By JEFF BURBRINK
Extension Educator, Purdue Extension Elkhart County

Jeff Burbrink

GOSHEN — One of the least used and most under-rated methods to control scale insects, and some species of aphids and mites on fruit trees, crabapple, euonymus and other woody plants is the oil spray. If you had problems with these critters before, now is a good time to prepare for the treatment. 

Today’s horticultural oils are the modern descendants of the dormant oils used by our grandparents to protect their fruit trees from overwintering pests such as scale. The old dormant oils could not be used on leaves because impurities caused damage. Today’s modern versions of these products are safer for plants under most conditions. They are typically refined mineral or vegetable oils. They typically include an emulsifier to help the product blend with water for accurate spraying. 

Most species of scales overwinter on the tree as nymphs and European red mites and aphids overwinter as eggs. They may appear to be inactive this time of the year, but like all living organisms, they must breathe. A dormant season application of horticultural oil covers the surface of the insect or egg with an impervious blanket, causing the pest to die of suffocation. 

Remember that oils are not directly toxic to the pests. They only work by suffocation.

Therefore, the better the coverage, the better control you will receive. These creatures like to hide in the deep cracks and crevices on the bark, so thorough coverage is a must. 

On both fruit bearing and ornamental fruit trees, oil sprays should be applied when one-half inch of green is showing at the bud tip, up to the stage known as tight cluster, which is when flower buds are preparing to open. Oil sprays should not be applied during, immediately before, or immediately after freezing weather. For best results, apply when temperatures are 45˚Fahrenheit or above, and not just before rain showers. Purdue’s data has shown that mite control is improved if oil is delayed to tight cluster stage rather than at one-half inch green tip. Earlier oil sprays are more effective than late sprays for San Jose scale control. 

There is another plus to this type of treatment. Oil sprays do not have a negative effect on predator mites, which are mites that feed on other creatures. If you are an organic grower, oil sprays are considered an acceptable solution to many pests.

It is important to note that oil sprays do not have any effect on the two-spotted spider mite, one of the more common mites we have in the area, especially in droughty years. Two-spotted spider mites do not overwinter on the trees. They typically start out in the grassy areas, and as the drier summer weather sets in, the two-spotted spider mite will migrate to more favorable locations, like your trees. 

For more information from Purdue Extension on controlling scales on ornamentals, visit this website. For a complete fruit tree care schedule, download Purdue’s Managing Pests on Home Fruit Plantings, visit this website.

Oil sprays can be purchased at most garden centers or farm supply stores in the area. Be sure to follow the label directions. There are a few plant species that do not tolerate oil sprays, and the label rates often vary dependent upon conditions.