I have just taken over management of the small landscaped yard for my condominium and we have two trees (weeping birches I’ve been told) in the front that appear to have been infested with aphids. The trees are about 15 feet tall and are located between the building and the sidewalk to the entrance. They have southern exposure. There’s a few evergreen bushes around the trees, no grass.
I am not familiar with aphid controls, so have done some internet research, including your very useful
site. We want to avoid using pesticides, so from what I’ve read, the best control is insecticidal soap.
Before I try to spray this on the trees I have a few questions I was hoping you could answer.
1. Can you verify that this is aphid damage?
2. It seems to me that the amount of white material on the undersides of the leaves has decreased in the last month. Given that it is getting late in the growing season, is it still worth treating the trees?
3. Does insecticidal soap seem like a good treatment in this situation, and if so do you have any application tips to make sure the undersides of the leaves are treated?
4. Do you have any recommendations for preventative actions to decrease the impact of aphids on these trees in the future?
Birches are commonly afflicted with aphids, and the aphids suck sap and secrete honeydew, which can be a nuisance, and is usually why homeowners contact us. Unfortunately, if your birches are overhanging a sidewalk, it is probably getting sticky from the honeydew. Otherwise, you could probably ignore the problem (except in the most severe infestations).
You can try spraying the aphids off the leaves with a strong jet of water. You can also encourage natural predators. Avoid over-fertilizing, or exposing the trees to lawn fertilizer, for example, as this will lead to succulent new growth which attracts aphids. Make sure the trees are not under any stress, as aphids are more likely to feed on a weakened
tree. You may be able to avoid using the insecticidal soap as a control. If you do use it, you are correct that you need to reach all leaf surfaces, which is labor-intensive. Some of these soaps can cause damage, so it is always a good idea to test any such spray on a small area before coating the whole plant. An article by Colorado State University Extension provides information on insecticidal soaps. Aphids go through many generations in a year, and their eggs can overwinter.
Toxic-Free Future (formerly known as Washington Toxics Coalition) has created a document on managing aphids in the landscape.
Here are additional links on aphid control:
Aphids from University of California at Davis
Managing Aphid Problems without Pesticides from the Northwest Coalition for Alternatives to Pesticides