Physical Action

Since the action of Accotec surfactant formulations is unique amongst insect control agents it is described here in some detail. This can best be done at the molecular level where the physical nature of the process can be shown. In this way, a single molecule of monolayer surfactant can be represented schematically as having a long water-repellent (hydrophobic) 'tail' attached to a more compact water-attracting (hydrophilic) 'head'. In consequence of this asymmetry a collection of these molecules on a water surface will spontaneously form an oriented sheet of molecules with tails in the air and heads in the water. This is a monomolecular layer or 'monolayer'.

Diagrammatic representation of highly magnified surfactant molecule at air/water interface

When a bubble of air which is encapsulated in a larval or pupal respiratory syphon or trumpet rises to a catchment surface, it will break through to the atmosphere, whether the surface is clean or treated with surfactant.

Normal approach and withdrawal from clean water surface (larval and pupal stages)

However, at a monolayer-treated surface, when it detaches itself, it brings its own monolayer, transferred from the surface. Any further contacts result in the opposition of two monolayers, forming a typical stable foam structure (bilayer). This is stable because of the intervening layers of 'heads', which are difficult to remove and therefore inhibit respiration.

Detachment from monolayer-covered surface produces stabilised 'bilayer' on next approach

Monolayer molecules on the trumpet/syphon air-sac now have the opportunity to migrate to the hydrophobic plastron surface by attraction of the 'tails', thus the hydrophilic 'heads' bring water molecules with them. This eventually wets the plastron, reducing the air content of the syphon, until the air sac is completely displaced, resulting in the suffocation of the insect by drowning.

Increased wetting of the interior of trumpet/syphon with eventual displacement of respiratory air sac leading to death by asphyxia

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