April to May: Ash / Lilac Borer Larvae burrow into trunks and excavate galleries in young and stressed Colorado Ash trees, causing severe limb die back and possible tree death.

May to June: Ash Sawfly In Colorado, green larval worms feed on the leaves of Ash trees, sometimes defoliating the entire tree.

April to July: Bronze Birch Borer Larval feeding destroys tissue of trunks and branches. Preventative treatments are effective for control.

July to Aug: Cottonwood / Poplar Borer Affects weakened Cottonwood, Poplar and Aspen trees, causing drill like holes in trunk. Many times Colorado trees infested with these bugs die off.

April to July: Crown Borer Will attack the lower trunk of Peach and Plum trees. Evidence of these pests comes in the form of oozing gummy sap . Can cause decline or death of tree.

April to July: Elm Bark Beetle The carrier of Dutch Elm Disease they destroy the tissue under the bark. American and English Elms can become infested.

Feb to Sept: IPS Engraver Beetle. A common group of bark beetles that infest pine and spruce trees.

March to Sept: Kermes Scale Heavy infestations cause "flagging", where the leaves on branch tips turn off-color, droop and often drop off.

June to July: Poplar Scale has only been found associated with aspen in Colorado. Infestations on aspen typically occur on the trunk and larger branches, which produce a characteristic bubbling symptom on bark. Decline and some death of aspen has been associated with this insect in Colorado with street trees and trees in poor sites particularly susceptible.

May to June: The Tussock Moth caterpillars are voracious eaters capable of defoliating entire forests.

April to Aug: Larvae of the Zimmerman pine moth damage trees by tunneling under the bark, making gouging wounds. Most injuries occur at branch crotches.