Understanding Scale Insects in Colorado

When Colorado homeowners notice sticky leaves, black sooty mold, or patches of dying branches, they often assume it’s a fungus or lack of water. In many cases, the real culprit is a pest you almost never see: scale insects.

Scale are silent feeders—small, immobile insects that attach themselves to twigs, branches, or leaves and drink the sap from your trees. If left untreated, scale can weaken or even kill a tree over time. As arborists, we see scale infestations every year in Colorado, especially on stressed trees.

Understanding what scale is, why it thrives here, and how to manage it can help you protect one of your most valuable landscape assets—your trees.

What Are Scale Insects?

Unlike the active insects you see crawling or flying, scale insects spend most of their lives motionless, hidden beneath a hard shell or waxy covering. They attach themselves to plant tissue and feed continuously.

There are two major groups:

1. Soft Scale

• Produce sticky honeydew
• Often covered by a thin, waxy coating
• Lead to black sooty mold
• Common on shade trees and ornamental plants

2. Hard Scale (Armored Scale)

• Covered by a hard protective shell
• Do not produce honeydew
• Often more difficult to control
• Can blend into bark or needles easily

Both types damage trees by feeding on sap and weakening the plant over time.

Why Scale Is a Problem for Trees

Scale insects feed by inserting needle-like mouthparts directly into a tree’s vascular tissue. Think of them as hundreds or thousands of tiny straws draining energy from the tree.

Over time, this can cause:

• Leaf or needle yellowing
• Early leaf drop
• Branch dieback
• Thinning canopy
• Reduced growth
• Increased vulnerability to other pests (borers, mites)
• Decline leading to death in severe cases

Scale doesn’t usually kill a tree quickly—it slowly starves it, season after season.

Signs of Scale Infestation

Scale insects are easy to miss until the infestation is advanced. Here’s what to look for:

Visible Symptoms

• Leaves with pale spots or yellowing
• Sticky residue (honeydew) under the tree
• Black sooty mold growing on leaves or patio furniture
• Ants crawling up and down the trunk (they harvest honeydew)
• Small bumps that don’t brush off easily
• Twig or branch dieback starting at tips

How to Identify Scale

Scale often looks like:

• Rough bumps on twigs
• Tiny oystershell-shaped flakes
• Little balls or discs attached to needles
• Flat, oval or round dots that feel glued on

If you can flake it off with your fingernail—and it’s not part of the bark—it may be scale.

Why Scale Is So Common in Colorado

Colorado’s climate encourages scale outbreaks for several reasons:

1. Trees Are Often Stressed

Water scarcity, alkaline soil, compacted soil in new developments, hot summers, and sudden cold snaps create stress. Scale targets stressed trees first.

2. Dry Weather Kills Natural Predators

Many beneficial insects that would control scale in humid climates struggle here. Lady beetles, lacewings, and parasitic wasps exist in Colorado—but drought and heat reduce their impact.

3. Urban Microclimates Favor Scale

Rock landscaping, reflective surfaces, and low humidity create ideal environments for scale, especially on parkway trees, parking lots, and street plantings.

Trees and Shrubs Commonly Affected in Colorado

We see scale on many species, but the most frequent hosts include:

Shade Trees

• Maple
• Linden
• Ash
• Elm
• Honeylocust

Evergreens

• Spruce
• Pine
• Fir

Ornamental Trees

• Hawthorn
• Crabapple
• Dogwood

Shrubs

• Cotoneaster
• Lilac
• Euonymus (burning bush)
• Grape vines

Scale species vary, but the damage pattern is similar.

How Scale Spreads

Scale insects don’t move far on their own. They spread through:

• Crawlers blown by wind
• Birds brushing against trees
• Pruning tools
• Infected nursery stock
• Ants transporting crawlers

Once scale establishes, numbers can explode quickly.

How Arborists Manage Scale

Treating scale the right way requires timing, tree health knowledge, and correct products. Spraying at the wrong time wastes money and has little effect because adult scale are protected by their shells.

Here’s how we manage it:

1. Improve Tree Health

Healthy trees can withstand pest pressure better.

We focus on:

• Watering deeply and infrequently
• Reducing soil compaction
• Mulching correctly (no mulch volcanoes)
• Avoiding excessive nitrogen fertilizer
• Pruning dead material to support air flow

The goal is to remove the stress that made the tree vulnerable.

2. Target the “Crawler” Stage

Scale is most vulnerable right after eggs hatch and before a shell forms. These tiny “crawlers” are exposed—and that’s when treatments are most effective.

This period varies by species and season, which is why professional timing matters.

3. Horticultural Oils

Dormant or summer horticultural oils:

• Smother scale
• Penetrate the shell on some species
• Are low impact on beneficial insects if timed correctly

This is one of the most effective, environmentally friendly approaches for many scale species.

4. Systemic Treatments

For tough species, we sometimes use systemic insecticides delivered through:

• trunk injection, or
• soil applications

These products move inside the tree and target feeding scale insects without spraying the canopy.

This is especially helpful for:

• large shade trees in tight spaces
• high-value trees
• infestations that aerial sprays can’t reach

Why DIY Sprays Don’t Work

Many homeowners buy insecticides hoping for quick results. The problem is:

• Adult scale are protected by a shell, so contact sprays often don’t reach them.
• Timing is critical—spray at the wrong time and the problem remains.
• Some broad-spectrum sprays kill natural predators and make the problem worse next year.

This is one pest where a strategic approach matters more than force.

How Homeowners Can Help

You don’t have to manage scale alone, but you can take important steps:

1. Water Correctly

Deep, slow watering makes trees more resilient. Avoid shallow daily lawn watering—the tree never sees that water.

2. Mulch Right

2–4 inches of wood chips around the base of the tree (not touching the trunk) helps conserve moisture and improve soil biology.

3. Watch for Signs

Inspect your trees in:

• late spring,
• mid-summer,
• and early fall.

If you see sticky leaves or unusual bumps, call an arborist early.

4. Avoid Stress

Protect root zones during construction and landscaping. Stress invites pests.

When to Call an Arborist

You should get a professional involved if:

• you see dieback or thinning canopy,
• there’s sticky sap on cars, patio furniture, or under the tree,
• you notice black sooty mold on leaves,
• ants are consistently crawling up the trunk,
• parts of the tree look unhealthy despite watering,
• you’re not sure what’s causing the decline.

A certified arborist can:

• identify the scale species,
• time treatments for maximum effectiveness,
• inspect soil and root health,
• and create a long-term plan.

Final Thoughts

Scale insects are common and damaging pests in Colorado, but they don’t have to cost you a tree. With proper diagnosis, well-timed treatments, and a focus on overall tree health, most infestations can be controlled or eliminated.

The key is early awareness and smart action. The sooner you act, the easier it is to protect your trees and avoid long-term harm.