ON SOME LIMITING FACTORS IN THE USE OF SATURATED PETROLEUM OILS AS INSECTICIDES

Abstract

I. Saturated petroleum oils (petroleum oils from which all, or nearly all, unsaturated hydrocarbons have been removed, usually 98% or more), when used as insecticides on plants, have been shown to be largely non-toxic. Careful studies of the action of such oils have, however, shown persistent disturbances in the metabolism of the plant which may be harmful. Such disturbances are primarily of a physi-cal nature. The application of oil is accompanied by a sharp drop in the transpiration rate, varying directly with the viscosity of the oil: the starch content of treated foliage is greatly reduced during the following 5 wk. and then rises rapidly to a point above normal; respiration is greatly increased for a month or more until much of the absorbed oil is translocated.[long dash]II. Field observations show that the heavier oils are absorbed by the leaves while laboratory studies show that the treated leaf becomes functionally active after about 60 days. By means of a special system of staining, the leaf was made trans-parent without disturbing the oil, so that the distribution of the oil in the plant could be followed. The epidermal cells absorbed the oil very rapidly. On the lower surface of the leaf, penetration was principally through the stomata. After penetration, the oil becomes green from absorbed chlorophyll and may be seen within the cell. It accumulates in the leaf veins, enters the vascular system, and, in the citrus tree, the pith and wood.[long dash]III. Penetration of oil into foliage varies with both the type of leaf and the character of the oil. Leaves adapted to xerophytic conditions are more resistant to penetration than mesophytic leaves. A light oil such as kerosene naturally penetrates more quickly than a viscous oil of 110 seconds, Saybolt. The penetration of oil into foliage leads to such serious disturbances as may cause yellowing or death of leaf tissue and even twigs.