Question
I understand that certain borers can infest white birches during/around pruning.  Is this true, and when is the best time to prune white birches in San Diego?  Mine are over 15 years old and beautiful, but need a little thinning. I apply Bayer's All-in-One Tree and Shrub Insect Control  once a year as leaf buds swell and haven't noticed much insect activity. I'd like to keep it that way. Any suggestions would be greatly appreciated.  Thanks, Paulette

Answer
Prune only when the tree is dormant if possible. This puts less stress on the tree. It can also reduce exposure to disease. Avoid pruning between May and August. This is peak time for the lethal bronze birch borer, a small beetle, and the females are attracted to fresh pruning wounds.

Look for and remove diseased, damaged or dead branches first. Remove these using pruning or lopping shears. Removing these branches is the principal type of pruning done with birch trees.

Prune for controlled light when removing live branches. You want to avoid pruning more than 25 percent of the live canopy. First study how removal of a branch will increase the penetration of light to the ground. Too much light into the root zone reduces the level of moisture. Birch trees like both damp ground and moist. This can put stress on the tree, making it more susceptible to disease.

The Bayer product is a good treatment for borers so I would continue that. Sounds like you are doing great with what you have done.