# Scale insects on a native tree or shrub, and when oil works

> What the bumps on twigs and sticky leaves mean, how soft and armored scales differ, when dormant or summer oil kills them, and when lady beetles do the work.

Published 2026-11-06. Tags: Decay & damage, Protection & care. By Blue Whale Trees, a native tree nursery in the United States. Web version: https://bluewhaletrees.com/articles/scale-insects-on-native-trees-and-shrubs/

Small brown, gray or white bumps crusted on a twig, a leaf or a needle are most often scale insects. These sap feeders spend most of their lives in one spot under a waxy or hard cover. Lady beetles and tiny parasitic wasps usually keep them below the point of harm. Horticultural oil kills only the scales it wets. It works on soft scales wintering on bare twigs, sprayed before the buds open, and on the [crawlers](https://bluewhaletrees.com/articles/glossary/index.html.md#crawler), the young that walk out for a few weeks each year.

![A row of brown, domed oak lecanium scales on a gray oak twig between broad green oak leaves](https://bluewhaletrees.com/img/articles/scale-insects-on-native-trees-and-shrubs-1-1000.jpg?v=252d00dd "Oak lecanium scales on an oak twig on August 12, 2017. Lecanium scales winter as young females on the twigs, where a dormant spray in early spring reaches them.")
*Photo: Even Dankowicz, [CC BY 4.0](https://creativecommons.org/licenses/by/4.0) ([source](https://commons.wikimedia.org/wiki/File%3AOak_Lecanium_Scale_imported_from_iNaturalist_photo_30467972_on_16_March_2024.jpg))*

## Soft scales and armored scales

Scale insects come in two groups, and the group decides what works. Soft scales are convex and about 1/8 to 1/4 inch long, and magnolia scale reaches 1/2 inch. They drink the sap in the phloem and pass the surplus as [honeydew](https://bluewhaletrees.com/articles/a-tree-dripping-sap-or-foam/index.html.md), the sticky drip that black sooty mold grows on. Most winter on the twigs as young females.

Armored scales are smaller, about 1/16 to 1/8 inch, flat and often the color of the bark they sit on. They feed on single cells and make no honeydew, yet they do more harm, killing twigs and, in a heavy infestation, whole plants. Most winter as eggs under their mothers' covers.

Scale can build for several seasons before anyone notices, and a dying limb is often the first sign of armored scale. A gray-green crust spread flat over the bark is [lichen](https://bluewhaletrees.com/articles/lichen-and-moss-on-bark/index.html.md), which does the tree no harm.

Lift a few bumps with a pin. A dead scale looks dry, discolored and often loose, though one killed recently can take a couple of weeks to dry out. A stick drawn along a crusted twig quickly shows whether many are alive, most plainly in colored kinds such as magnolia scale.

## The scales of native trees and shrubs

These are scales of native trees and shrubs, listed in the order their crawlers appear. The months are from Pennsylvania and Minnesota, and the crawler weeks are the time for a summer spray:

| Scale | Kind | Natives it feeds on | Crawlers out |
| --- | --- | --- | --- |
| Oystershell scale | Armored | Dogwoods, maples, ashes and willows | Late May to early June |
| Euonymus scale | Armored | [Eastern wahoo](https://bluewhaletrees.com/plants/eastern-wahoo/index.html.md), [strawberry bush](https://bluewhaletrees.com/plants/strawberry-bush/index.html.md) and [American bittersweet](https://bluewhaletrees.com/plants/american-bittersweet/index.html.md) | Late May and June, again late July and August |
| Elongate hemlock scale | Armored | [Eastern hemlock](https://bluewhaletrees.com/plants/eastern-hemlock/index.html.md), [Carolina hemlock](https://bluewhaletrees.com/plants/carolina-hemlock/index.html.md), firs such as [balsam fir](https://bluewhaletrees.com/plants/balsam-fir/index.html.md), and spruces | Late May and early June, then on into September |
| Lecanium scales | Soft | Oaks, maples and many other trees | June and July |
| Tuliptree scale | Soft | Tuliptree, magnolias and now and then [linden](https://bluewhaletrees.com/plants/american-basswood/index.html.md) | August into mid-September |
| Magnolia scale | Soft | Magnolias only, [cucumber magnolia](https://bluewhaletrees.com/plants/cucumber-magnolia/index.html.md) among the most attacked | Late August and early September |

We do not grow tuliptree. In South Carolina, crawlers often come with the spring flush of new growth, and near the coast some scales hatch over a longer period. From a little before the month, wrap double-sided tape around an infested twig or shake a branch over white paper, and look once a week until crawlers show.

## When oil works, and when it does not

Horticultural oil is a refined oil mixed with water. Dormant oil and summer oil name the season it is sprayed, not two products, and most oils are labeled for both. It acts only on what it wets and only until it evaporates, so it does little harm to the natural enemies that hold scale down.

### Dormant oil, before the buds open

A dormant spray goes on in late winter or early spring, before the buds open, with 24 to 48 hours above freezing to follow, preferably above 40 degrees Fahrenheit. A spray that coats every young twig kills the soft scales wintering there, lecanium, magnolia and tuliptree scales among them. It does much less against armored scales. The label sets the rate, often 3 to 4 percent oil in water for a dormant spray and 2 to 3 percent in summer.

The University of Minnesota Extension advises oil for soft scales in late fall as well. Cornell Cooperative Extension in Nassau County advises no oil after early September in the Northeast, since plants have not hardened off and resting insects outlast the oil.

### Summer oil, on the crawlers

Crawlers have no cover yet, so summer oil or insecticidal soap kills those it wets. Spray when the tape first catches crawlers, and again where hatching runs on for weeks. Spraying the crawlers is the best way to reach an armored scale, and summer oil works well on euonymus scale. Adults under their covers live through it, and so do young still hidden under the mother.

### Where oil burns

Oil can burn leaves in heat and high humidity and on a plant short of water, and it must not be used with sulfur. Maples, hickories and [black walnut](https://bluewhaletrees.com/plants/black-walnut/index.html.md) can be hurt by a dormant spray, and some azaleas by a summer one. [Beech](https://bluewhaletrees.com/plants/american-beech/index.html.md), [redbud](https://bluewhaletrees.com/plants/eastern-redbud/index.html.md) and spruce show a tendency toward harm from a dormant spray.

## Natural enemies, and when to leave scale alone

Lady beetles, lacewings and tiny wasps feed on scale. The wasps grow up inside the scales and leave exit holes in the covers. A lady beetle black with two red spots was brought to the eastern states to feed on euonymus scale.

![Shiny black lady beetle with a red spot on a green leaf among narrow white and brown euonymus scale covers](https://bluewhaletrees.com/img/articles/scale-insects-on-native-trees-and-shrubs-2-1000.jpg?v=be3e8363 "A lady beetle brought to the eastern states to feed on euonymus scale, among the scale's narrow white male covers and brown female ones, photographed in 1992.")
*Photo: Tim McCabe, USDA-ARS, [Public domain](https://commons.wikimedia.org/wiki/File%3AChilocorus_kuwanae_eating_Unaspis_euonymi.jpg) ([source](https://commons.wikimedia.org/wiki/File%3AChilocorus_kuwanae_eating_Unaspis_euonymi.jpg))*

Insecticides kill these enemies too. Ants that collect tuliptree scale's honeydew guard it from predators and parasites. Cottony maple scale is held down so well by its enemies that it seldom needs treatment. A spray is for a valuable plant in real danger:

> "Avoid using insecticides unless the plant is very valuable and in danger from the scale." (Clemson Cooperative Extension, *Dogwood Diseases and Insect Pests in South Carolina*)

Heat and stress tip the balance. On [willow oaks](https://bluewhaletrees.com/plants/willow-oak/index.html.md) in Raleigh, North Carolina, Emily Meineke and colleagues found oak lecanium scale 13 times more abundant in the city's hottest parts than in cooler ones, though wasps parasitized the same share:

> "Here we show that the abundance of a common insect pest is positively related to temperature even when controlling for other habitat characteristics." (Meineke, Dunn, Sexton and Frank, *Urban Warming Drives Insect Pest Abundance on Street Trees*, 2013)

Insect damage to a [flowering dogwood](https://bluewhaletrees.com/plants/flowering-dogwood/index.html.md) is usually minor, but it can be serious on one in full sun with little water. Nitrogen fertilizer helps elongate hemlock scale multiply. A soil-applied systemic insecticide reaches [the nectar and pollen](https://bluewhaletrees.com/articles/native-trees-for-bees/index.html.md) of a flowering tree, and the soil treatments for hemlock woolly adelgid do not control elongate hemlock scale.

## What to do through the year

Match the bumps to the table, then:

- **Winter.** Check young twigs and the undersides of evergreen leaves, and lift a few bumps to see if they are alive.
- **A light crust on a healthy tree.** Leave it to the lady beetles and wasps.
- **Before the buds open.** Where live soft scales crowd a tree losing twigs, spray dormant oil on a mild day, coating every twig.
- **The crawler weeks.** Set tape on an infested twig, and spray summer oil or insecticidal soap when crawlers show.
- **Through the season.** Prune out the worst twigs, brush big magnolia scales into soapy water in July, and water through drought.

Blue Whale Trees answers questions about a tree you already have through its [support page](https://bluewhaletrees.com/support/index.html.md). Say which tree it is, where the bumps sit and when they appeared. Photos of the whole tree and a close-up can follow by email once we reply.

## Common questions

### What are the small bumps on my tree's branches?

Most often scale insects, sap feeders fixed in one spot under a waxy or hard cover. A flat gray-green crust is lichen, which is harmless.

### When should I spray dormant oil for scale?

In late winter or early spring, before the buds open, with 24 to 48 hours above freezing to follow. It works on soft scales such as lecanium and magnolia scale, and much less on armored ones.

### Does horticultural oil kill scale in summer?

It kills the crawlers it wets, so spray in the weeks they are out, at the lower summer rate. Adult scales under their covers live through it.

### Will scale insects kill my tree?

Seldom a healthy one, since lady beetles and parasitic wasps keep most scale in check. Heavy infestations of armored scales or of magnolia scale can kill branches and, in time, whole plants.

## Sources

- [Scale insects](https://mortonarb.org/plant-and-protect/tree-plant-care/plant-care-resources/scale-insects/): The Morton Arboretum: crawlers as the mobile young without a cover, soft and armored scales and how each winters, honeydew and sooty mold, oystershell scale's hosts, lecanium scales' size and hosts, magnolia scale's 1/2 inch, dormant oil before the leaves appear and its weakness on armored scales, soap on crawlers only, natural enemies, and adult armored scales protected from sprays.
- [Scale insects on Minnesota trees and shrubs](https://extension.umn.edu/garden-and-home/yard-and-garden/yard-and-garden-insects/scale-insects): University of Minnesota Extension (Jeffrey Hahn, reviewed 2021): the sizes of soft and armored scales, phloem against single cells, honeydew from soft scales only, bark-colored armored scales and their greater harm, how each winters, numbers that build unnoticed, limb death as a first sign, natural enemies, crawlers watched with tape and white paper weekly, soaps and oils that act on contact and spare natural enemies, dormant oil only against scales wintering as young or adults, late fall oil, the crawler months of oystershell and lecanium scales, their hosts, and cottony maple scale held down by its enemies.
- [Magnolia Scale](https://extension.psu.edu/magnolia-scale): Penn State Extension (Gregory A. Hoover), with *Tuliptree Scale*, *Elongate Hemlock Scale* and *Euonymus Scale*: magnolia scale on magnolias only, cucumbertree magnolia among its favorites, its crawlers in late August or early September, and heavily infested trees injured or killed; tuliptree scale on tuliptree, magnolia and linden, its crawlers from mid-August to mid-September, dormant oil against it and the ants that guard it; elongate hemlock scale on hemlocks, firs and spruces, its crawlers from late May into September, nitrogen fertilizer and the adelgid's soil treatments; and euonymus scale on euonymus and bittersweet, its two crawler generations, its covers and the lady beetle brought in against it.
- [Magnolia Scale](https://www.umass.edu/agriculture-food-environment/landscape/fact-sheets/magnolia-scale): UMass Extension Landscape, Nursery and Urban Forestry Program (Zoe Robinson and Tawny Simisky, 2023): wintering nymphs on young twigs, crawlers in late August, double-sided tape for crawlers, light infestations brushed into soapy water in July, pruning, the exit holes of parasitoid wasps, crawlers protected under the female, and a dormant oil coating the young twigs.
- [Dogwood Diseases and Insect Pests in South Carolina](https://hgic.clemson.edu/factsheet/dogwood-diseases-insect-pests/): Clemson Cooperative Extension, Home & Garden Information Center (HGIC 2003, updated 2026): scales as small brownish, grayish or blackish bumps, insect damage to dogwood usually minor and serious in full sun with little water, the quotation, insecticides that kill scale's enemies, tape for crawlers, and crawlers with the spring flush and over a longer period near the coast.
- [Horticultural Oils for Ornamental Plants](https://content.ces.ncsu.edu/horticultural-oils-for-ornamental-plants): NC State Extension (Steven Frank, James Baker and Stephen Bambara, reviewed 2023): horticultural and summer oils as one, oil mixed with water at 1 to 4 percent by the label, oil's low effect on other insects, and summer oil's strength against armored scales such as euonymus scale.
- [Horticultural Oils](https://ccenassau.org/resources/-horticultural-oils-as-insecticides): Cornell Cooperative Extension Nassau County, Home Grounds Fact Sheet (reviewed 2003), with the Branching Out post *Scope it Out!* (Dan Gilrein, Cornell University, 2022): dormant and summer oil as timing, labels for both seasons, oil refined and mixed with water, oil that acts until it evaporates, the rates of 3 to 4 and 2 to 3 percent, no oil after early September, burn in heat, humidity and drought, sulfur, the plants sensitive to oil, armored scales harder to reach with dormant oil, dead scales that look dry, discolored and loose and take weeks to dry, and a stick drawn along a twig.
- [Horticultural Oils: Answers for All](https://ipm-cahnr.media.uconn.edu/wp-content/uploads/sites/3216/2023/04/Dormant-Oils-Answers-for-All.pdf): UConn Extension Integrated Pest Management Program (Evan Lentz): dormant oil before bud break, and 24 to 48 hours above freezing, preferably above 40 degrees Fahrenheit, after spraying.
- [Urban Warming Drives Insect Pest Abundance on Street Trees](https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0059687): E.K. Meineke, R.R. Dunn, J.O. Sexton and S.D. Frank, PLOS ONE 8 (2013): oak lecanium scale 13 times more abundant on willow oaks in the hottest parts of Raleigh, parasitism the same in hot and cool sites, and the quotation.

## Related

- Next step: [Write to us](https://bluewhaletrees.com/support/index.html.md#article=scale-insects-on-native-trees-and-shrubs)
- [A tree dripping sap, sticky drops or white foam: honeydew, slime flux and bleeding wounds](https://bluewhaletrees.com/articles/a-tree-dripping-sap-or-foam/index.html.md)
- [Borers in a young or stressed tree: the signs, the trees they pick, and what keeps them out](https://bluewhaletrees.com/articles/borers-in-a-young-or-stressed-tree/index.html.md)
- [Lichen and moss on bark: harmless, and what they say](https://bluewhaletrees.com/articles/lichen-and-moss-on-bark/index.html.md)
- [Caterpillars on a tree: tents, webs and bare branches, and when to leave them](https://bluewhaletrees.com/articles/caterpillars-on-a-tree/index.html.md)