Horticultural Oil

Now is a great time to apply horticultural oil for control of Fraser fir pests. Horticultural oil is the workhorse of organic pest control of Christmas trees. But even if you aren't growing organic certified trees, it provides a good way to control most pests. It can control balsam woolly adelgid (BWA), balsam twig aphid (BTA), spruce spider mite (SSM), hemlock rust mites (HRM), and elongate hemlock scale (EHS).

Oil burn on needles

How oil works. Oil controls pests by smothering them. Therefore, you have to get complete coverage of the tree so that pests on all surfaces are coated with the oil solution. For pest control, a 2% solution of oil (that is 2 gallons in 100 gallons) is required. At this concentration, however, the tree may be damaged. Needles may turn brown and drop off the tree.

Types of oil. Not all oils are alike. To get the best pest control without foliage burn, use a highly refined horticultural oil. These are often called summer oils. Do not use a dormant oil as these will cause foliage burn. Look for oils with at least 92% unsulfonated residues on the label. But the higher, the better! It will list this value under the active ingredients.

Encapsulated oil -- a new type of oil. There is a new type of oil on the market called Saf-T-Side oil. The encapsulation process results in an oil that mixes with water with water and won't cause burn. This is more expensive than regular oil, but you will not need good agitation in the spray tank.

Applying oil. Typically a high pressure sprayer will give better coverage. If a limited number of trees are being treated, you may also use a backpack mistblower. Treat trees from opposite directions to get the best coverage. If you are controlling BWA, you have to wet the tree all the way to the trunk.

The encapsulated oil can be used with any type of sprayer. Other oils should only be used if there is good agitation in the spray tank. A paddle type agitator is better than by-pass pressure recirculating into the tank because the agitation is continuous. The oil and water can separate in the hose if you stop spraying for awhile, so if you do spray what's in the hose back into the tank to remix the solution.

Twig aphid egg killed by oil

What oils will and will not do. Horticultural oil has no residue. It only kills on contact at the time of application. Control of some pests is excellent, but control of other pests is not as good. However, if oil is used each year, pest numbers will continue to be suppressed.

A 2% solution of oil applied in mid-March will do a good job of controlling the following pests:

• All stages of HRM -- actually oil works as well as expensive miticides when controlling rust mites
• BTA eggs
• SSM eggs
• BWA crawlers, nymphs, and adults though the adults are covered with white wool and are therefore harder to wet.
• Cinara aphids

Control of the following pests is not as good. A 2% solution of oil will only suppress the following:

• BWA eggs – however, there shouldn’t be any BWA eggs present in mid-March
• BTA nymphs or adults. Therefore be sure to treat before the twig aphid eggs start to hatch
• EHS -- this is really too early in the year to get good EHS control. You can treat for this pest in the summer with oil, but problems with burning foliage increase in hot weather.

Be sure to keep scouting through the spring to make sure treatments have worked.