Colorado’s Front Range Pest Calendar

Great Western Tree Care – An Arborist’s Guide for Homeowners

One of the most common questions I hear as a certified arborist is: “When do certain tree pests show up in Colorado?”

In Colorado, timing matters. Many insect pests are only vulnerable during specific life stages. Treating at the wrong time can waste money and do little to stop the problem, which is why understanding the pest calendar helps homeowners know what to watch for—and when to call for help.

Below is a month-by-month overview of the most common tree pests along the Front Range and the general timeframe when they become active.

Note: Weather creates variation. Warm springs shift activity earlier, cold springs delay it. Higher elevation is later, lower elevation earlier.

March–April: Early Season Activity Begins

As temperatures warm, overwintering insects start to wake up.

Aphids (Early Colonies)

• Begin hatching from overwintering eggs
• Often start on fruit trees, maples, lindens, ash
• Still small and not producing heavy honeydew yet

Spruce Spider Mites (Spring Generation)

• Active in cool weather
• Damage appears as flecking on spruce needles
• Often go unnoticed until summer

Scale Insects (Egg Hatch for Some Species)

• Certain scale species have early crawler stages
• Young crawlers are the vulnerable stage

Ips Bark Beetles (Warm Days Over 50°F)

• Ips beetles emerge early in spring
• Attack stressed pine and spruce
• Especially attracted to fresh pruning wound or storm damage

Elm Leafminer

• Early larvae feed inside emerging elm leaves
• Creates brown blotches within leaf tissue

May–June: Peak Emergence for Many Pests

Warmer weather accelerates insect life cycles.

Aphids (Major Population Growth)

• Honeydew begins (sticky leaves, cars, patios)
• Sooty mold follows
• Ants farming aphids is a common sign

Leafrollers & Leafminers

• Feeding inside leaves of ash, apple, elm, hawthorn
• Leaves curl and brown patches appear

Sawflies

• On spruce and pine needles
• Often mistaken for caterpillars
• Can strip needles quickly

Spider Mites (Temperature Dependent)

• Cool-season mites decline
• Warm-season species may begin

Caterpillars

• Western tent caterpillars and cankerworms
• Eat leaf tissue on shade trees
• Visible silk nests on certain species

Elm Leaf Beetle

• Feeding begins on elm trees
• Skeletonizing leaf tissue

June–July: Peak Summer Pest Season

This is when most visible damage appears.

Japanese Beetle (June–August)

• Major issue in many Front Range neighborhoods
• Feeding on roses, lindens, grapes, fruit trees
• Active on warm sunny days
• Adults visible mid-June through August

Scale Crawlers (Various Species)

• Soft scale crawlers are active
• Best time for horticultural oil treatment

Spider Mites (Hot-Season Outbreaks)

• Particularly on spruce, juniper, aspen
• Heat + drought = explosive population growth
• Fine webbing and stippled foliage

Pine Needle Scale

• Egg hatch early summer
• Crawlers settle on needles

Ips & Mountain Pine Beetle Flight

• Ips often has multiple generations
• Attacks stressed pine
• Mountain pine beetle flight typically begins mid-summer in higher elevations

Zimmerman Pine Moth (Historic Problem Areas)

• Larvae tunnel in branch collars of Austrian and Scotch pine
• Early summer stage is when damage becomes visible

August–September: Decline or Shift in Pest Pressure

Summer pests continue, others subside.

Spider Mites (Peak in Hot Dry Weather)

• Two-spotted mites on deciduous trees
• Repeated damage can cause early leaf drop

Aphids (Late Season Generations)

• Final colonies form
• Honeydew increases again on linden, hackberry, maple

Fall Webworms

• Webbing at branch tips in late summer
• Looks like bagworms but seasonal timing is different
• Often just cosmetic late in the season

Scale (Second Crawler Peaks for Some Species)

• Depending on species, a second vulnerable stage may occur

Borers (Attack Injured/Stressed Trees)

• Bronze birch borer,
• Lilac/ash borer
• Flatheaded borers active when trees are stressed from summer drought

October–November: Activity Drops, Planning Begins

Cool temperatures slow pest cycles.

Spider Mites (Cool-Season Mites Return)

• Spruce spider mite becomes active again in cool weather
• Damage shows on needles heading into winter

Scale (Overwintering Phase)

• Many species overwinter as nymphs
• This is when dormant oil planning begins

Egg-Laying for Next Spring

• Aphids and mites lay eggs for next year
• Treatments need to focus on prevention rather than cure

December–February: Dormant Season Opportunities

Most insects are inactive, but it’s a chance to set up for next year.

Dormant Oils

• Best time for horticultural oil on scale, aphid eggs, and mites
• Helps reduce spring populations

Prune During Dormancy

• Removing dead or infested branches reduces pest habitat
• Less chance of attracting Ips beetles with fresh cuts

Soil Health Work

• Compost, mulch, and root zone improvement help trees resist pests naturally

Why Timing Matters So Much

Each pest has a life stage where it is vulnerable:

• Aphids & mites: early population growth stages
• Scale: tiny crawler stage
• Borers: before they enter the tree’s cambium
• Beetles: during flight periods
• Caterpillars: early feeding stages

Spraying during the wrong phase doesn’t impact the pest—but may harm beneficial insects like lady beetles and parasitic wasps which naturally control outbreaks.

An arborist’s job is to understand when the pest is exposed and time treatment accordingly.

What Homeowners Can Do Each Season

Early Spring

• Inspect buds and new leaves
• Look for early aphid colonies
• Plan dormant oil treatments

Late Spring (Peak Growth)

• Watch for leaf curling, sticky honeydew
• Inspect spruce and pine for mites

Summer

• Look for beetles feeding on leaves
• Check for scale bumps on twigs
• Monitor trees stressed by heat/drought

Early Fall

• Check evergreens for mite damage
• Look for late aphid populations
• Plan preventive treatments for next year

Winter

• Schedule dormant oil
• Prune dead and diseased wood
• Improve soil and mulch

Healthy Trees = Less Pest Pressure

Pests target stressed trees first. The best long-term protection isn’t just insect control—it’s tree health.

Trees in Colorado struggle with:

• Compacted clay soil in new developments
• High pH, iron lock-up
• Inconsistent irrigation
• Drought cycles
• Improper planting depth
• Mechanical injury from mowers
• Poor species selection

When a tree is stressed, its natural defenses drop—and pests take advantage.

A tree with healthy roots and consistent moisture is far less attractive to beetles, borers, mites, or scale.

Final Advice: Don’t Guess—Check the Timing

If you suspect a pest issue, the best thing to do is identify the pest and the timing before treating.

A certified arborist can:

• Confirm the pest species
• Determine the life stage
• Recommend environmentally responsible treatments
• Prevent damage before it spreads
• Improve tree health so pests are less likely to return

Trying to treat the “wrong month” is one of the most common mistakes we see.