Some form of horticultural oils have been used to control pests for hundreds of years. Their most well known name is dormant oils. However, the oils we have on the market today make that a misnomer as many are labeled to be used throughout the “green” season. Dormant spray is a name for one of the times you can treat and horticultural oil is a better name for the actual pesticide. Horticultural oils are technically classified as a pesticide because they are intended to control certain insects and mites.
These oils used to be much heavier weight and so could only be safely applied during their dormant season. Timing of the spray is now based more on the insect you are trying to control than on the timing allowed by the oil. The most commonly used horticultural oils in use are highly refined petroleum based products mixed with additives which will allow the oil to mix more thoroughly with water. While ambient temperature still plays an important role in the timing of horticultural oil sprays, the stress level of the plant may be more indicative of how the plant will respond. Even a plant that is not normally sensitive to oil sprays may show signs of damage if it is already stressed by a lack of water.
It is important to realize that even though oil sprays are environmentally friendly they are not a cure all. They only work on those insect/ mite stages that have little to no motility. This is because the spray puts a thin film of oil over the pores or (spiracles) through which they breathe and has also been noted to disrupt some cell membranes allowing desiccation to occur. This is actually considered a mechanical mode of attach which makes it unlikely that a pest will be able to develop a resistance to them and minimizes any chance of beneficial insects being affected. Any insect that is more mobile will disrupt that film of oil thus cancelling the effect. Horticultural oils will generally evaporate fairly rapidly, aka they will have no residual benefits. Once they evaporate they will not affect any future generations of the insect you are trying to control.
The most commonly controlled insects are mites and varieties of hard shelled (armored) scale. Horticultural oils have also been able to get a head start controlling some scale crawlers, aphids and even some powdery mildews. Dormant sprays are best done when the weather is starting to warm up but prior to bud break as the oil can have a phytotoxic effect on the tender shoots. Many insects will start to become biologically active at this time and will start to respirate more rapidly which will make them more susceptible to the oil coating. A good rule of thumb is to avoid spraying at any time during leaf expansion. Labeling requires most oils to only be sprayed when the temperature is under 90 degrees. Which in our area means most of the year.
If you have any questions about a horticultural oil spray or anything you’re seeing in your landscape please give our office a call at: 208-523-5296.