Advocacy: IPM Alerts

IPM Alert: Apply Horticultural Oils This Week

Monday, February 6, 2017  

The Short Window Has Opened for Horticultural Oil Applications

From Stanton Gill, Extension Specialist in IPM and Entomology, University of Maryland

February 6, 2017

There is a brief window of time between today and tomorrow (Tuesday and Wednesday) to apply dormant horticultural oil. We are expecting a couple of warm days (above 55 °F) with nighttime temperatures above freezing, during which horticultural oil can be effective. The dormant rate horticultural oil appears to work fairly well in controlling overwintering arthropod pests such as scale, mites, and eggs of overwintering aphids. Oils are usually used at rates of 0.5 to 1% in summer and 2 to 4% in the dormant (winter) season.

Over the last couple years, armored and soft scale insects in landscapes have been an increasing problem. One of the least expensive materials for controlling scale insects is horticultural oil. It also kills mite eggs that overwinter on plants and aphid eggs tucked in cracks and crevices of woody plants. Although pesticide prices have skyrocketed over the last 20 years, horticultural oil is still relatively reasonable in price. So why aren’t you using it?

How does horticultural oil work?

Essentially, all commercially available horticultural oils (e.g., UltraPure Oil®, Scalecide®, Suff-X oil, Saf-T-Side®) are refined petroleum products (also known as mineral oils), with the impurities of the oil that are associated with plant injury—such as aromatic and sulfur compounds—removed. Filtration, distillation, and dewaxing complete the production of the finished base oil, and an emulsifying agent allows the oil to mix with water. Some plant-derived oils also are used. Whether petroleum or plant-derived, horticultural oils all basically work the same way—they coat the air holes (spiracles) through which insects breathe, causing them to die from asphyxiation. Similarly, oils applied to egg masses inhibit oxygen uptake and decrease hatching success. In some cases, oils also may act as poisons, interacting with the fatty acids of the insect and interfering with normal metabolism.  As oils have grown in popularity, the marketplace has been flooded with several plant-derived vegetable and herb-based oils that also can be used as insecticides.

One precaution: Do not apply oils during freezing weather because it can cause the emulsion to break down and produce uneven coverage. You basically have today and tomorrow to apply. After this period, the window closes.