Tree Pests and Diseases

One in eleven trees in Denver is an ash, and all are vulnerable to emerald ash borer (EAB), an invasive, non-native insect. Denver is working to preemptively identify, treat and/or replace the city’s ash trees to preserve the urban canopy. The EAB is small enough to fit on a penny and is identifiable by its dark, metallic-green color and coppery-red or purple abdomen, which can be seen under its wings. Learn more about reporting signs of EAB or proactively replacing your ash tree with a City Forester-approved tree.

Not sure if you have an ash tree? Using the TreeKeeper Interactive Map, you can identify the species of public right-of-way trees adjacent to your Denver property. Instructions for using TreeKeeper:

1. Follow the link.
2. Type your address in the search bar in the upper left corner (or search using map navigation).
3. Click on any visible green diamonds around your house. A pop up will open and tell you the species of the tree and the address it is associated with.

Click here to learn more about treatment options.

Click here to learn more about the right-of-way ash tree treatment schedule.

Dutch Elm Disease (DED) is a fungal disease transmitted by the elm bark beetle and/or root contact between infected and non-infected elms. Stressed elm trees are more prone to be attacked by the elm bark beetle, so it is best to keep your trees as healthy as possible with proper watering and other best care practices. Signs of DED include wilting, yellowing leaves in the tree-top, branch die-back and decline of the tree in later stages.

There are preventive treatment options available, and it may be possible to save an infected elm tree if caught in the very early infection stages. There is potential for this disease to have a great impact on Denver's elm tree population, so if an infected elm is found on a property, the Office of the City Forester will issue an order to remove the tree within 14 days in an effort to contain the spread. Wood debris must be disposed of properly, so unfortunately wood from the removed tree cannot be kept or stored.

Please be proactive and contact a licensed tree care professional to discuss options for your elm tree(s). To apply pesticides, a contractor must be licensed by the State of Colorado Department of Agriculture. Search for a contractor using the State of Colorado's Pesticide Applicator Search tool.  Enter the name of the contractor and select 'Denver' in the list of counties.

If you have questions, please send an email to forestry@denvergov.org.

Learn more about DED:

• Colorado State University Extension
• University of Minnesota Extension

Search for a Licensed Pesticide Applicator.

The spotted lanternfly is an invasive plant-hopper that feeds on a variety of plants and trees, including tree of heaven, fruit trees, maples, oaks, pines and many more. This non-native species was first detected in the United States in 2014.The spotted lanternfly can cause significant damage such as wilting, oozing, leaf curling and sometimes even death to plants and trees. 

Timeline:

• Spotted lanternfly egg masses typically appear September through June, one generation per year.  
  • Young egg masses are naturally camouflaged by what appears to be a smear of mud. Look for them on trees, rocks, vines, posts, vehicles and essentially anywhere. It’s much easier to get rid of them at this stage!  
  • Older egg masses become inch-long columns of brownish seed-like deposits. These contain anywhere from 30-80 eggs!  
• Eggs hatch April – June. 
• Adult spotted lanternflies emerge in July. They are black with brightly colored hindwings (red, black and white) and gray forewings with black spots. Spotted lanternflies are capable of flight, but are more often found jumping.

Additional Spotted Lanternfly Resources:

• Colorado Department of Agriculture Identification Guide
• Penn State Extension’s Management Guide
• U.S. Department of Agriculture Animal and Plant Health Inspection Service Overview