I found scale insects on my 3 large holly shrubs last spring. I sprayed with...
I found scale insects on my 3 large holly shrubs last spring. I sprayed with an organic insecticide until early fall when I noticed they were not getting better. I sprayed with a Bayer insecticide. This spring, they were worse, so I continued with the Bayer and another insecticide (both said they were good for scale). Since then, two are completely dead and the 3rd is almost dead. Now, I've seen sooty mold and scales on two large Nellie Stevens hollies. What else can I do?
There are multiple types of hollies and they can have multiple different problems and pests, some more serious than others. It is very important to correctly i.d. the problem or pest before treatment for multiple reasons. For instance, to successfully treat for scale insects, most pesticide applications must be carefully timed for the crawler stage of their life cycle because at other times they are protected by their shells or outer coatings.
If you can send us photos attached to this reply we can help you correctly identify the pest you are seeing on your hollies. Certainly the sooty mold is suggestive of a sucking insect like cottony camellia scale. A heavy infestation of this scale can cause spring leaves to be light green, but dieback is not common, though sooty mold and sticky honeydew are often objectionable. At this point in time, look on the under sides of leaves above where you see sooty mold as the immatures remain on the underside of the leaves through winter. At this point, wait for the time when the leaves fall off deciduous trees this fall (start of the dormant period) and apply a dormant rate of horticultural oil to all tree surfaces, especially the undersides of those higher leaves.
I took a few pictures to show the scale insects on the underside of the leaves, as well as on the entire bark and inside of the trees. The two large hollies in the pictures are a 'Nellie Stevens' and a 'Blue Girl'. Both have LOTS of sooty mold and the white insects as well. I need to find the best and fastest way to protect these shrubs so that we don't lose them, like we did the other 2 and possibly a third holly shrub. So far, these are the ONLY plants in my yard that are infected. There are no deciduous shrubs or trees infected. Lastly, will horticultural oil be strong enough to control these insects? If so, where do you find it and is it mixed?
This is cottony camellia scale. Like most insects, it is somewhat of a specialist and only infests certain plants. This scale is especially fond of holly.
Here is the information about it on our website: http://extension.umd.edu/learn/cottony-camellia-scale.
As you will read, you need to spray during the dormant season and also next spring. Yes, horticultural oil is effective, but timing is crucial. Because you have such a bad infestation, multiple applications may be in order. Remove dead and dying plants that are simply providing a source of more scales.
Any good garden center, farm and home store, or well-stocked hardware store will have horticultural oil. It usually comes in a concentrated liquid and you dilute it differently, depending on whether you want a dormant oil (thick) or a summer horticultural oil (thinner.) You may be able to find it ready-to-use also.