Boxelder

(Acer negundo — also called Ash-leaf Maple)

Boxelder is a fast-growing, drought-tolerant maple often considered “weedy,” but it remains common across Colorado. It is prone to breakage, borers, boxelder bugs, leaf spots, and environmental stress in urban soils. Despite this, it can survive harsh conditions where many other species fail.

Seasonal Pest & Disease Calendar

Problem Type Active Season
Boxelder bugs Insect Late Summer–Fall (peak)
Aphids Insect Spring–Fall
Scale insects Insect Spring crawlers; visible year-round
Flatheaded & roundheaded borers Insect/borer Summer
Leaf spot / anthracnose Fungus Spring–Summer
Verticillium wilt Fungus Spring–Fall
Environmental stress (drought, soil compaction) Abiotic Summer–Fall

Pest & Disease Guide (Homeowner-Friendly)

Boxelder Bugs

Common symptoms
  • Swarms of black/red insects on house or trunk in fall
  • Minor leaf feeding
  • Nuisance but harmless to tree
Treatment & management
  • Seal home gaps
  • Power wash bugs off siding
  • Insecticides rarely necessary for tree health

Aphids

Common symptoms
  • Sticky honeydew
  • Sooty mold
  • Leaf curling
Treatment & management
  • Hose-rinse foliage
  • Encourage lady beetles
  • Systemic treatments for large outbreaks

Scale Insects

Common symptoms
  • Hard bumps or cottony masses on twigs
  • Yellowing leaves
  • Branch dieback if heavy
Treatment & management
  • Treat crawler stage in late spring
  • Apply horticultural oil late winter
  • Prune infested twigs

Flatheaded & Roundheaded Borers

Common symptoms
  • D-shaped or round exit holes
  • Boring dust
  • Dieback of branches
Treatment & management
  • Reduce stress—borers target weak trees
  • Deep water during drought
  • Professional preventive sprays if needed

Leaf Spot / Anthracnose

Common symptoms
  • Brown or black spots
  • Irregular leaf margins
  • Early leaf drop in wet years
Treatment & management
  • Rake and remove leaves
  • Improve airflow
  • Fungicide rarely needed

Verticillium Wilt

Common symptoms
  • Sudden wilting on one side
  • Brown streaks in sapwood
  • Progressive branch dieback
Treatment & management
  • No cure—manage stress
  • Prune affected limbs
  • Mulch & deep watering help prolong life

Environmental Stress / Decline

Common symptoms
  • Scorch on leaf edges
  • Sparse canopy
  • Frequent limb breakage
Treatment & management
  • Deep water every 2–3 weeks in summer
  • Avoid soil compaction
  • Structural pruning recommended

When to Worry vs. When It’s Cosmetic

When to Worry (action recommended)

  • Sudden wilting on one side (possible Verticillium wilt)
  • Borer exit holes or significant branch dieback
  • Large-scale scale infestation with sooty mold
  • Repeated heavy limb drop (structural hazard)
  • Canopy thinning year over year

These should be evaluated by a professional arborist.

Mostly Cosmetic (monitor only)

  • Boxelder bugs congregating in fall
  • Light aphid honeydew
  • Minor leaf spotting
  • Occasional scorch during heat waves
  • Early leaf drop from drought

These issues often resolve with better watering and minor cleanup.