GARDENING: Let’s take a look at common tree pests |

GARDENING: Let’s take a look at common tree pests

Curt Swift
Free Press Gardening Columnist
Courtesy / Curt Swift
Staff Photo |

At the end of the day when I park my truck under the catalpa that hangs over the street in front of my home, I know I will need to turn on my windshield wipers the next morning to remove the fine droplets that accumulated during the night. If I have the misfortune of parking under a scale-infested American elm, or an aphid-infested aspen, I know I will need to scrub the window the next morning to rid it of the accumulation of sticky leaves and sticky droplets of honey dew.

My catalpa, like all other trees, releases volatiles and plant exudates during the night. The tree actually belongs to the city because it is planted in the area between the sidewalk and the road, but since I fertilize, water and spray it for insects, I consider it more mine than the city’s. I’ve even had Teddy’s T4Tree crew prune it and the hackberry next to it. So, I guess it really is my tree. And a great tree it is.

The mist raining onto my trunk from the catalpa is from glands in the leaves called hydathodes. These glands are responsible for the droplets of liquid you sometimes see at the tips of leaves of houseplants and the crystalline deposits that accumulate along the edges of leaves. This exudation is the result of a natural process called guttation. These glands help with water movement through the tree. They also release excessive water pressure in the plant, reducing cell damage but this is not always effective. When excessive amounts of water build up in leaves, cells become engorged with water and may even burst giving the appearance the leaf is infected by a rust fungus. This condition is called edema (aka oedema).

When I park under elms infested with the European elm scale, the coating on my windshield is from guttation as well as honeydew excreted by the scale. In addition to producing this sappy substance, this scale kills the terminal leaders of the tree, increasing the need for costly pruning. Guttation fluid is from the water-conducting tissue and has some salt, sugar and organic compounds dissolved in the water. The honeydew from the scale insect is mostly sugar water. Think of diluted maple syrup dripping on your windshield and you have a general idea of the extent of the problem when your trees are infested with sap-emitting insects like soft scales and aphids.

Most trees have their own complement of insect pests and need to be examined on occasion to determine if there are problems needing treatment. Small trees with aphids can be washed down with a forceful stream of water from a hose but scale insects are a different matter as they are often tightly affixed to the stem. There are bio-pesticides available that provide effective control if applied at the proper stage of the insect. There also are systemic insecticides available to control such pests, but you need to make sure you use the correct one for the insect. If you suspect a problem or need a home visit you can always contact the CSU Extension office and see if someone from that office can make a house call or give me a call at 970-778-7866 to schedule a visit. You can also contact a pesticide applicator who specializes in trees.

Many people don’t even realize their trees are infested with sap-exuding insects until they start tracking sticky leaves into their home or have to pick sticky leaves out of the fur of their pets. Problems that are obvious to me are not always obvious to homeowners or property managers. For example, the other day I visited a yard where the aspens were infested with borers. The trees had large gouges in the trunk where the borers had excavated much of the wood tissue. There were even pupal cases of the insect sticking three-quarters of an inch out of the trunk. The aspens had shiny leaves due to the honeydew being excreted by aphids and galls on the twig caused by a fly. Identifying problems like these and providing you solutions really makes my day. Kind of weird but true.

Dr. Curtis E. Swift is a retired horticulture agent with the Colorado State University Extension. Reach him at or check out his blog at He owns Swift Horticultural Consulting and High Altitude Lavender.

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