White Prunicola Scale (suspected)

Question: Hi! I believe our dwarf North Star cherry has been infested with White Prunicola Scale. We were away for a little over a week and came back to it covered in snowy white (see photos). Today, we blasted all visible white "stuff" with water from the hose, washing it off but not damaging the bark. The tree is weeping a bit, indicating its stress, but the leaves look healthy and I see no twig dieback. I am wondering what the best organically approved spray application might be at this time of year and again as the tree loses its leaves and goes dormant for the winter? (There appear to be adult scale attached to some branches.) I have also read that Purple Scale Predator ladybirds (Lindorus lophanthae) may be helpful. Any advice would be welcome -- we love this little tree and it produces fruit we enjoy!

responder: Alex Bunten, UVM Extension Master Gardener Volunteer  Replied August 27, 2024, 1:59 PM EDT
response: Hello Jessica,  Thanks for reaching out to UVM Extension Master Gardener Helpline.  Sorry to hear about the scale affecting your cherry. It looks like it really took hold while you were away. It would appear you're right that it's White Prunicola Scale. We answered a very similar question last year and will link that here but also copy the recommendations of our Plant Diagnostics Clinic Director Ann Hazelrigg here:  "I would spray with horticultural oil asap to kill crawlers if they are still active. It is an organic product which is good. This winter/late spring when the tree is still dormant I would spray with dormant oil, another type of organic oil that will smother overwintering scale. Then I would try to spray the hort oil again at the times that the factsheet proposes. If you want us to check to see if crawlers are active, you could send in some samples through the summer. I suspect you will need to repeat these controls for a few years to get this under control. This fact sheet tells how to check to see when crawlers are active. The gardener is welcome to send in a sample and I will have the entomologist confirm which kind of scale it is." I hope that helps! Good luck!

responder: The Question Asker Replied August 27, 2024, 2:05 PM EDT
response: Thank you very much for the response! I picked up all-season horticultural oil yesterday and plan to spray it this evening. I am also planning to try a round of ladybird predators; I'll let you know if it has a noticeable effect, in case that's helpful to others.  Thanks again!JessicaOn Tue, Aug 27, 2024 at 1:59 PM Ask Extension <<personal data hidden>> wrote:

responder: An Ask Extension Expert Replied August 28, 2024, 12:52 PM EDT
response: Hi Jessica: The entomologist thought this was likely the prunicola scale. She said:  "I suppose the first thing would be to scrape off as much as they can and perhaps apply soap and oil. The scale is protected under the fluff so just a simple spray over it won’t help. However, the time to spray for scale is when the crawlers hatch from the eggs, which is in the spring. It looks like the owner has let this infestation go for a while." She should be ready to do the soap and oil next spring when the crawlers are active. The application now won't do much, but cant hurt. I hope this additional info. is helpful.