Don’t Let Tree Pests Bug Your Colorado Trees This Spring!

Aphids: These tiny, soft-bodied insects are often found in clusters on new growth, such as leaves and stems. They feed on plant sap, causing leaves to curl, yellow, and become sticky with honeydew (their sugary waste product). Honeydew can also attract sooty mold. You might notice ants crawling on your trees, as they feed on the honeydew aphids produce.

• Where to look: Undersides of new leaves, tips of branches.
• Damage: Leaf curling, yellowing, stunted growth, sticky residue.

Spider Mites: Although technically not insects, these tiny arachnids thrive in dry conditions and can multiply rapidly, especially during warmer spring days. They feed on plant cells, causing a stippled or bronze appearance on leaves. You might also see fine webbing on heavily infested areas.

• • Where to look: Undersides of leaves. Look for fine webbing.
  • Damage: Stippled or bronze leaves, leaf drop in severe infestations

Pine Bark Beetles (Early Stages): While more notorious for their activity later in the summer, some pine bark beetle species can begin their activity in the spring as temperatures rise. Look for small holes in the bark of pine trees and sawdust-like frass (insect excrement) at the base of the tree or in bark crevices. Early detection is key for managing these destructive pests.

• • Where to look: Bark of pine trees, especially stressed or weakened trees. Look for small entry holes and sawdust.
  • Damage: Yellowing or browning needles, eventual tree death if infestations are heavy.

Elm Leaf Beetle: These small, yellowish beetles with black stripes emerge in the spring and feed on elm leaves, creating small, irregular holes. Their larvae also feed on the undersides of leaves, causing more significant damage.

• • Where to look: Elm leaves. Look for small holes and larvae on the undersides.
  • Damage: “Shot-hole” appearance of leaves, skeletonized leaves.

Emerald Ash Borer (EAB): While perhaps not as visibly active on the foliage in early spring, the Emerald Ash Borer remains a critical concern for ash tree owners in the Denver area. This invasive beetle has been confirmed in Colorado and poses a significant threat to all species of ash trees. Adult beetles emerge later in the spring and summer, leaving distinctive D-shaped exit holes in the bark. Larvae feed under the bark, disrupting the tree’s ability to transport water and nutrients, eventually leading to tree death within a few years.

• • Where to look (Signs of Infestation): Look for thinning canopy, yellowing or browning leaves (even if the rest of the tree appears healthy), D-shaped exit holes (about 1/8 inch wide) in the bark, S-shaped galleries under the bark (visible if bark is peeled away), and increased woodpecker activity (feeding on larvae).
  • Damage: Canopy dieback starting from the top, bark splitting, D-shaped exit holes, eventual tree death. Prevention and early intervention are crucial.
  • Know Your Ash Trees: Identify any ash trees on your property (they have opposite branching and compound leaves with 5-9 leaflets).
  • Monitor for Symptoms: Regularly inspect your ash trees for the signs of EAB mentioned above.
  • Consider Preventative Treatment: For high-value ash trees in areas where EAB has been confirmed, preventative insecticide treatments applied by a certified arborist can be highly effective. These treatments are most effective when applied before an infestation occurs.
  • Report Suspected Infestations: If you suspect your ash tree may be infested with EAB, contact the Colorado Department of Agriculture or a certified arborist immediately. Early detection is crucial for slowing the spread.
  • Consider Removal and Replacement: If an ash tree is heavily infested, removal is often the best course of action to prevent the spread to other healthy ash trees. Consider replacing ash trees with non-host species to maintain your landscape’s tree canopy.