Each spring is different in Denver. Last year we had a wet spring, and an Elm Leave Beetle infestation occurred. In 2022, we had a dry early spring with little snow; it rained a lot in late May and early June. Tree aphids thrive in these conditions, and these soft-body insects are now growing in large numbers on Spruce and Maple trees. No matter the tree, a tree aphid will likely munch on it. Most do not injure plants, but some tree aphids cause problems.
- Over 350 aphid species are found in Colorado; most feed on specific plants or trees.
- Some aphid species curl new leaves.
- Aphids excrete a messy, sticky fluid called honeydew.
- Natural predators such as ladybugs, flower fly larvae, lacewing larvae, and parasitic wasps keep aphids in check most of the time.
- Insecticides, insecticidal soaps, and jet spraying plants can control aphids.
Aphids Life History
Aphids have an oval body and a pair of cornicles (pipe-like structures) protruding from their backs. Most think all aphids are green, but their colors range from pale yellow to dark, nearly black. Some aphids use waxy threads to obscure their body and are known as “woolly aphids.” The Adelgidae, a close cousin of aphids, are found on pine and spruce trees.
Like ants, most aphids, regardless of their form, are females. Aphid females produce genetically identical daughter aphids through parthenogenesis (asexual reproduction), which is common in invertebrates. Aphids become full-grown in about two weeks. Adults can have three to five young per day in the summer. At the end of the season, eggs hatch into males and females for reproduction. After mating, the females lay eggs that overwinter in crevices and branches until the following spring, and the life cycle resumes. Colonies become infestations in the right conditions, such as we are having in Denver this summer.
How Do You Control Aphids?
Most aphids do not hurt plants or trees and go mostly unnoticed. However, large tree infestations can be a nuisance because of the copious amounts of sticky honeydew covering everything below. Aphids in large numbers can also damage trees. Below are ways to control these insects.
- Jet Hosing
Jet hosing shrubs and trees wash off any aphids on plants that the water hits. Jet hosing injures aphids, and the ones knocked to the ground cannot find their way back onto their host plant.
- Dormant Oils
Dormant oils come from petroleum or cottonseed, safflower, and soybean oils and kill aphid eggs on fruit trees, Aspen/Poplars, Willows, Pines, Lindens, Maples, Oaks, Hawthorn, and Viburnum in the winter. They are effective against other soft-bodied insects that overwinter in the egg stage. Dormant oils do not work on insects that do not overwinter as eggs, such as the Ash Leafcurling Aphid.
- Insecticidal Soaps and Pyrethrins
Non-persistent contact insecticides do not last long on plant foliage and only kill insects that come in contact with the product. Insecticidal soap is made from potassium salts of fatty acids and is effective on soft-bodied insects such as mealybugs, scales, and aphids. These soaps do not harm plants but penetrate insects’ cuticles, causing cell collapse and desiccation. Pyrethrins are natural insecticides found in chrysanthemum flowers. The insecticide excites the nervous system of insects leading to paralysis and death. Since these compounds do not persist, they pose a minimal hazard to pollinators and natural enemies of aphids. They have low toxicity to mammals and birds.
- Contact Insecticide Sprays
We do not recommend using general insecticides because they kill the natural enemies of aphids and pollinators. Do not apply persistent insecticides when plants bloom to avoid harming bees.
- Systemic Insecticide Sprays
Systemic products move into a tree using the vascular system to spread protection to all parts of the plant. These products provide better control of aphids than conventional spray insecticides. Acetamiprid is less toxic to bees and is effective for two weeks after application.
- Soil-Applied Systemics
Imidacloprid applied to the soil is picked up by plant roots and moved through the plant, concentrating in the newer leaves. It takes about two weeks until the product protects the tree. Imidacloprid does kill bees, so do not apply it when plants are flowering or soon to be flowering.
Aphids Found in Denver
Aphids tend to be tree specific. Several species of aphids may attack the same tree targeting different tree sections. Below is a list of aphids found in Colorado.
|Name||Scientific Name||Trees Affected|
|American Popular Leaf Aphid||Chaitophorus populicola||Cottonwood, Aspen, and Silver Popular|
Speckled Popular Aphid
Cottonwood, Aspen, and Silver Popular
|Giant Conifer Aphids||Cindara||Pines, Juniper, Spruce|
|Woolly Apple Aphid||Eriosoma lanigerum||Elm, Apple, Crabapple|
|Monterey Pine Needle Aphid||Essigella||Pines|
|Pine Needle Aphid||Eulachnus||Pines|
|Leafcurl Ash Aphid||Meliarhizophagous fraxinifolii||Green Ash|
|American Walnut Aphid||Monellia caryae||Walnut|
|Linden Aphid||Myzocallis tiliae||Linden|
|Western Dusky-Winged Oak Aphid||Myzocallis alhambra||Bur Oak|
|Norway Maple Aphid||Periphyllus lyropictus||Norway Maple|
|Woolly Beech Aphid||Prociphilus fagi||Beech|
|Reddish Brown Willow Bark Aphid||Pterocomma bicolor||Willows|
|Woolly Elm Aphid||Eriosoma amiercanum||Elm, Serviceberry|