The life cycle of a magnolia scale insect is dominated by a long period during which it just looks like the proverbial "bump on a log," but do not let their innocent appearance fool you, because they are sucking fluids out of your plants. There are two general groups of scale (although there are thousands of different species): the armored-body types and the soft-body types. Magnolia scale belongs to the latter group.
Due to their "inanimate" appearance, people are often caught off-guard when they finally witness evidence of an infestation. Worse, they may misdiagnose the situation as a mold problem or even an ant problem; ants are attracted to the honeydew excreted by magnolia scale, as is explained in this article on organic ant control.
There is an insecticidal spray you can apply to fight the problem, but for effective magnolia scale control, it is essential that you understand the scale insect's life cycle, because that cycle is what determines when to spray with insecticide to control magnolia scale.
The Life Cycle of the Magnolia Scale
Scale insects pass through stages (phases) in their life cycle called "instars." The Penn State Department of Entomology explains the life cycle of these insects.
The insect overwinters at the tips of the tree's branches in what is known as the "nymph" instar. These nymphs start feeding in spring and reach maturity in the middle of the summer, at which time mating occurs. "Females later give birth to living young called crawlers in late August or early September." Soon after, the crawling stops and they will cling to the very twigs upon which overwintering will take place -- thus bringing us full circle.
When to Spray With Insecticide to Kill Scale Insects
To control magnolia scale successfully, you must spray the insecticide (for example, malathion or acephate) at the right time in this insect's life cycle (namely, when it is in the crawler stage). These insects are more vulnerable to sprays at that particular time in their life cycle than at other periods, and that time is generally late summer. The Cornell Entomology Department explains that, once the scale insect progresses past the crawler stage in its life cycle, "the exoskeleton hardens, making it less susceptible to contact insecticides."
Since the insects are at the crawler stage in late summer, then, that is the best time to spray (aim for September). If you wish to try an organic insecticide, try neem oil.