Wikipedia
White oil
White oil is a homemade insecticide spray used for controlling a wide range of insect pests in the garden. The spray works by blocking the breathing pores of insects, causing suffocation and death. It is effective in the control of aphids, scale, mealybug, mites, citrus leaf miner and smooth skinned caterpillars.
"White oil" is also an alternative name for mineral oil.
Usage examples of "white oil".
For perhaps three seconds the row of white oil-drums ahead seemed to remain static, a watery indeterminate barrier - the limit between safety and a horrible death.
Had they worked, Crane told his staff, the blue pox would have floated to the top of each well as a white oil.