Asked September 5, 2019, 2:19 PM EDT

I have a holly tree which is infested with red spider mites and also has scale. It has been recommended to me that I spray with horticultural oil. Is this the best treatment? If so, is there a specific type of horticultural oil to use and when and how often should I spray? Is there any other treatment that you would recommend? The tree is located close to (almost touching) the side of my house which is covered with painted wood siding. I don't know if this is within your expertise, but is the horticultural oil likely to cause a problem is it gets onto the wood during the spraying?

In addition, I have a yucca bush which is infested with yucca plant bugs (halticotoma valida). I have been using ortho 3-IN-1 Insect Mite & Disease spray (active ingredients Sulfur and Pyrethrins) with limited success. What do you reommend? I've read that malathion is good. Should I try that or is something better? I would prefer to use an insecticide rather than horticultural soap or oil because, I believe that the soap and oil does not have residual effect and must be sprayed directly onto the insects.

Baltimore County Maryland

1 Response

We would like to see photos of your holly to better understand your situation.
Shots of the whole tree and also the insect infestation you describe would be helpful. You can attach up to three at a time directly to this reply by using the Choose File tabs below.

Before we talk about your yucca plant bugs, we need to mention what we call IPM (Integrated Pest Management) and pesticides.
IPM is a way of managing landscapes in the healthiest and least toxic way. The vast majority of insects are not pests that bother plants (or you) but are helpful to control the ones that are. All of them together are a part of a healthy ecosystem or web of life.
People have different ideas as to what is an acceptable amount of insect damage in their landscapes, which we refer to as the threshold. For instance a nursery trying to sell perfect looking plants has a lower threshold for damage than a homeowner trying to have a healthy environment but still have an pleasing aesthetic .

Everything we do here is aimed towards using science-based research and the use of least toxic alternatives to manage problems.
I bring this up because older chemicals like malathion are not recommended any more. The long residual is not a good thing, in that it will kill everything, including non-target beneficial insects that help control the pest insects.
We also rarely recommend combination products (3- in-one) as that is application of pesticides that are not specifically needed.

In the case of your yucca bugs, the suggested application is insecticidal soap.
Follow label instructions carefully, and avoid spraying on very hot days.