Junipers used in bonsai are typically hardy plants that have few problems with pests and diseases. If you're lucky, you will never need to worry about diagnosing and treating problems. But if you do have insect problems with junipers, scale is one of the more common.
What is Scale?
Scale are tiny insects that live nearly their entire life as stationary 'bumps' on trees, shrubs and other plants. During their stationary phase they suck sap from the host plant, weakening the growth. In small numbers scale do little harm, but when allowed to multiply they can cause significant damage to bonsai. Taxonomically of the family Diaspididae, there are more than 3000 known species, many of which interact with only specific genera or even specific species of plants.
This YouTube Video contains all the same information plus a bunch more detail and more images and video of this pest:
Scale Species to Host Specificity
Juniper and the closely related plant genera such as Chamaecyparis, Cupressus and Hesperocyparis are prone to infection by a number of species of scale, most prominently Carulaspis juniperi and Carulaspis minima. Illustrating the specificity of the relationships, there is also Cryptomeria scale, Pine Needle Scale, Hemlock Scale, and Redwood Scale among many others. While each species of plant and each species of scale have a specific relationship, it is difficult to distinguish the species of scale using the naked eye, or even with a 20x hand lens. However, if you have junipers with scale on them, there is little chance that those same scale will infect a nearby redwood, cryptomeria or hemlock since these typically host different species of scale.
Field Identification
To find armored scale on any of the above listed conifers or others, scout regularly using a 10-20x hand lens. The white dots are large enough to be visible to the naked eye, but the smaller yellow instars (juvenile stationary stages) and the crawlers (juvenile mobile stage) are not. If you snip a small piece of foliage from the plant and lay it on a flat surface you can use a sharp tool and the hand lens to dissect one of the white bumps. Typically the inside just looks like a dot of goo, but in late winter and early spring you will start to see eggs when doing this. If the white shell falls off easily or is dry underneath, that female has already died.
Unlike some scale insects, these small scale do not produce honeydew, and hence are not farmed by ants. If you see ants, it is common that you actually have aphids or scale, but not this type.
Scale Lifecycle
All of the scale species we're discussing here are "armored scale": the stationary females create a wax coating over themselves as protection from predators and other environmental factors. The adult females lay eggs a single time under the protective shell, dying in place afterward. In colder climates this happens once per year, with the female laying eggs in late winter or early spring. In warmer climates there can be two life cycles per year for some species. The eggs hatch under the shell of the female, then emerge as crawlers, traveling a minimal distance up the stems toward the fresh growing tips. Once they find a new spot they settle and are known as "settled crawlers". The distinction is mostly of age, but also the timing when they begin to create the armored shell. The emergence and crawling stages can last for 3-6 weeks. Once settled, females will not move again; and as they mature through stages, the covering starts out yellow, with new rings of white added around it, finally resembling a fried egg with the yellow in the middle and white around. Males remain mobile and are typically more elongated rather than round.
Physical Removal
In bonsai we have the opportunity to pay very close attention to our plants. Scale can be controlled, or heavy infestations reduced significantly using a nylon brush (like an old toothbrush) to physically remove the wax covering from the females. While this is a tedious process on large plants, regular inspection and removal of small infestations can be effective in treatment. Because the crawlers and instar stages are less visible keep in mind that removing all the white may not eliminate the problem. Follow up the physical removal with one of the treatments below. If physical removal is impractical due to the number of plants involved see below.
Insecticide Treatment Problems
Most resources on armored scales mention that you can only control them during the crawler and settled crawler stages as the adults are not susceptible to contact insecticides and are known to develop immunity to systemic insecticides over time. Because the instar and adult stages account for 80% of the lifecycle this makes management more difficult; if you miss the crawler stage you have an entire year to wait in some cases.
In the nursery industry scale insects are a major source of damage. Bayer, Syngenta and other agri-chemical companies continue to develop new treatments that are effective at least initially for many scale pests, regardless of species and regardless of timing or life-stage. Unfortunately, agri-chemicals are both expensive and not readily accessible to most bonsai enthusiasts.
Treatment Options
Soap sprays and Oil sprays are known to smother insects. The egg and crawler stages of scale can be effectively managed using either but require some careful application to ensure complete coverage. These treatments only work if they cover the insects, so spraying multiple times may be required. Both soaps and oils can cause phytotoxicity in junipers and some other conifers like spruce; follow label dilution directions and temperature/condition instructions carefully. Even with these precautions you will typically see a change in color in the foliage as the oils remove the glaucous surface feature, resulting in a bright green rather than a blue-green. The change in color will last only a short time with soaps but will last much longer with oils. The new growth will emerge as the natural color again.
Insect Growth Regulators (IGRs) are chemicals similar to insecticides, but that inhibit the growth rather than killing the insect. This class of chemicals is still effective in treating scale while somewhat less problematic for beneficial pests. The chemical reduces the adult growth, as well as egg laying by females, reducing the population. However residual action from application doesn't last more than a few weeks in most cases, so multiple applications may be needed.
Neonicotinoids are a class of chemicals that you're probably already using, perhaps without knowing it. All the Bayer tree-care products originally contained the most used of the class - Imidacloprid. These chemicals are under increasing regulatory pressure in the US and many other countries as they are suspected to be impacting bee populations as well as insect populations in general. Safari is a brand name of Dinotefuran that through 2024 was still one of the most effective knock-down controls for juniper scale. Note that these chemicals should often be applied as a soil drench rather than a foliar spray - the chemicals are absorbed by the roots and transported effectively to the foliar tissue where the scale consume them.
Starting in January 2025 in California 'Neo-Nics' are only available to licensed professionals rather than to the public. They are also heavily regulated in NY, VT and across much of Europe. In terms of scale control we are seeing them become less effective even at higher dosages. The combination of regulations and waning effectiveness mean that these chemicals are less attractive for us as bonsai growers.
Scale have natural predators - mainly tiny wasps that parasitize the stationary insects. General predatory insects like Green Lacewings and Ladybugs will also feed on scale. While these can help with keeping populations low, they will not solve the problem quickly, nor in the exact place you want.
Treatment Recommendations
"All-Seasons" oils are currently our go-to for scale control, they are highly-refined horticultural oils with minimal residual sulfur, making them less likely to damage plant tissue. Even if you've had juniper tips browning after oil application, you'll likely find these products get the job done without damaging your trees:
Have a small collection - pick up a bottle of "Bonide All-Seasons" oil
Have a larger collection - consider a larger "Tri-Tek" container.
Apply the oil at the dilution listed on the label as a foliar spray. Thoroughly spray the top and bottom sides of the foliage, rotating the tree as needed to ensure complete coverage. It's a good idea to apply the spray more than once, a few days apart. After treatment, you should see many of the white covers simply fall off from watering or a gentle tap to the branches.
We do NOT recommend you apply Neem oil or other oils to junipers during the hot or cold weather as they often do cause damage. Similarly, older oils like "Volck" and "Dormant Oil" can damage junipers and spruce extensively. If you want to use these, read the directions, then test on a sacrificial plant, or a small area to see if damage occurs.
Other common pests for Junipers include Spider Mites, Tip blight, and Cedar-Apple Rust.