ABSTRACT
Using Melophagus ovinus, the sheep ked, as test insect, it was found that certain organic solvents of diphenylamine, such as the cresols, benzyl alcohol and 4-methyl-cyclohexanol, greatly increase the rate of action of this insecticide. Others, strcn as carbitol and methyl benzoate, gave little or no improvement in the time of kill. The degree to which a solvent induces rapid penetration of an insecticide is referred to as its ‘carrier efficiency’.
The influence of the physical properties of the solvents on carrier efficiency was investigated. It was found that a high carrier efficiency could be correlated with a high rate of penetration through beeswax, a high partition coefficient of the solvent between beeswax and water and a high solubility of insecticide in a solution of the solvent in water. The volatility of the solvent and the solubility of insecticide in solvent were also contributory factors.
Mixtures of two solvents, each showing no carrier efficiency but together possessing all the essential physical properties, were tested and showed a carrier efficiency considerably higher than that of either constituent. This is taken as supporting evidence that carrier efficiency depends on certain physical properties of a solvent.
Using a range of solvents shown to exhibit various degrees of carrier efficiency with diphenylamine, comparable results were obtained with dixanthogen, ω-nitrostyrene dibromide and rotenone and showed that the synergy could be extended to other insecticides.
It is suggested that certain solvents increase the rate of penetration of contact insecticides through the insect cuticle :
By transporting the insecticide through the lipoid elements of the epicuticle to the interface between this layer and the water permeating the exocuticle.
By concentrating the insecticide at the interface between the epicuticle and the exocuticle, as the. solvent passes into the exocuticle, and thus increasing the diffusion gradient of the insecticide across that interface.
By increasing the solubility of the insecticide in the water permeating exo-and endo-cuticles and thus, by raising its partition coefficient between solvent in the epicuticle and water in the exocuticle, further increasing its rate of diffusion, not only across this interface, but also through exo- and endo-cuticles to the hypodermis.
It was shown by Webb (1945 a) that ground derris root dusted on to keds in which all the spiracles were sealed penetrated the cuticle slowly at 30°C. but not at all at 20°C. Derris resin extracted from the root and ground was also used in later experiments (Webb, 1945 b), when it was found that penetration of the cuticle was rapid at 30°C. but very slow at 20°C. The difference in behaviour of the ground root and the ground resin, at 30°C., was attributed to the higher concentration of the toxic agent present in the latter preparation. It seems that this suggestion may be incorrect, since a powder containing only 0·25 % of rotenone, and unable to penetrate the spiracles, caused death of keds in as short a time as 6 hr. at 30°C. In spite of the high concentration of derris resin in ground root, most of this is probably contained in cellular elements and is, therefore, unable to come into contact with the surface of the insect. If the available resin is composed solely of particles liberated from parenchyma cells of the root during the process of grinding, then, as these are carried freely by air currents, it would be expected that ground root would act rather as a respiratory than as a contact insecticide. Where ground resin or rotenone is used, every particle of the insecticide is free to come into contact with the cuticle and, in a powder containing 0·25 % rotenone, this may be far greater in amount than the free particles in a sample of ground derris root, although in the latter the concentration of resin may be very much higher.
Since this paper went to Press it has been shown by Wigglesworth in Rhodnius prolixus Hemiptera) and by one of us (J. E. W.) in Eomenacanthus stramineus (Mallophaga) that the pore canals do not end at the base of the epicuticle, but pass through the epicuticle as far as the outer wax layer. Thus after an insecticide has penetrated the wax layer and has entered the aqueous cytoplasmic contents of the pore canals its passage into the body should be facilitated by streaming of the protoplasm within the canals.
Further details of this work on the structure of insect cuticle will be published elsewhere.
