Dormant Oil Sprays

Dormant Oil Sprays

Part of Ep. 401 Waiting for the Snow to Melt

Apply dormant oil to trees in the early spring to control insects living on the trees during the winter.  Entomologist Dan Mahr explains the differences between dormant and summer oil insecticides.

Premiere date: Feb 29, 1996

TRANSCRIPT+
Wisc Gardener Transcript: 

Shelley:
I'm with UW-Madison Extension entomologist, Dan Mahr to talk about dormant oils. For years, every early spring, late winter, I go out there and faithfully apply dormant oil to all my apple trees. Is that the best use of my time?

Dan:
It would be a good use of time if you had insects that could be controlled with dormant oils.

Shelley:
Let's back-up. What is a dormant oil?

Dan:
A dormant oil is, simply, a petroleum oil. It is of particular heaviness of oil, so that it gives insecticidal activity. But still, it's light enough so that it doesn't burn the plants.

Shelley:
I've heard of summer oils, too.

Dan:
Correct, there are two different types of oils: dormant oil is heavier, gives better insecticidal activity, but it causes more plant damage; summer oils are a lighter weight oil. So, a dormant oil gives better insecticidal activity, but it's applied when the plants are dormant hence the name, dormant oil.

Shelley:
And it works by smothering the insects?

Dan:
That's correct, it suffocates the insects. So, to apply it, we have to get very good coverage of the branches, twigs and all parts of the tree to make sure we have thorough coverage of the insect's body.

Shelley:
You said, it only worked on specific insects. What should I be looking for?

Dan:
It works specifically on the eggs of some insects that actually over-winter on the tree such as spider mite eggs, some leaf roller eggs. It also works on certain types of scale insects. We have a couple of examples, here. Spider mite eggs-- eggs of european red mite, which is a common apple pest-- the eggs are usually laid in the end of the branches out by the tips and buds. And here, we have some specific examples. These eggs are very tiny, only about 1/50th-of-an-inch in size, bright red in color and oval. It would be helpful to have a ten-power magnifying glass. If they're causing problems, the eggs will be very numerous.

Shelley:
So, once we see them, we'll really see them!

Dan:
That's correct. Here's another example. This is oyster shell scale-- one of the scale insects that cause problems on trees. These are larger, but they're still quite small. They look like little bumps on the bark of the tree.

Shelley:
They look like bark.

Dan:
They have this typical clam shell or oyster shell shape. They're about 1/16th of-an-inch in size. These are actually the adult scale insects that we can control with the oils.

Shelley:
Those light brown crescent things?

Dan:
Correct. Those can occur anywhere on the bark, even on the trunk. It's important to get thorough coverage of the entire tree.

Shelley:
Speaking of coverage, how do we apply this oil?

Dan:
The oils can be applied in a regular home garden sprayer such as this compressed air sprayer. We simply mix it with water, by label directions and shake it up very well, because oil and water tend to separate. So, we have to keep it well-shaken during the course of application. Every couple of minutes, shake it up real well again, so that it doesn't separate.

Shelley:
Again, we have to get the whole tree moist to cover any area where those insects might be.

Dan:
That's correct.

Shelley:
And there's a temperature limit?

Dan:
That's correct. In order to keep from causing problems to the tree, dormant oils have to be applied at temperatures above 40 degrees, but usually below 70 degrees. Read the label, because different manufacturers might have different specific temperature guidelines. That information will be on the label.

Shelley:
There are certain trees that you can't use dormant oils on.

Dan:
Correct.

Shelley:
Okay, thanks Dan. So, know your enemy, read the label and then, dormant oils are an easy and effective way to get rid of over-wintered insects.

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