Apply Horticultural Oil to Control Scale Insects!

We have a couple of good temperature days in which to apply horticultural oil to control scale insect pests… today and tomorrow.

FROM: Stanton Gill, Extension Specialist in IPM and Entomology, University of Maryland, February 6, 2017:

If you have time today or Tuesday, there is a brief window during which to apply dormant horticultural oil. We have a couple of warm days (above 55 F) and when the nighttime temperatures do not sink below freezing in which horticultural oil can be effective.

Over the last couple years, armored and soft scale insects in landscapes have been an increasing problem. The dormant rate horticultural oil appears to work fairly well in controlling overwintering arthropod pests such as scale, mites and eggs of overwintering aphids. 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.

How does oil work?  An application of horticultural oil immediately creates a physical barrier to respiration by clogging the spiracles, or breathing pores, along the sides of abdomens of adults and larvae. Similarly, oils applied to egg masses inhibit oxygen uptake and decrease hatching success. Oxygen demands of all life stages decrease with decreasing temperature, however, so winter applications must be at a higher rate than those of summer oils to ensure sustained coverage. Horticultural oils also interact with cell membranes, interfering with their function and possibly creating toxins.

Horticultural oils typically consist of highly refined petroleum oils combined with an emulsifying agent that 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. In some cases, oils also may act as poisons, interacting with the fatty acids of the insect and interfering with normal metabolism. Essentially, all commercially available horticultural oils (e.g., UltraPure Oil®, Scalecide®, Suff-X oil, Saf-T-Side®), Pure Green 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. Oils are usually used at rates of 0.5 to 1% in summer and 2 to 4% in the dormant season. 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.

Traditionally, most landscape managers have applied horticultural oils in the late winter to early spring, typically when the temperatures are above 55 °F. 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 the next two days to apply. After this period, the window closes. 

NFG 2/6/2017


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