Globe Willow and Aphids

Question:

My three large globe willow trees were infested with tiny black bugs last year. These bugs were very tender and, if touched, left a reddish-colored spot. I was told that they were aphids. They covered the trees until the limbs were black with insects and a sticky sap covered the stems, leaves, and grass beneath the trees. In the late afternoon, they would be crawling on the concrete patio. I tried using a lot of different insecticides to get rid of them; it almost seemed that they were being nourished by these chemicals. I dread going through another summer with them. What can I do?

Answer:

Your identification is correct. Aphids are soft-bodied insects that will infest globe willow trees and many other plants. They produce the syrupy "honeydew" you described, and they can be hard to control. If you choose to use insecticides, you will need to make applications every few days to "clean up" any insects you missed. Even a few surviving insects will rapidly reproduce and repopulate the tree with aphids.

After three to five applications of insecticides, wait a while to see if the aphids reappear. Be sure to read and follow the directions on the insecticide label. As you have already observed, insecticides sometimes fail to totally eliminate the pests. There is an alternative to the insecticides that you can try. This non-chemical approach will also not totally eliminate the pests but may reduce their numbers sufficiently to make them a non-nuisance.

In this alternative treatment you will use strong jets of water and, if necessary, insecticidal soap. Use the soaps carefully because they can injure the leaves on your tree. By washing the insects from the tree frequently (blasting them out with strong jets of water from the garden hose), you will disrupt their feeding and reduce the production of the sticky honeydew.

Not using insecticides will allow the development of populations of predatory insects that make their "living" by eating aphids and other garden pests. We want to encourage these beneficial insects and not poison them with insecticides. Beneficial insects include ladybird beetles, syrphid flies, parasitoid wasps, lacewings, and others. There are many things that eat aphids; that's why aphids reproduce so rapidly.

Aphids also have "friends" in the insect world. Ants like to feed on the honeydew, so they often "herd" and protect aphids. To prevent the ants from interfering with the beneficial insects that eat aphids, you can put a band of sticky substance around the base of your trees. This may be a strip of old-fashioned fly-paper or other sticky material to keep the ants from climbing the tree to protect the aphids. The ants will become trapped in the sticky trap. After enough dead ants or dust and debris covers the sticky material, you will need to replace or renew the sticky surface.

Use of non-insecticidal methods of controlling aphids is often more effective because there are so many insects that feed on aphids. In addition, some of these "aphid eaters" are also pollinators for herbs, flowers, and vegetables. So, the good-guys are doubly good. Take care of the beneficials and they will take care of your aphid problem.

Also, please join us on Southwest Yard & Garden, a weekly program made for gardeners in the Southwest. It airs on KRWG in Las Cruces Saturdays at 11:30 a.m., repeating Thursdays at 1:00 p.m.; on KENW in Portales on Saturdays at 10:00 a.m.; and on KNME in Albuquerque on Saturdays at 9:30 a.m.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

Links:

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the Desert Blooms Facebook page.

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