Dormant and verdant or summer oil applications have been used safely on many types of plants. For oil to be effective, the material must coat the pest or egg, thus complete coverage is essential for optimum results. Some oil labels caution the applicator about using oil in combination with or immediately before or after a spray containing certain chemicals.
Concern exists over the potential phytotoxicity from oil applications on green and growing plants.
Many recent research studies have shown no phytotoxicity except to a few sensitive species under the most adverse spraying conditions of high heat and relative humidity. Some species which may be sensitive under certain conditions include Acer rubrum 'October Glory', Acer saccharum 'Green Mountain', Quercus rubra , Juglans nigra , and some cultivars of Juglans ailanthifolia and J. cinerea . Phytotoxic symptoms may appear as twig dieback, leaf burn, and killing of new growth. Oil removes the blue coloration from the current season's growth on plants such as Picea pungens var. New, untreated foliage is not affected by previous applications.