MSU Extension

The horticultural oils offer a valuable option in insect control yet they are probably the least explored alternative. Ever since the mid 80’s when I worked extensively with oils, I have been a proponent of their use. I guess they have proven to be an example for the axiom that it is better to prevent the problem than to deal with it once it is established.

The horticultural oils or “dormant” oils as they are often referred to, can and should be used as very effective control materials for insects that overwinter on the bark, under bark scales, in crevasses, etc. Oil programs are aimed primarily at scale species, adelgids, aphids, mealybugs, mites, plant bugs (eggs and nymphs), psyllids, eggs of certain moths (fall cankerworm, webworm…), leafhoppers, leafrollers (eggs and young larvae), Gypsy moths (eggs and young larvae) and fall web worms (eggs and young larvae). Once on the target pest, oil damages the membranes and the waxy layer of the exoskeleton resulting in suffocation. Timely application of oil will lead to fewer applications of miticides and insecticides later in the season. Oils fit well in IPM and resistance management programs. Toxicity is not a major issue since oil evaporates relatively quickly leaving very little residue. At the time of oil application there is usually not much else active in the orchard; predators are not in the picture yet! 

What are horticultural oils?

The horticultural oils are mainly refined petrolium products. More recently the list has expended to include extracts from the seed of the neem tree (Azadirachta indica) and some vegetable oils such as cottonseed and soybean oils.

Oil effectiveness

In the past, oils had different degrees of refinement. The problems associated with the oil-sprays are most often due to the purity of the oil and rates applied. There are several criteria that need to be considered when determining the effectiveness of oil.

The unsulfonated residue or degree of refinement to remove sulfur impurities of 92% and above is preferred. The higher the percentage the more refine the oil. These highly refined oils are known as “Superior Oils.” There are several trade names for them like, Sun Ultra-Fine Oil, Sunspray, Superior 70 Oil, Supreme Oil, Trilogy (neem product), etc.

Gravity or density reading is referring to the weight of the oil. Paraffinic oil is less dense than aromatic hydrocarbon type.

Viscosity is the most important parameter when selecting the oil for spraying fruit trees. Viscosity is expressed in seconds and represents the time needed for the drop of oil to pass through a standard opening. This in turn relates to the length of time that the oil is exposed to effectively cover the plant before breaking down. The values for dormant oils are from 90 to 150 seconds. For the summer or verdant oils the values are from 65 to 90. To be on the safe side, oils 65 to 70 seconds should be used for summer sprays.

Over time, the term “dormant oil” spray has become synonymous with horticultural oil spray. Initially, the term “dormant oil” referred to a heavier and not as well refined product that was applied during dormant season before bud break or bud swell. Improvements in refining have produced a superior product that is safer to the plants enabling extension of the use-season.

How do oils work?

Superior oils work as contact insecticides and miticides. They can be used by themselves or in combination with other insecticides providing synergistic effect. It is essential to provide excellent coverage so that the targeted pest gets in contact with the oil to have satisfactory control – a minimum of 100 gallons of water per acre is recommended. Oils are mixed with emulsifying agents that allow them to make a milky solution when added to the water. The mixture has a good tank-life, usually up to several days. However, it is best to use fresh mixture and fill the tanks with the amount needed for each day.

Before spraying, oils in the drums should be checked for proper emulsion formulation. This could be easily tested by mixing 2% volume/volume solution of oil in 1 gallon of water, shaking well and letting it sit for 5 minutes. If the solution turns milky, the oil is good and can be used. If the oil separates and stays on the water surface, the emulsifier is gone and the mixture is not safe. The oil should not be used.

Pest control falls into two basic categories: 1) interference with egg development, and 2) reducing the insect or mite population after hatching. In the first instance, the oil may prevent normal oxygen exchange through the egg covering, hardening of the outer membrane and preventing hatching, possibly dissolving the outer covering, by penetrating into the egg itself, oil may cause the coagulation of the protoplasm. When the oil gets in contact with the larval or adult stage of the insect or mite, it may interfere with the insect’s respiratory activity by blocking the tracheal openings on their bodies or, possibly, create imbalance in hormonal activity.

Best time to spray oil?

Oil sprays are best known for their use in “dormancy,” though they could be used throughout the “green” season. With dormant sprays there is a dilemma whether it is better to spray in fall or in spring. In either case there are a couple of things to consider: frost potential and the problem of determining dormancy.

How can we be sure that the trees are truly dormant? Leaf drop does not necessarily mean that the trees are dormant. If mild conditions prevail in the fall, the cells in the wood and cambial layer may still be active. Spraying oil will block and seal the pores and hinder the normal gas exchange killing off the tissue. In our northern climate it is important to have a minimum of 48 cumulative hours of below-freezing temperatures before we can safely apply oil at the dormant rates. Normally, it will take a few days with the temperatures in low twenties or in the teens to satisfy this requirement and stop any activity on the cellular level in the trees.

In spring, just a week of higher than normal temperatures will trigger the “coming-out” of dormancy. Oil applications at this time followed by mild weather are not a problem. However, if the period of above-normal temperatures is followed by a sudden drop in temperatures after the trees have been sprayed, significant tissue damage will occur.

Using oils in the “green” stage during summer is recommended for scale, aphid, and mite control. Superior oils are relatively safe and most fruit trees will tolerate

the application at summer rates. Summer oil rates do, however, depend on the overall health of the trees, temperatures, air humidity and “accompanying” materials in the spray tank.

Severely stressed plants should not be sprayed with the oil. Generally, if there is some stress involved, the rate should be cut down to 1¼%. Normally, it is safe to spray 70 seconds oil at 1½% rate. If there is temperature above 75-80^°F the rate should be dropped to 1¼ to 1%. It is not advisable to spray oil when the temperature is much above 80^°F. High humidity (above 90%) may result in increased injury potential while low humidity allows for faster oil evaporation and lower injury potential. If there is an insecticide in the spray mix, drop the oil concentration to 1%.

When working with oils, it is important to pay attention to the pressure and agitation. Spray should be done at lower pressure (never to exceed 300PSI). Otherwise, the oils can be “driven” into the tissue, which can result in significant phytotoxicity “burn” injury and potential dieback. Damage may and will also occur if the sprayer agitation is not working properly. Under this scenario, the oil will tend to separate and result in non-uniform concentrations on the plant.

Hot mixes

Oil is not compatible with Captan, Sulfur or any other sulfur-containing compound. It is necessary to provide a safe interval (two weeks) between the oil application and use of any of the cited compounds. Otherwise, phytotoxicity will occur.