|Capnodium sooty mold on a holly.|
Durham Co. Master Gardener
Oils are complex. They can be an extra virgin companion to focaccia breads, a fishy supplement in the fight against cardiovascular disease, or a devastating killer to marine wildlife with decade-long impact.
Or, the same oil can be a killer to voracious, piercing and sucking insects and sooty molds, while playing savior to the holly bush on which the two are attacking. Enter the role of horticultural oils.
Horticultural oil is considered a practice of Integrated Pest Management (IPM) for controlling insect damage to plants. They consist of ingredients and a mechanism comparatively less harmful to the environment than commercial grade pesticides1. One of the main advantages of horticultural oils is that they dissipate through evaporation, and leave little residue on the plant, which in turn leaves little chemical impact to the plant’s soil and ecosystem. Plants (and pests) need to be thoroughly coated with the horticultural oil to be truly effective, however, once dried the oils will cease to have the desired insecticidal effect1. Another advantage that is making oils a popular tool in IPM is that they are generally easy to apply with ready-to-use formulations, can be applied with existing spray equipment, and can be combined with other commercial pesticides to extend performance to a broader range of pests. More notably, unlike commercial pesticides, insects have developed no immune responses to horticultural oils4!
Horticultural Oils Potential Hazard to2:
Water quality (aquatic wildlife)
Natural enemies (beneficial)
People and other Mammals
Not acutely toxic
Toxicity category2: III - Apply only during late evening, night, or early morning
Horticultural oils were first used centuries ago to control mites and scales on fruiting trees3. They were first developed as a thick, winter or dormant-season oil treatments (before bud break) to kill eggs of caterpillars, mites and insects, such as scales and aphids, that spent the winter on the plant4. Today, gardeners and commercial growers alike can use horticultural oils on not only fruit trees, but shade trees and woody ornamental plants to kill scale, aphids, mites, and other soft-bodied insects1. In addition, lighter formulations horticultural oils have been recently developed for use throughout the growing season for flowers, vegetables and other herbaceous plants. These oil products are commercially labeled “summer,” “superior,” or “supreme oils”1.
The main difference between heavy dormant oils and lighter summer oils is that dormant oils have 50 to 90% unsulfonated residues4 (50 to 10% unsaturated hydrocarbons resulting from the plant’s chemical reaction with sulfuric acid in the oil), and they tend to damage green plants and tender stems. Summer oils have 92 to 96% unsulfonated residues (8 to 4% unsaturated hydrocarbons) and they are much safer to use on leaves and stems4.
How do horticultural oils work? First the pest must be thoroughly coated with the oil. The density of the oil treatments subsequently blocks the respiratory “spiracles” through which insects breathe, causing them to die from asphyxiation. Oils have also been clinically shown to negatively interact with insect body fatty acids, thus interfering with some insects’ metabolism3. Horticultural oils also clog the piercing and sucking stylets in the Homoptera insect order, thus blocking the transmission of some plant viruses by aphids3. Lastly, horticultural oils can help smother established populations of powdery mildew and sooty molds4 allowing the gardener to more easily hose off the infestation.
DIY and penny-savers might be tempted to create their own a horticultural oil, but getting the correct viscosity and insecticidal compounds is a bit more complicated than pouring some Wesson™ or extra virgin into a spray bottle with a gush of water and shaking it up. Commercial horticultural oils are synthetic, refined petroleum products, commonly also known as “mineral oils”. Impurities in the oil which are associated with plant injury, such as aromatic compounds and compounds containing sulfur, nitrogen or oxygen, are removed3. Filtration, distillation and de-waxing process complete the production of the finished base oil; from there the base oils is combined with an emulsifying agent that allows it to mix with water. This mixture usually is used at about a 2 percent dilution3. If organic and simple (not to mention less fussy and scientific) is more the philosophy of a residential gardener, then he can create a homemade horticultural oil with one gallon of water to four tablespoons of cottonseed oil2. Cottonseed oil is generally considered the best insecticidal of vegetable oils. Soybean oil is another choice which has been deemed fair-to-good control of some insects and mites1.
- Avoid using oils on plants that tend to be oil-sensitive. Avoid drift onto sensitive plants.
- Do not apply when temperatures are excessively high (above 100 degrees F) or low (below freezing). High temperature limitations are primarily related to the drought-stress status of the plant. Plants under stress may be damaged. Those not stressed are much less likely to be damaged by an oil application. Dry conditions without plant stress generally reduce risk of injury by oil, because evaporation is more rapid.
- Do not apply oils during freezing weather. This can cause the emulsion to break down and produce uneven coverage.
- Do not apply oils if plant tissues are wet or rain is likely. These conditions inhibit oil evaporation. High humidity (above 90 percent) also may contribute to injury risk, while low humidity generally reduces it.
- Do not spray when shoots are growing.
- Avoid treating plants during the fall until after winter hardening has occurred. Fall treatments have sometimes caused increased susceptibility to winter injury.
- Do not apply oils in combination with sulfur or sulfur-containing pesticides such as Captan or Karathane. They can react with oils to form phytotoxic compounds. Because elemental sulfur can persist for long periods, label directions on most oils prohibit their use within 30 days of a sulfur application.
- Some species are more sensitive to oils including: black walnut, Cryptomeria, Douglas fir, hickories, junipers, cedars, Japanese and red maples, redbud, smoke tree, Alberta spruce
(2012). UC IPM Online. The Regents of the University of California. Retrieved from, http://www.ipm.ucdavis.edu/TOOLS/PNAI/pnaishow.php?id=39