I had a bad case of aspen scale and have sprayed volec oil 2 years in a row. Trees have responded pretty well but some have changed the bark from smooth to wrinkled. Do I need to do it again this year, should I skip a year, or what do you suggest. Trees are about 23 years old and some are quite tall. Some are only 15 feet cause excel keeps topping them every two years. Thanks
That roughening of the bark is a very common response to oystershell scale having been present. Even if the insect has been controlled, the bark continues to have that roughened appearance. Not sure why aspen responds this way, but it is a permanent change in the way the bark will grow from now on.
As for whether you need to continue that depends on whether there are still live scales present. It is a bit difficult to tell if they are still alive, since the scale cover remains attached even after the insect underneath is dead. The best way to tell is to crush some of the scales you think might still be alive and see if any fluid is present. Or you can flip them and see if you can see the tiny white eggs. Eggs are the stage they would be in at this time.
If the scales are all dead or there are very few you can stop treating. Often aspen, once recovered and the bark has thickened, will have a fair amount of resistance to the insect.
The sheet we have on oystershell scale is at: http://www.ext.colostate.edu/pubs/insect/05513.html