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Why pyrethrins are toxic to insects

The six esters known collectively as pyrethrins are produced in the Chrysanthemum plant, Chrysanthemum cinerariaefolium. Pyrethrins are found at particularly high concentration within flower structures known as achenes which are located in the flowerhead of the Chrysanthemum.

Pyrethrins induce a toxic effect in insects when they penetrate the cuticle and reach the nervous system. Pyrethrins bind to sodium channels that occur along the length of nerve cells. Sodium channels are responsible for nerve signal transmission along the length of the nerve cell by permitting the flux of sodium ions. When pyrethrins bind to sodium channels, normal function of the channels is obstructed thereby resulting in hyperexcitation of the nerve cell and, consequently, a loss of function of the nerve cell. The shutdown of the insect nervous system and death are most often the consequences of insect exposure to pyrethrins.

Synergists

Insects have evolved detoxification mechanisms to interfere with toxins in their environment, like pyrethrins.  Synergists are added to insecticides like pyrethrins to circumvent the detoxification mechanism of insects.  Synergists are chemicals which directly increase the toxicity of insecticides.  When the synergist PBO, piperonyl butoxide, is added, a lower concentration of pyrethrins is required to achieve insect control.

The diagram provided shows the dynamic of pyrethrins binding to sodium channels in the absence (top) and presence (bottom) of PBO.  In the far  left of the diagram, the light blue structures are the binding sites for pyrethrins on the sodium channels.  The yellow diamonds represent active pyrethrins.  Moving towards the right, the next structures are the insect detoxification enzymes, represented here by blue discs.  In the absence of PBO (top), pyrethrins are detoxified by the insect detoxification enzymes and the inactive form of pyrethrins is represented by the light brown diamonds.  In the presence of PBO (bottom), the insect detoxification enzymes (blue discs) are blocked by PBO, represented here by dark blue structures.  In the presence of PBOs pyrethrins are not bound and detoxified by the insect detoxification mechanisms.  When these insect detoxification enzymes are blocked a higher concentration of pryethrins reaches the binding site on the sodium channel, as can be seen at the lower left of the diagram. 

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Uses

Pyrethrins are the active ingredient in many products used in many markets such as:

Home & Garden
Products that control insects in and around the home or garden

Animal Health
Products that control nuisance pests on animals or around animal premises

Crop Protection 
Products that protect growing crops from crop damaging insects

Public Health 
Products that protect the public with government mosquito abatement programs

Professional Pest Control   
Products that control pests in residential and commercial accounts

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