Not far from my house, I spied what appeared to be black paint on the trunk of a magnolia. Now, tree bark is supposed to be brown, tan, or grey in most cases, but not black. I suspected the devious work of a real sucker, the magnolia scale (N. cornuparvum). A close inspection confirmed my suspicions and revealed an encrustation of dozens of scales on every branch.
Magnolia scale comes from a group of insects known as soft scales, so named for the soft texture of their outer skin. Compared to most insects, these creatures move very little. The adult females hunker down on twigs and branches and engage in the business of producing young. In autumn each female may give birth to hundreds of young; really a sucker is born every minute. Crawlers move away from their mom, find their own little piece of bark, and settle in for the winter.
When spring arrives, the young scales become active. The flow of nutrients in the tree provides food that supports growth and development of the scales. Like aphids, scale insects have sucking mouthparts that are inserted into the plant. Much the same way you sip a drink through a straw, they withdraw nutrients from the vascular system of the tree with their proboscis. Excess fluids taken in while feeding are excreted by the scale in the form of sticky liquid called honeydew.
When honeydew falls on bark and leaves, it serves as a substrate for the growth of a fungus called sooty mold. While this fungus does not cause disease in the tree, it does stain the bark and leaves deep brown or black coloration. Honeydew is a prized food of many insects such as wasps, and infested trees can be a frightful swarm of activity. Heavy infestations of magnolia scale can reduce foliage and flower production. Plants appear leggy with dead branches.
I have found magnolia scale becoming much more common. It could be due to warmer winters failing to kill off crawlers. Magnolia is often attacked if planted in full sun, surrounded by little vegetation, roads, parking lots, and buildings. Reducing plant stress by proper placement and irrigation may reduce susceptibility to infestation. Excessive nitrogen fertilizer may increase infestation by making the plant more nutritious to scales and reducing natural defenses.
In some cases, it may be necessary to treat a tree with insecticides. Imidacloprid can be applied as a drench around the root zone of infested plants. This water-soluble insecticide is taken up by the roots and transported throughout the plant where it is ingested by sap feeding insects. The time to apply is several weeks before crawlers are active, in early May, for best results.
So, if you see a tree with dark brown or black bark, inspect the branches and see if these small suckers are to blame.