Question
I live in New England and have a Magnolia in the front yard, this is the second year in a row we've had this same problem.   The tree appears to have what looks like slime mold and sooty mold and there are these round white scales on the branches (they look similar to barnacles found on ocean rocks) - not sure what that is.  There are bees and ants among a few other insects crawling all over the branches and leaves.  How can I treat this tree?

Answer
Sounds like scales or aphids. These insects suck the plant juices form the plant and secrete a substance called honeydew which is high in sugar content. This is sticky and the ants harvest the honeydew. You usually see sooty mold growing on the honeydew.
Scales are under a shell like structure and look like small turtles shells stuck to the branches. These will not move if touched.

Aphids will move when touched and sometimes are covered with waxy material.

Aphids can be controlled with a systemic spray on the foliage and branches. Contact insecticides currently registered for aphid control include: acephate (Orthene),  bifenthrin (Talstar), chlorpyrifos (Dursban),  diazinon, malathion, nicotine sulfate, pyrethrum, , rotenone, resmethrin, and tetramethrin + sumithrin.

A systemic insecticide can be used that the tree takes the insecticide into the root system and to the leaves and twigs and kills the aphid when they suck the plant juices.
Imidacloprid (Bayer Advanced Garden Tree and Shrub Insect Control) is a newly available systemic insecticide that can provide aphid control on trees for several months following application to the soil under the tree.

Scales can be controlled by the use of horticulture oils in the winter months or using systemic insecticides. The scales are difficult to control after the insect has constructed the scale. Systemic insecticides are absorbed and moved within plants. If the plant is infested with a scale species susceptible to systemic insecticide, systemics are particularly useful where it is not practical to spray because plants are large or spray may drift to unintended areas. Depending on the product, systemic insecticide may be applied by spraying foliage, or applying on or into soil beneath trees and shrubs, where the insecticide is absorbed by roots.

There are some newer insecticides that will control the scales that are applied to the soil and the roots transport the insecticide to the branches and leaves and when the scale sucks the juices it kills the insect. Certain persistent insecticides that move systemically in the plant may provide control through the fall. Imidacloprid (Bayer Advanced Garden Tree and Shrub Insect Control) is a newly available systemic insecticide that can provide scale and aphid control on trees for several months following application to the soil.  

Check with your local nursery/garden type store for these insecticides.