I have a cotton wood tree. On the stem of the leaf there is a nest looking thing growing with tiny white looking mites or spiders. How do I get rid of them? They are all over the leaves.
The Gardener’s Answer
Hello, Elizabeth in California: It is difficult to give you a positive diagnosis without being able to see your tree, but from what you have described it sounds like your cottonwood tree (Populus fremontii) may be infected with aphids that form galls on the petiole. The petiole by definition is exactly what you have described as the stalk that connects the leaf to the stem. Fortunately, these insects do not typically harm the overall health of the tree but will cause premature leaf drop, and the aphids secrete a sticky substance known as honeydew that can be a nuisance but will not kill your tree. These insects are most populated during the late summer/early fall and can be controlled by spraying a dormant oil next spring before bud break. For now, you should make sure to clean up all fallen leaves to ensure they do not over-winter and attack your tree again next year. It is best to have a certified arborist come out and give you a positive diagnosis or you can always take a sample to your County Cooperative Extension Office to have the horticulture agent take a look.