Homegrown: Aspen trees
We planted aspen trees several years ago and they have done well. We started with three trees and they now number 30 or so.
We are now threatened by tiny little “eggshells” on some of the trees. We’ve been told it is a parasite and can kill the trees.
What can we do? Are there chemicals to fight this problem? It would devastate us to lose these trees.
— Dick and Marj
Sounds to me like you have some oystershell scale on your trees. As the name implies, these little devils look like masses of miniature oysters stuck on the bark of the twigs, branches and trunk of the tree.
They’re pretty small, about 1/8-inch long. They’re a brownish gray and can sometimes be difficult to spot against the color of the bark.
They suck the sap from the tree and will damage and even kill branches.
This time of the year, what you’re seeing are the dead remains of last year’s scales. They’re all female and they only live for one year, but before they die they lay a bunch of eggs which are under that hard shell. The eggs typically start hatching in late May or early June and will continue hatching for a couple weeks.
The young scale are called “crawlers” and they’re small, about the size of a pinhead, and usually a yellowish color.
They crawl out and find a suitable place to settle down. Typically, within a week, these crawlers start to secrete the waxy shell that protects them so well.
Controlling oystershell scale is a bit of a challenge and usually needs to be maintained over several years to completely get rid of them.
The first thing you can do early in the spring is prune out any badly infested branches that are losing leaves and scrub off the remaining overwintering shells with a plastic scrub pad.
This will remove most of the egg masses and expose the rest. Then, apply a dormant oil spray to try to kill as many of the exposed eggs as you can.
Doing this will greatly reduce their numbers and make later control measures easier.
The dormant spray must be applied before the tree buds start growing for the year. Since they’re already budding, skip the oil for now. Scrubbing off the shells is still very helpful. Just be careful when you’re scrubbing that you don’t damage any young bark.
The next step in controlling these devils is applying some insecticide sprays when the crawlers emerge. The exact timing of those sprays will vary from year to year.
As I mentioned earlier, the hatch is usually in late May or early June, and it’s best to continue to check your trees for the crawlers’ presence. They are tiny, but they are visible with the naked eye if you look closely.
Once you start to see them, start spraying. Since the crawlers lack that hard shell, most insecticides will work well.
The best choices probably are permethrin, acephate or cyfluthrin. I’d spray twice about 10 days apart.
If you don’t want a chemical insecticide, All Season Spray Oil (not a dormant oil) or insecticidal soap work great. However, you’ll probably want to spray once or twice a week for best control.
The soil applied systemic insecticide imidacloprid doesn’t do that good of a job on this particular insect. It doesn’t concentrate well in the bark tissues where these guys feed.
It’s too bad because it would make applying your control measures a lot easier. Understand, it does give some control but relying completely on it to solve the problem will disappoint you.