Vegetable oil is a benign insecticide that works well in modest amounts. To deter pests from devouring your plants, spray them on the leaves. It should not, however, be poured on the ground. The oil prevents oxygen and nutrients from reaching the body. As a result, it will have to put in more effort for the plant to live. Although a large amount of oil would be required to kill the plant completely, there are other options for disposing of your cooking oil.