Q: What is the best treatment for the bugs causing this mold. I am afraid it is killing my magnolias.

A: Your magnolia has a severe infestation of one of the several species of scale insects. The motionless insects feed under plant leaves, sucking sap from the leaf. They then exude a syrupy liquid called honeydew that falls on leaves below. Honeydew is a great food for sooty mold, which covers the top of the leaves, making them unsightly.

There are three steps to control:

1. Spray as much of the tree as you can now with neem oil, trying hard to spray the underside of leaves.

2. Apply a systemic insecticide (click for sources) now (October) to the ground around the tree

3. In March, wrap several twigs with duct tape, sticky side out. Check the tape flags every few days. When they start catching orange/yellow crawlers, spray the tree with garden insecticide.

In May, check the leaves to see if most of the formerly white critters have turned brown (meaning they are dead). If you still have a lot of the white things (and particularly if they seem “juicy” when squeezed with your fingers), repeat the steps above.