For the past couple of years, Bryan Davis, IPM technician in Ashe, Alleghany and Watauga Counties, and I have been playing around with using horticultural oil for twig aphid control. Bryan first heard of this technique when he gave a talk in Maine. Tree growers up there regularly use this method for twig aphid control. But their weather, pests and trees are different than ours. Would this same technique work here?
The answer has been an overwhelming -- YES. Spraying trees in mid-March with a 2% oil solution has resulted in excellent twig aphid control without causing burn. That is, if the agitation is sufficient. And what do I mean by excellent control? In one test plot conducted in Avery County last year with the help of Jerry Moody, there were no curled needles at all. In Bryan Davis' own trees where he was getting good enough aggitation, there was again, virtually no twig aphid curl.
So how is oil controlling twig aphids? This is a twig aphid egg that has been sprayed with oil. It is the black thing in the middle that kind of looks like a raisin. It looks a lot different from a healthy egg which is plump and has white, waxy rods scattered across it. This egg will not hatch in the spring.
A 2% oil solution is also quite effective at controlling hemlock rust mites. In fact, oil is every bit as good as some of our other miticides such as Sanmite and Envidor. Oil will also reduce problems with spider mites, and control balsam woolly adelgid.
The trick is to apply the oil before the twig aphids have started to hatch, as it is not as effective against the aphids as their eggs. Early to mid March is a safe time to apply. By the end of March, some aphids will be present. I haven't gone as early as February with a treatment, but may if the weather cooperates.
There are 2 down sides to using oil. One is that you have to use a high pressure sprayer. Though I haven't tried it personally, I don't think that there will be good enough coverage to get control of pests with oil applied with a mistblower.
The other down side is potential for burn. Frasers are very sensitive to burn by oils and pesticides. If there isn't good enough agitation, even dormant trees will be burned. After all, oil and water don't mix. However, there may now be a solution.
Even if you don't get burn, there is a distinct change in the color of the trees. It's not that the trees look bad, but on a sunny day you can definitely tell that the trees have been sprayed with oil.
Saf-T-Side oil is an encapsulated oil that mixes readily with water. It is more expensive than regular oil, but it appears to be much safer. I tried it last summer on a hot day in July for elongate hemlock scale control, and though several other products burned the trees, this oil did not. I haven't tried it for twig aphid control in the spring yet, but I plan to this spring. I don't see why it wouldn't work. After all the July spray with oil gave the same results you would expect with a regular oil -- about 65% control. I'll let you know how it works.
This method of pest control will work well with organic Christmas trees. If you try it, be sure and scout from mid to late April to make sure twig aphids aren't showing up.