Pesticides: Modes of Action

Most people know that insecticides kill insects. However, the way in which these chemicals work is a mystery to most of us. How an insecticide works is called its mode of action. A complete understanding of the mode of action of an insecticide requires knowledge of how it affects a specific target site within an organism. The target site is usually a critical protein or enzyme in the insect, but some insecticides affect broader targets. Although most insecticides have multiple biological effects, toxicity is usually attributed to a single major effect.

Larvicides and Adulticides — Organophosphates

Organophosphorus insecticides affect the nervous system. These insecticides are synaptic poisons. The synapse is a junction between two nerves or a nerve connection point (hence the name synaptic poison). Specifically, organophosphorus insecticides bind to an enzyme found in the synapse called acetylcholinesterase. This enzyme is designed to stop a nerve impulse after it has crossed the synapse. Organophosphorus insecticides bind to and prevent the enzyme from working. Therefore, poisoned synapses cannot stop the nerve impulse. Consequently, continued stimulation of the nerve occurs as observed with pyrethroids. Poisoned insects exhibit tremors and uncoordinated movement.

Larvicides — Growth Regulators

These chemicals are typically referred to as insect growth regulators or IGRs. IGRs act on the endocrine or hormone system of insects. These insecticides are specific for insects, have very low mammalian toxicity, are non-persistent in the environment, and cause death slowly. Most of the currently registered IGRs mimic the juvenile hormone produced in the insect brain. Juvenile hormone tells the insect to remain in the immature state. When sufficient growth has occurred, the juvenile hormone production ceases triggering the molt to the adult stage. IGR chemicals, such as methoprene mimic the action of juvenile hormone and keep the insect in the immature state. Insects treated with these chemicals are unable to molt successfully to the adult stage, and cannot reproduce normally.

Larvicides — Bacteria

Bacillus thuringiensis var. israelensis (Bti) is a naturally occurring bacterium that produces a crystalline protein toxin (crystal) and a spore. The larval activity of Bti formulations is due to the presence of the protein toxin. The spore has no larvicidal activity. For mosquito larvae, many factors are necessary to produce the toxic effects of Bti crystals. If the crystals are available in sufficient quantity, to suffer toxicity and die, a larva must : 1) Capture and ingest the crystals, 2) Possess a digestive tract with a highly alkaline pH, 3) Possess the enzymes capable of liberating the toxic proteins, and 4) Possess the gut membrane receptors, compatible with the solubilized toxins. Bti-based products are not insecticides of contact. The active ingredient (crystals) must be ingested to show a toxic activity. This very specific mode of action makes it very safe for non-target organisms present in the same environment.

Bacillus sphaericus (Bs) is also a naturally occurring, spore-forming bacterium found throughout the world. At the time of sporulation, Bs produces crystallin proteins (as in Bti but different) toxic for many species of mosquito larvae upon ingestion.

Larvicides/Pupicides

Very refined oil or surfactants can be used as larvicides/pupicides. These products have the ability to kill both larvae and pupae. The District is not currently using any oils but is using surfactants. The surfactant used is also called monomolecular film (MMF). Using conventional spraying methods, the invisible monomolecular film quickly spreads over standing water habitats. The film reduces the surface tension of the water making it difficult for the mosquito larvae and pupae to attach to the surface which causes them to drown. Emerging mosquitoes are unable to fully emerge and will drown. Mosquito larvicide and pupicide are effective on all species of mosquitoes that breed in standing water and require the air/water interface in their lifecycle.

ULV Adulticides — Pyrethroids

Pyrethroids are synthetic chemicals whose structures mimic the natural insecticide pyrethrin. Pyrethrins are found in the flower heads of plants belonging to the family Compositae (e.g. chrysanthemums). These insecticides have a unique ability to knock down insects quickly. Synthetic pyrethrins (also known as pyrethroids) have been chemically altered to make them more stable. Pyrethroids are axonic poisons (they poison the nerve fiber). They bind to a protein in nerves called the voltage-gated sodium channel. Normally, this protein opens causing stimulation of the nerve and closes to terminate the nerve signal. Pyrethroids bind to this gate and prevent it from closing normally which results in continuous nerve stimulation. This explains the tremors exhibited by poisoned insects. They lose control of their nervous system and are unable to produce coordinated movement.

Pyrethroids are most of the time used with piperonyl butoxide (PBO) which is a synergist that is usually incorporated within the final products. PBO enhances the effect of pyrethroids by inhibiting an enzyme (cytochrome P450) produced by the insect to break down the pesticides. The PBO allows the insecticides to be effective with less active ingredient than would otherwise by required.

References :
Insecticides Used in the Urban Environment: Mode of Action. S. M. Valles and P. G. Koehler, http://edis.ifas.ufl.edu/IN077.
Cognis. http://www.mosquitommf.com/AgniqueBrochureWeb.pdf