Hi - There are different types of scale insects, with different life cycles, and therefore they have different timeframes for the most effective treatment. If you can please share with us some photos of the shrubs and the scale insects you are seeing, that will help us determine which type and the best course of action. You can upload several photos here and we would be happy to take a look. 

In the meantime, here is our information on some of the most common types of scale we see in home landscapes, and how to manage them. https://extension.umd.edu/resource/introduction-scale-insects

Christa

Thank you for the photos. These appear to be Indian Wax Scale, a fairly common species that can feed on an array of different shrubs and trees. Usually they cause the plant stress and potentially some leaf loss, but dieback is not a regular occurrence unless the population is very high (or has been present for a long time) or the plant is suffering from another unrelated stress factor. Nandina are fairly tough plants, but can be stressed by overly-dry or overly-wet soil, too much shade, or over-pruning (if sheared, since they don't branch the way most shrubs do, this can limit regrowth). These stresses can contribute to leaf loss, so it can be difficult to determine whether the scale are solely responsible for that symptom.

The simplest approach for now is to just pick the scale off and dispose of them. If you do not wish to touch them (and don't have tweezers or some other way to pluck them off), you can cut the Nandina stems down to just about the soil line. Plants with healthy roots and which have been established for a few years should regrow new stems fairly easily, and since the scale don't live among the roots, this will eliminate most or all of them for now. More information about management options are available on the linked page and our general scale information page(s) that Christa included. Pesticide use is discouraged unless no other technique is successful. Additionally, Nandina is considered an invasive species in our region, though admittedly not all varieties produce the berries by which they can spread. If the scale are a recurring problem or you do not wish to treat for them, you could replace the plants with a different evergreen or flowering shrub.

As for the issue of scale appearing on various plants around the yard, sometimes plant stress predisposes them to colonization by pest insects. Stress can take the form of temperature, soil moisture, or root confinement. Added heat from nearby walls or pavement, which can not only reflect heat during a summer day but also will radiate it out overnight, reduces the plant's ability to "rest" from the daytime stress. Soils in these environments tend to be drier, which can tax roots and halt leaf growth, and limited root space from a nearby building wall or compacted soil underneath paving can also suppress root growth or make plants less resilient when faced with a challenge. Soils staying too wet due to an influx of water from a nearby roof downspout outlet or other source of drained water can kill roots on sensitive plants. Lastly, if pesticides are used with some regularity in the yard, such as aerial spraying for mosquitoes (even with organic formulations), the harm to insects which help prey on scale and other pests can lead to outbreaks afterwards. While these factors might not entirely apply to your yard's conditions, these are some of the most common causes of pest outbreaks and plant struggles.

Southern Magnolia may have had Indian Wax Scale, though it can contract other, similarly-shaped scale instead, such as Tuliptree Scale. (While not nearly as white as Indian Wax Scale, dead adult Tuliptree Scale can turn paler than their usual dark brown color.) Unless the tree was quite young and the scale population high, it's unlikely the scale alone caused drastic leaf loss or plant death on the Magnolia. In that case, a problem with root health is suspect, such as accidental over- or under-watering or planting the tree too deeply. If this or a replacement Magnolia exhibits worrying symptoms in the future, feel free to send us photos and we can try to diagnose it.

Miri

Monitoring plants for signs of recurrence is really the only practical way to detect a scale population rebound. Pre-treating the plants with insecticide is not recommended, in part because any systemic chemical (the only type that would be useful in that situation) could be present in flower nectar and/or pollen later in the season, which would put pollinators at risk of harm. Plus, such a treatment might also impact harmless insects using the plant as a food source (though not as a pest).

Often, though not exclusively, pest insect outbreaks follow a period of plant stress, such as from over- or under-watering, high heat, over-fertilization (perhaps not stressful by itself, but this can force growth the plant can't defend), or other environmental conditions that don't suit its preferences. While the weather is of course outside of our control, monitoring soil moisture during  dry periods and avoiding an influx of water from sources like a nearby roof downspout outlet can help reduce stress related to root moisture levels. Heat reflected off pavement or hard surfaces like walls can be mitigated by siting plants far enough from the radiating warmth to allow for some air circulation to cool the plant. While it may not be practical to move established plants if they're in such a situation, keeping it in mind for the future can help. Spider mites are a plant pest that commonly experiences population booms where plants are more hot and dry than the rest of the yard due to nearby structures trapping heat or impeding airflow.

Since scale crawlers (the hatchlings) are tiny and easily moved on things like bird feed, squirrel fur, and even the wind, it's impossible to keep them excluded from certain plants long-term, though catching a new population early makes a big difference in treatment ease and success. Additionally, not all scale populations will need intervention, since a number of beneficial insect predators and parasites can consume them and keep numbers in check. A landscape having diverse plantings that include many flowering plants (annuals, perennials, shrubs, trees) providing pollen can help to boost beneficial insect numbers, since many of them use flower resources as alternate sources of food when not hunting prey.

Miri