Understanding Spider Mites

If you’ve ever noticed dusty, bronzed foliage on a spruce, pale speckles on leaves, or extremely fine webbing between needles or under leaves, there’s a good chance you’ve encountered spider mites. They’re one of the most common and damaging landscape pests in Colorado—and they’re tiny enough that most homeowners don’t realize they’re there until damage is already visible.

As certified arborists, we see spider mites every year, especially during hot, dry periods. The good news? Once you know what to look for and how they work, you can protect your trees with smart, simple actions.

What Are Spider Mites?

Spider mites are microscopic arachnids, not insects. Even though they’re related to spiders and ticks, they feed on plant tissue, not other insects.

They live on the underside of leaves or on needled foliage. With specialized mouthparts, they pierce plant cells and suck the contents out. Each feeding spot creates a tiny pale speck. When mites feed in large numbers, foliage becomes:

• yellowed or bronzed,
• dry,
• and prone to dropping early.

Their small size means they can build up huge populations before you ever notice them.

Why Spider Mites Are a Big Problem in Colorado

Our climate creates the perfect conditions for mites:

1. Hot, Dry Weather

Mites thrive in heat, especially above 85°F. Unlike many pests, dryness doesn’t hurt mites—it encourages them.

Meanwhile, natural predators like lady beetles and predatory mites need a bit more humidity, so hot, dry weather shifts the advantage to spider mites.

2. Urban Heat Islands

Colorado cities have:

• reflective rock landscaping,
• asphalt,
• glass,
• stucco,
• stone mulch,
• and very little humidity.

These surfaces trap heat and create microclimates that push mite populations into overdrive.

3. Dust

Spider mites love dusty foliage. Dust clogs pores of the leaf, reduces the tree’s natural defenses, and protects mites from predators. Spruce trees planted along busy roads are especially vulnerable.

4. Stressed Trees

Mites target trees already weakened by:

• drought,
• poor soil,
• freeze damage,
• or other insect activity.

A stressed tree produces fewer natural defense chemicals, making it easier for mites to feed.

Tree Species Most Affected

Spider mites can attack many plants, but we most commonly see them on:

Evergreens

• Colorado Blue Spruce
• Norway Spruce
• Spruce hybrids
• Fir species
• Some pine species

Spruce mites are especially problematic in late winter and early spring.

Deciduous Trees and Shrubs

• Honeylocust
• Linden
• Maples
• Burning Bush
• Roses
• Many annuals

Different mite species peak at different times of year.

What Damage Looks Like

On Evergreen Trees

Typical symptoms:

• Gray or bronze needles
• Foliage looks dusty or dirty
• No obvious insects when inspecting
• Fine webbing between needles
• Needle drop from inner canopy outward
• Thin, dull appearance

Damage often shows up after winter and continues into spring.

On Deciduous Trees

Typical symptoms:

• Yellow or pale leaf stippling
• Leaf edges dry or curl
• Leaves feel gritty underneath
• Webbing in leaf joints
• Early leaf drop
• Overall faded color

Damage is often visible mid-summer.

How Spider Mites Work

Spider mites reproduce very quickly:

• Eggs hatch in days
• Full lifecycle can be under 2 weeks
• Populations double and triple under heat

A tree that looked fine in early June can look severely damaged by August if the outbreak goes unnoticed.

Why Spruce Trees Decline So Fast

Cold-season mites are active in cool weather (spring/fall), while warm-season mites thrive in heat. A spruce can be hit by two waves of mite pressure in one year:

1. Early season (cold season mites),
2. Late season (warm season mites).

This repeated damage exhausts the tree.

How to Check for Spider Mites

The “Paper Test”

One simple way to detect mites:

1. Hold white paper under a branch.
2. Tap the branch sharply.
3. Look for tiny dark specks falling on the paper.
4. Gently smear your finger across the specks.

If the specks smear greenish or rusty, you’re crushing mites.

If they remain firm like dust, it’s likely debris.

How to Manage Spider Mites

1. Don’t Overreact with Pesticides

Many broad-spectrum insecticides:

• Kill beneficial predators,
• But don’t kill mites effectively.

This can create a mite explosion, because predators die but mites rebound faster.

Avoid quick fixes without proper diagnosis.

2. Water Smartly

Deep watering reduces drought stress, which improves the tree’s natural resistance. But avoid:

• frequent, shallow lawn sprinklers,
• overhead irrigation during hot afternoons.

Trees need soak cycles, not misting.

3. Reduce Dust

If your spruce is near a road or construction zone:

• Lightly rinse foliage with plain water in the morning every couple of weeks.

This removes dust and helps natural predators.

4. Use Miticides (When Needed)

If mites are confirmed and populations are high, an arborist may recommend:

• targeted miticides applied at the right time of season.

Miticides are different from insecticides. Timing matters:

• Cold-season mites are treated in spring/fall,
• Warm-season mites are treated in summer.

Using the wrong product or wrong timing wastes money and increases risk.

5. Encourage Predators

Beneficial organisms like:

• predatory mites,
• lacewings,
• lady beetles

…naturally help control spider mites. Avoid chemicals that kill them.

Why Some Trees Never Recover

If mites damage newest growth year after year, trees:

• lose stored energy,
• thin out,
• invite bark beetles,
• and go into a decline spiral.

We often see blue spruce die not because of mites alone, but because miticide never happened, predators died, drought stacked on top of mite damage, and then beetles moved in.

Mites are often the first domino.

When to Call a Certified Arborist

Contact a professional if:

• You see bronzing on your spruce,
• The “paper test” confirms mites,
• More than one season of damage is visible,
• Nearby trees show similar symptoms,
• You aren’t sure whether it’s mites or disease.

An arborist can:

• identify the mite species,
• recommend timing for treatment,
• check soil conditions,
• assess drought stress,
• and create a long-term plan rather than a quick spray.

Final Thoughts

Spider mites are small, but they’re one of the most impactful pests in Colorado landscapes. The key to managing them isn’t just spraying—it’s understanding why they thrive here and making your yard less welcoming:

• reduce drought stress,
• reduce dust,
• avoid broad insecticides,
• irrigate correctly,
• and respond early.

With the right strategy, you can keep your spruce, shade trees, and ornamentals healthy even in tough years.