The influence of volatility and water solubility of systemic insecticides on their movement through soil and absorption by plant roots

SUMMARY When applied in the soil at the time potato tubers were planted, phorate and disulfoton were more effective than dimethoate and menazon in controlling aphids on the foliage of the crop in dry years, but all four were equally effective in wet years. The reason was sought by comparing the ability of the roots of wheat seedlings to absorb these systemic insecticides from air and from water. When absorbed from solution in water, all four insecticides were almost equally effective in killing Rhopalosiphum padi fed on the leaves, but when the roots were in contact only with the vapours of the insecticides, phorate and disulfoton were much more effective than dimethoate or menazon. This is thought to be the first demonstration that sufficient of the vapours of systemic insecticides can be absorbed by the roots of a plant to render the leaves toxic to insects feeding on them. These results and the physical properties of the insecticides suggest that only insecticides as volatile as phorate and disulfoton can move through dry soil to the roots of plants, and that even in moist soil phorate and disulfoton probably move mainly through the air rather than through the soil water. The more soluble and less volatile dimethoate and menazon are probably absorbed in amounts able to make the foliage toxic to aphids only when they can move to the roots through water.