By Megan Gross, Colorado State University Cooperative Extension, Horticulture
A nuisance aphid has been covering the leaves of aspen, cottonwood, and willow trees with honeydew this summer.
The culprit is the clear winged aspen aphid Chairophorus populifoliae. It tends to peak from late summer through early fall. While this aphid usually doesn't impact the overall health of the tree, the honeydew certainly can be annoying.
Clear winged aspen aphids
Chairophorus over winters as eggs on its summer host plants, which include various species of aspen, cottonwood and willow. These eggs hatch in the spring and, if environmental conditions are favorable for the aphids (low predator populations, adequate moisture and moderate temperature), they will re-infest the foliage of aspen and cottonwood. To knock down the over-wintering egg population, you can apply an insecticide in late winter or early spring on a day when the temperature is above freezing.
A commonly used product is "dormant oil," available at most garden centers and hardware stores. The oil blocks air holes through which insects breathe, causing death by asphyxiation. Oils pose few risks to people or most desirable species, including beneficial natural enemies of aphids, such as lady bugs and green lacewings.
This type of oil is applied when plants are dormant during February or March on a day when temperatures are above freezing. Because oils act on contact, adequate coverage is essential for good control. Spray thoroughly from the branch tips back about 18 inches. This is where aphids overwinter. Always read and follow label instructions, where you will find more detailed information about how to apply the product.
Another oil based insecticide, horticulture oil, can be applied during the growing season on aphids that are out in the open, not curled up in leaves.
Photographs courtesy of Megan Gross.
© CSU/Denver County Extension Master Gardener 2010
888 E. Iliff Avenue, Denver, CO 80210
Date last revised: 01/05/2010