Leaf miner damage isn't too devastating, but it can weaken the trees to the point that birch borers move in. Once borers arrive, you might as well surrender. It's done at this time to catch the miners as they move up the trunk to lay eggs on the leaves. Or you can spray it directly on the leaves later in the season. Keep in mind that Cygon is pretty potent, and not always effective for this type of infestation.
Healthy trees have a better chance of fighting off disease and insect damage.
Thanks for your question. I was busy in the fall and never got them planted. If the bulbs are firm, solid and not corky or mushy, try it. Gail Beckie Fox Try circling susceptible plants with a ring of pea gravel or crushed egg shells. The critters don't like crawling over anything that's scratchy.