 25. Examples of carbamates once widely used by PMPs include carbaryl (Sevin), bendiocarb (Ficam), and propoxur (Baygon). Like the OPs, some carbamate registrations still exist, but their allowable use patterns have been greatly diminished.
 

Chemical Class: Neonicotinoids. This includes the active ingredients imidacloprid, dinotefuran, thiamethoxam, clothianidin and acetamiprid. Neonicotinoids are synthetic materials modeled after the natural, plant-produced insecticide nicotine. Neonicotinoids target the insect nervous system by binding to the acetylcholine (Ach; a neurotransmitter) receptor on the post-synapse nerve cell (see Figures 2A and B above). Under normal conditions, Ach binds to this receptor for only milliseconds (1/1,000 of a second) at a time, resulting in short and controlled nerve stimulation. The neonicotinoids bind to the Ach receptor for very long periods, approximately minutes or greater. This mode of action results in nerve hyper-stimulation. Insects exposed to a neonicotinoid insecticide exhibit symptoms of tremors and hyperactivity, much like pyrethrins, pyrethroids and fipronil.
 

Chemical Class: Spinosyns. This includes the active ingredient spinosad. Spinosyns (also known as "Naturalytes") are chemicals produced by the soil bacterium Saccharopolyspora spinosa. Spinosyns are acquired by fermentation of S. spinosa cultures, then by purification and modification of the active chemical components produced by the microbe. Spinosyns bind to and stimulate the Ach receptor on the post-synapse nerve in a manner similar to but slightly different than neonicotinoids (see Figures 2A and B above). Spinosad intoxication is characterized by excitation of the nervous system, leading to involuntary muscle contractions, tremors and paralysis. Other minor modes of action for spinosyns have been determined, such as blockage of the GABA receptor (see Figures 2A and B above). GABA (gamma amino butyric acid) is an important neurotransmitter that stimulates chloride channels located in the central nervous system.
 

III. Neurological Target Site: Chloride Channels

Chemical Class: Phenylpyrazole. This includes the active ingredient fipronil. Fipronil was discovered in 1987. It received its first registration by the early 1990s for the control of agricultural pests in Europe. One of the first fipronil registrations in the U.S. urban pest management market was for termite control. Fipronil acts on the insect nervous system by binding to and blocking the GABA receptor on the post-synapse nerve cell (see Figures 2A and B above). This blockage prevents GABA from binding to the receptor site, which then prevents the influx of chloride ions into the post-synapse nerve cell. Because chloride ions limit and balance the electrical activity within nerve cells, blocking chloride influx leads to rapid, uncontrolled nerve firing throughout the nervous system. Fipronil-treated insects exhibit tremors and shaking similar to that seen in pyrethrin- and pyrethroid-treated insects.
 

Chemical Class: Avermectins. This includes the active ingredients abamectin, emamectin benzoate and ivermectin. The avermectins were originally isolated from soil bacteria from the genus Streptomyces. Older avermectins, such as abamectin, are used in their natural form; however, newer materials, such as emamectin benzoate, are partially natural and synthetic. Ivermectin is another natural avermectin. It has uses for endoparasite control in pets and companion animals. These materials are similar to phenylpyrazoles in that they bind the chloride channels that are regulated by the neurotransmitter glutamate (see Figures 2A and B above). While phenylpyrazoles block chloride channels, the avermectins stimulate them, resulting in constant and unimpeded chloride ion flow into nerve cells. This results in complete inactivation of nerve cells and a loss of neurological function. Poisoning symptoms in insects are similar to those caused by indoxacarb and metaflumizone (limp paralysis).

INSECTICIDES THAT DO NOT TARGET THE INSECT NERVOUS SYSTEM

I. Muscular Calcium Channel Toxins

Chemical Class: Diamide. This includes the active ingredient chlorantraniliprole. The diamide insecticides are technically not neurotoxins; however, they act on muscular calcium channels that are under direct control of the nervous system. Diamides bind and stimulate muscular calcium channels, causing uncontrolled calcium release and resultant muscle contractions (see Figures 2A and B above). Early stages of diamide exposure in insects appear as rigid or "contractile" paralysis. In later stages of exposure symptoms are very similar to inhibitory neurotoxins like the oxadiazines, semicarbazones and avermectins.
 

II. Insect Growth Regulators

Insect growth regulators (IGRs) used by the pest management industry include the juvenile hormone analogs and the chitin synthesis inhibitors. IGRs do not act on the nervous system. They are insecticides that disrupt critical physiological functions associated with normal insect growth, development and reproduction (egg production). IGRs are typically not acutely (immediately) toxic to adult insects. Adult insects exposed to IGRs usually suffer no adverse consequences, and typically live a normal lifespan. Because they target unique biochemical pathways found only in insects and related arthropods, IGR-containing products generally have low mammalian toxicity (i.e., large LD50 values). However, like all pesticides, IGRs must be handled safely and applied with a great deal of care and consideration for non-target organisms. For example, the developmental physiology of many aquatic invertebrates is similar to that of insects. Because of this, aquatic arthropods are susceptible to some IGRs.
 

Chemical Class: Juvenile Hormone Analogs. This includes the active ingredients hydroprene, methoprene, pyriproxyfen and fenoxycarb. The first category of insect growth regulators important in managing urban and structural pests are juvenile hormone analogs (JHAs). JHAs mimic a naturally occurring chemical in immature insects called juvenile hormone. Juvenile hormone is an important regulator of insect growth and development, including the normal maturation process. The presence of juvenile hormone in immature insects keeps them from becoming adults — thus the name.

During the life of an immature insect, the quantity of juvenile hormone in the insect's blood is relatively high. When present, immature insects are prevented from maturing because juvenile hormone prevents them from developing toward adulthood. As immature insects progress through their life cycle, however, the level of juvenile hormone in the blood is reduced through a decrease in its production and by juvenile hormone-degrading enzymes. With less juvenile hormone present, the insect can then proceed naturally toward adulthood.

In adult insects, juvenile hormone plays various roles in directing reproductive maturation, such as sperm production in adult males and egg production in adult females. In social insects such as termites, juvenile hormone plays an important role in caste differentiation; for example, high juvenile hormone levels in worker termites cause them to develop into soldiers.

Although the exact mechanism is unclear, experimental evidence suggests that JHAs may bind to juvenile hormone-degrading enzymes, the juvenile hormone receptor itself or a combination of both factors. Whatever the mechanism, JHAs maintain unnaturally high levels of juvenile hormone within the insect body at a time when it should not naturally be present. This abnormality has dire consequences on insect survival and reproduction, severely disrupting the insect's development and/or altering its reproductive physiology.

Figure 3. The only outward sign of the impact of JHA exposure on German cockroaches is adults that have twisted, curled or crinkled wings. German roach adults with twisted wings are sterile. Photo: K. Heinsohn

Death or sterilization often results from exposure to JHAs. For example, fire ant queens exposed to JHA-based baits stop producing eggs and/or colonies experience a shift in caste composition. The developmental physiology of immature mosquitoes and fleas exposed to methoprene is severely altered, resulting in death or severe developmental abnormalities that eventually lead to death.

German cockroaches exposed to JHAs during the last-instar molt into adult males that are physically incapable of mating or adult females with deformed ovaries. The result is that adult cockroaches in the population are sterile. As reproduction ceases, the population slowly declines as sterilized adults die of natural causes and are not replaced by nymphal cockroaches. Interestingly, sterile adults have twisted, curled or crinkled wings, which is the only visual sign of JHA exposure (see Figure 3). Twisted-wing adults may also be darker in color than normal adult cockroaches and are sometimes slightly larger than unexposed cockroaches. As is typical of IGRs, exposure of adult cockroaches to JHAs has no impact on adult survival.

Chemical Class: Chitin Synthesis Inhibitors. The second category of insect growth regulators are chitin synthesis inhibitors (CSIs). This includes the active ingredients diflubenzuron, hexaflumuron, noviflumuron and lufenuron. Like the juvenile hormone analogs, chitin synthesis inhibitors do not act on the insect's nervous system. They disrupt an important biochemical pathway responsible for the synthesis of chitin. Chitin is a critical chemical component found in arthropod exoskeleton. As part of the process of molting, chitin is synthesized and incorporated into the insect's new exoskeleton. Scientific evidence suggests that the mode of action for chitin synthesis inhibitors is to block an important enzyme, called chitin synthase, which is directly responsible for the conversion of certain chemicals into chitin. In the absence of this enzyme, chitin cannot be synthesized. The prevention of chitin synthesis is fatal for the affected insect.

Figure 4. Larval fleas feed on the dried blood defecated by adult fleas as they feed on their vertebrate host. If this food source contains the chitin synthesis inhibitor lufenuron, then larval fleas cannot properly molt and die when they molt. Adult female fleas that have fed on lufenuron-impregnated host blood do not produce viable eggs, but are themselves unaffected. Photo: D. Suiter

CSIs used by the structural pest management industry include diflubenzuron, developed decades ago for the control of agricultural pests and now used in several baits for termite control, and hexaflumuron and noviflumuron developed specifically for the control of termites in a baiting system. The subterranean termite worker caste is a continually molting immature form that makes up the majority of the social group. Workers cooperate to help maintain group stability and to keep social groups alive and viable. Termite baits that contain chitin synthesis inhibiting insecticides block chitin formation in molting termites exposed to the active ingredient.

Lufenuron is a chitin synthesis inhibitor used for flea control. It is sold directly to consumers by veterinarians for the control of fleas on companion animals. It is delivered orally and absorbed directly into the animal's bloodstream. Because fleas are obligate blood feeders (i.e., blood proteins are required for fleas to produce eggs), consumption of lufenuron-tainted animal blood by adult female fleas results in the production of eggs that fail to hatch (or first instar larvae that die soon after hatch) since insect eggs contain chitin. In addition, flea larvae are also killed by lufenuron. Adult fleas excrete large quantities of partially digested host blood. This high protein excrement is a primary food source of flea larvae during their development. Consumption of this lufenuron-tainted, dried blood is lethal to larval fleas when they molt (see Figure 4).
 

III. Inhibitors of Energy Production and Non-Specific Cellular Disruptors

Chemical Class: Amidinohydrazone. This includes the active ingredient hydramethylnon. Hydramethylnon is a cellular poison. It prevents the mitochondria within cells from doing their job — which is to produce energy for the cell and the organism to conduct its normal activities. Insects exposed to hydramethylnon die slowly as energy is depleted and not restored. Affected insects essentially are depleted of the energy needed to sustain normal bodily functions, causing them to die. Insects poisoned by hydramethylnon, as well as the diamide insecticide chlorantraniloprole, display limp paralysis much as the inhibitory neurotoxins noted previously.
 

Chemical Class: Pyrrole. This includes the active ingredient chlorfenapyr. Like indoxacarb, clorfenapyr must be converted by enzymes within the insect to an active form by a process known as activation. Once inside the insect, chlorfenapyr is converted to a new molecule (referred to as a metabolite) that is insecticidal. Interestingly, the metabolite is toxic to mammals as well; however, mammals lack the necessary enzymes to make the conversion from inactive to active insecticide. The mode of action of chlorfenapyr's active metabolite is much like that of hydramethylnon, i.e., it destroys the mitochondria's ability to supply energy to meet the insect's needs.
 

Chemical Class: Structural Fumigants. This includes the active ingredient sulfuryl fluoride. In the structural pest control industry, sulfuryl fluoride is used to fumigate residential and commercial buildings. It is thought that sulfuryl fluoride inhibits energy production in cells but does not appear to have a specific target site — i.e., sulfuryl fluoride is considered a non-specific metabolic inhibitor that causes a deprivation of cellular energy. Fumigants can be hazardous to applicators and non-target organisms if mishandled or misapplied. Most modern fumigants do not have intrinsic warning properties such as color, repellent odor or taste. It is for these reasons that all fumigants have strict use guidelines that require substantial applicator training. In addition, a small amount of the warning agent chloropicrin (tear gas) is applied in residential and commercial buildings prior to the introduction of sulfuryl fluoride gas.
 

Chemical Class: Borates. This includes the active ingredients borax, boric acid and disodium octaborate tetrahydrate. For decades, borates have been known to have insecticidal properties. Although an essential micronutrient for both plants and animals, at higher concentrations boron can be toxic. As a micronutrient, it aids metabolism and promotes enzyme function. Boron-based active ingredients are exclusively oral toxicants — they neither exhibit contact toxicity nor act as cuticular desiccants as do silica gels and diatomaceous earth (see later in article). Borates must either be consumed in baits or groomed off the insect's body after having been picked up as a dust formulation.

Although the exact mode of action of boron-based active ingredients is not fully understood, available evidence suggests that these materials are general cellular toxins or non-specific metabolic disruptors (perhaps even mitochondrial disruptors). Boric acid is used both in dry dust formulations and as a bait active ingredient for cockroaches and ants. Although a feeding deterrent to some pests at high concentrations, boric acid exhibits excellent water solubility and is slow acting at low concentrations —characteristics that make it a desirable active ingredient in liquid and gel baits. Disodium octaborate tetrahydrate is an active ingredient in preventive wood treatments targeted at both wood-destroying insects and fungi.
 

IV. Insecticides that Act Via Desiccation

Chemical Class: Dehydrating Dusts. This includes the active ingredients silica gels and diatomaceous earth. Silica gels and diatomaceous earth are inorganic (i.e., do not contain carbon) dusts composed of silicon dioxide. Silica gels are synthetically produced, while diatomaceous earth is the fossilized, skeletal remains of minute microorganisms known as diatoms (i.e., microscopic algae). Their remains are composed of silicon dioxide. Large deposits of fossilized diatoms are unearthed, mined, and used for insect control, among a myriad of other uses. Both silica gels and diatomaceous earth adsorb the thin wax layer on insect exoskeleton. The wax layer normally prevents insects from losing water through their exoskeleton and desiccating. By adsorbing the wax layer, silica gels and diatomaceous earth increase the permeability of the exoskeleton, resulting in insect death by dehydration. Silica gels and diatomaceous earth are most effective against crawling insects in dry (low humidity) environments where free water is limited. Although the toxicity of inorganic dusts is low, care should be exercised in their use because of their ability to injure the human respiratory system if breathed.
 

Michael Scharf holds the O. Wayne Rollins/Orkin chair in molecular physiology and urban entomology at Purdue University, West Lafayette, Ind. Daniel Suiter is with the Department of Entomology, University of Georgia, Griffin.