Scale insects are small, sap-sucking insects that are covered by a shell-like waxy coating. They’re typically small (about 1/8 to ½ inch) and have either a hard shell that’s not attached to the body (armored scale) or a soft shell that can’t be separated from the body (soft scales).
There are about 8,000 species of scale insects, with different species preferring to feed on different plants. Several of these species are present in the Colorado Front Range region and feed on a variety of common trees and shrubs.
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.
Below are three of the most common and destructive scale insects in our area.
Oystershell Scale
Oystershell scale on aspen tree in Evergreen, CO
Oystershell scale (Lepidosaphes ulmi) is an armored scale that’s highly damaging to deciduous trees in the Evergreen, CO region. 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.
Susceptible Plants
Oystershell scale develops on deciduous trees and shrubs and is most commonly found on:
- Aspen
- Ash
- Cotoneaster
- Willow
- Lilac
Signs of Damage
Oystershell scales feed on the living cells of trunks and branches of deciduous trees. As the scale sucks fluids out of the plant tissue it slowly 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 Cytospora canker. You may also notice bark cracking on trees that have been damaged by oystershell scale.
Pine Needle Scale
Pine needle scale on pine
The pine needle scale (Chionaspis pinifoliae) is a small (1/8-inch), grey/white armored scale that feeds on evergreen needles.
Susceptible Plants
Look for pine needle scale on:
- Pine
- Spruce
- Fir
We’ve been seeing pine needle scale predominantly on spruce.
Signs of Damage
Pine needle scales suck sap from the needles, causing localized discoloration and premature needle shed. Prolonged outbreaks can kill branches and young trees, stressing the tree and making it vulnerable to Ips and mountain pine beetle infestation. Heavily infested trees may look like they’re covered in a fine dusting of snow or splattered with white paint. The scale insects are found mostly on the older needles on the inside of the tree.
Striped Pine Scale
Striped pine scale *
Unlike the oystershell and pine scales, the striped pine scale (Toumeyella pini) is a soft scale. It’s a little larger (about ¼ inch) and darker (light brown to reddish brown, with darker areas and a lighter stripe) than the other scales.
Susceptible Plants
While striped pine scale can attack all pines, they’re particularly damaging to Scotch pine and lodgepole pine. Lately, we’ve seen a lot of striped pine scale on ponderosa and Austrian pines that have been transplanted.
Signs of Damage
Striped pine scale feeding stunts development and can cause premature needle drop. Heavy infestation can kill branches and entire trees. This scale produces a lot of honeydew, which attracts wasps (so look for an increase in wasp activity around your pines). Black sooty mold often develops on the honeydew, covering twigs and branches with a chalky grey/black substance. You may also see large numbers of woolly aphids infesting the tree at the same time. As with the pine needle scale, infested trees are more susceptible to attacks by Ips and mountain pine beetles.
Treatment Options & Timing
Pine needle scale on spruce
We treat all scale with systemic insecticides prior to the crawler stage (the point at which the eggs hatch and the insects start to move about looking for a feeding site). At this point in their lifecycle, scale insects aren’t protected by a waxy cover and are more easily controlled by the appropriate insecticide; treatment becomes more difficult after they’ve moved on to the next stage of development. Systemic insecticides spread throughout the plant and are ingested by the scale as it feeds, quickly killing it.
Treatment timing is different for each scale insect.
- Oystershell scale eggs hatch in late May or early June.
- Pine needle scale egg hatch is expected late May thru mid-June.
- Striped pine needle scale eggs hatch beneath the protective coating of the female beginning in late May or early June and crawlers can continue to emerge for a month or more.
One treatment per year will usually reduce most hard scale populations to non-damaging levels.
After treatment, you will likely still see scale insects on your plants, even after the insects are dead. The protective shield 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.
Timing is critical when treating scale so if you suspect your trees are infested with scale, or if you notice any of the signs described above, give us a call at 303-674-8733 right away.
* Striped pine scale image courtesty of United States National Collection of Scale Insects Photographs, USDA Agricultural Research Service, Bugwood.org
