If you notice white or brownish raised bumps on the trunk and/or branches of trees that don’t look healthy, it’s probably scale insects.
These tiny, sap-sucking pests are only about 1/8 to ½ inch in size but can be quite destructive. Long-term and/or heavy infestations of scale can be extremely damaging to the plants on which they feed, leading to leaf/needle discoloration, defoliation, stunted growth, limb dieback, vulnerability to other pests and diseases, and even plant death.
Unfortunately, because they blend in so well with the bark of the plants they feed on, it can be difficult to catch a scale infestation in its early stages.
How Scale Insects Damage Trees & Shrubs
Scale insects feed by inserting a long, needle-like stylet (mouthpart) into the plant and sucking out sap. The heavier the scale infestation, the more sap is removed, and the more damage is caused to the host plant. As more and more nutrients and water are removed from the tree or shrub, you’ll notice yellowing leaves, stunted growth and branch dieback. Without treatment, the plant may eventually die.
Some types of scale excrete excess sap as a sweet, sticky waste product, called honeydew, that coats nearby leaves, branches and other surfaces below the scale infestation.
While honeydew itself isn’t harmful, it’s a food source for the sooty mold fungus. This fungus grows quickly to cover leaves, twigs, surfaces and objects beneath the tree with an unsightly, grey-black blanket. Honeydew also attracts wasps and bees so look for an increase in their activity around your trees.
Types of Scale Insects
There are two categories of scale insects:
- Armored scales. These types of scale insects are covered by a hard, waxy shell that’s not attached to their body. They do not produce honeydew.
- Soft scales. Rather than a shell, these scales protect themselves with a soft, waxy cover that is attached to their body. When feeding, soft scales excrete honeydew.
Different scale species (there are about 8,000 species of scale insects worldwide) prefer different plants. Several of these species are present in the Dayton area and feed on a variety of common trees and shrubs.
Below are the most common and destructive scale insects in southwest Ohio.
Oystershell scale (Lepidosaphes ulmi) is an armored scale that attaches itself to the trunk and branches of shade trees and shrubs. At maturity, an oystershell scale is only about 1/8-inch long. It’s brown or gray, shaped roughly like an oyster’s shell, and often blends in so well with the bark on which it’s feeding that it can be difficult to see.
Oystershell scale develops on deciduous trees and shrubs and is most commonly found on:
Signs of Damage
As the oystershell scale sucks fluids out of the plant tissue, it often kills the area around the feeding site. You’ll often see branches dying as a result of heavy infestation, and trees may be so weakened that they succumb to diseases or other pests. You may also notice bark cracking on trees that have been damaged by oystershell scale.
Lecanium scale is a soft scale that produces large amounts of honeydew as it feeds, attracting black sooty mold fungus. There are several different types of lecanium scale insects in southwest Ohio. Although they’re usually greenish-tan to chestnut-brown in color, they can differ considerably in color markings, making it difficult to identify specific types.
Lecanium scale typically appears on:
- Flowering fruit trees
Signs of Damage
Often, the first signs of a lecanium scale infestation are the appearance of large numbers of bees, wasps and ants who are attracted to the honeydew excretions. You’ll also notice sooty mold covering everything beneath the infested area.
Lecanium scale, if left undisturbed, can completely cover branches, leading to stunted growth and, eventually, death.
Pine Needle Scale
The pine needle scale (Chionaspis pinifoliae) is a small (1/8-inch), grey/white armored scale that feeds on evergreen needles. The insects are protected by an oyster shell-shaped wax cover.
Look for pine needle scale on:
- Pine (especially Scotch and mugo pine)
Signs of Damage
Pine needle scales suck sap from the needles, causing yellowing and premature needle shed. Prolonged outbreaks can kill branches and young trees. Heavily infested trees may look like they’re covered in a fine dusting of snow or splattered with white paint.
The Fletcher scale (Parthenolecanium fletcheri), also called arborvitae scale, is a soft scale most commonly found on arborvitae and yews. Interestingly, the species only exists as females (they reproduce by parthenogenesis, or asexual reproduction). Adult females are yellowish brown to tan, rounded, and less than 1/8 inch in diameter.
Signs of Damage
Like other soft scales, Fletcher scales produce an abundance of honeydew so look for signs of sooty mold. Infested plants may not grow as strongly as usual and heavily infested ones may have chlorotic (yellowish) needles or drop the prematurely.
Cottony Maple Scale
As its name suggests, cottony maple scale (Pulvinaria innumerabilis) affects mostly maple trees (predominantly silver and red maples).
The distinguishing characteristic of these soft scales is the white egg sac (ovisac) produced by female scales in summer. This ¼ to ½ inch roundish sac looks like a cotton ball and is quite conspicuous, especially when there are many scales on the host plant.
Cottony maple scale primarily attacks silver maple, as well as other species of soft maple.
It can also be found on:
- honey and black locust
- white ash
Signs of Damage
A cottony maple scale infestation doesn’t usually cause significant damage to the host plant, although the combination of “cotton balls” and sooty mold attracted by the honeydew can be an unsightly nuisance. In cases where a tree is very heavily infested, you may notice yellowing and prematurely falling leaves, or even some branch dieback. An exceptionally heavy infestation may kill stressed or weakened trees.
The magnolia scale (Neolecanium cornuparvum) is huge in comparison to other soft scales, measuring almost ½ inch across at maturity. Color varies as these scales develop, ranging from yellowish-brown to brown/tan to purplish-brown; in early summer, females are covered with a powdery, white waxy coating.
Some species of magnolia are commonly attacked and can be severely infested.
- Saucer magnolia
- Star magnolia
- Lily magnolia
Other magnolia species may sometimes be infested but are rarely damaged.
Signs of Damage
Other than sooty mold attracted to the honeydew excreted by feeding magnolia scale, you may notice stunted growth on twigs and branches that are heavily infested and fewer leaves throughout the tree canopy. Severe and long-lasting infestations can eventually kill the host tree.
Treatment Options & Timing
We use a variety of treatment options to control scale, depending on the type of scale, time of year, and stage in the scales’ lifecycle.
Scale insects are most vulnerable in the crawler stage of their development (after hatching, this is the point at which they start to move around the host plant looking for a feeding site). Without their protective cover, they’re more easily controlled by the appropriate treatment.
Fully mature scale are difficult to eradicate as they’re covered with a protective, waxy shield. Spraying them is generally ineffective at this stage.
Below are some general timeframes for the treatment of scale in the Dayton area.
Late Winter/ Early Spring (February, March)
- Dormant oil treatment - A heavy horticultural oil is sprayed on all tree surfaces to smother and kill overwintering female scales before they can finish their development.
Spring (April, May, early June)
- Soil injection - A systemic insecticide is injected into the soil around an infested tree. It’s taken up by the tree and distributed to areas where scale nymphs are feeding, killing them after they’ve ingested it.
- Bark spray - Insecticidal soaps or oils are sprayed on the bark where it kills the crawlers.
Summer (late June, July, August)
- Immediate treatment – Contact insecticides are sprayed on the tree to kill any crawlers that may be present.
Fall (September, October, November)
- Soil injection
After treatment, you'll probably still see signs of scale insects on your plants, even after the insects are dead. The protective waxy covering often stays attached to the tree, sometimes for several years. You’ll know that the insect is dead if it easily flakes off the bark when you crush it; live scales “bleed” when crushed.
Learn More About Scale
If you suspect your tree or shrub has a scale infestation, give us a call at 937-233-4118. We'll conduct a thorough inspection to identify the type of scale and recommend the best treatment options.