Various oils have been used to control insect and mite pests for hundreds of years. Recently, however, renewed attention has focused on the use of oils as a "natural" substitute for traditional insecticides.
What we call horticultural oils formerly went by the names dormant oils, superior oils, or 70-second oils. These names referred to the stage of plant development when the oil spray was applied (i.e., dormant) or technical details concerning how the oils were made. Recent refinements in the manufacture and formulation of oil sprays make today's products much more "plant friendly" than in the past. Because of the wide range of season-long applications that are available the name "dormant oil" is no longer adequate and the name "horticultural oil" is preferred.
Horticultural oils are highly refined petroleum products than can be mixed with water for application to plants for control of target insect and mite pests and without injury to the plants. Modern horticultural oils are not the vegetable, fish or whale oils that have other past and present uses.
Oil sprays are exceptionally safe to humans (the pesticide applicator as well as others). They have little if any effect on wildlife and nontarget insects in the environment (ladybugs, parasitic wasps, honey bees, etc.) Oil sprays are less toxic because of the method by which they kill target pests. Oils are not poisons. Instead, the thin film of oil covers the target insect or mite and plugs the spiracles or pores through which it breathes. The cause of death is primarily suffocation. Large, motile insects and animals that breathe by another method are not affected by these oils.
Another advantage of oil applications is the absence of objectionable odors. Also, compared to many insecticides, oils are usually inexpensive.
The major limitation to using horticultural oils is the very few pests that are effectively controlled by this treatment. Dormant oils are only effective against those pests that are thoroughly coated by the spray solution. This usually means small, immobile or slow moving pests that are exposed on the surface of the plant at the time of application will be controlled. In Iowa, this is a pretty short list in the home garden and landscape.
The following pests are good candidates for control by oil spray: pine needle scale, oystershell scale, euonymus scale, aphids, spider mites and small pine sawfly larvae. Many of our most annoying and common garden pests such as apple maggot, cabbageworms, white grubs and others are not effected by oil spray.
Since oil sprays only work by contacting and covering the target pest, thorough application is essential. Missed leaf undersides or small cracks in tree bark provide a safe refuge for pests.
If you use oil sprays, read and follow all label instructions. Oils must be mixed exactly at the right dilution rate to prevent plant damage. This is especially true for summer uses of the oils. Sprays must be evenly applied at the correct temperature and only to healthy, unstressed plants. The appropriate temperature range for oil sprays is warmer than 40 degrees F but no hotter than 90 degrees F.
Proper timing is critical for success when using oils. Dormant oils should be applied in late March or April before leaves or flowers show signs of breaking dormancy; that is, before "bud break." A common mistake is to apply 'dormant' oil sprays too early (on the first warm day in February or March) before insects are actively respiring and susceptible to the oil's suffocating effects. Wait until as close to bud break as possible before applying oil sprays. For summer use, oils are effective against insects that are "soft and slow." Oils will not control late instar immatures or adult stages. Effective monitoring to discover pest populations in the early instar stage will be necessary for effective control.
This article originally appeared in the February 19, 1999 issue, p. 16.