Witch Hazel Cone Gall Aphids
We have one witch hazel shrub badly infested with cone gall aphids, although another 6-8 witch hazels in the vicinity have none. Is it possible that one shrub can act as a “sink” for the aphid infestation, or is it more likely that the next generations will spread to the other witch hazels, this season or next? The cones are much bigger now than when the attached picture was taken in early May. What is the best way of getting rid of them (Towson U campus)? Thank you for your help.
Baltimore County Maryland
The aphids that cause these galls (Hormaphis hamamelidis) have a very complicated life cycle, but part of it is spent on River Birch (Betula nigra). As River Birch is a native tree as well as being fairly common in landscaping, it is likely this alternate host is close enough to this aphid colony to be supporting them. Removal of either plant is impractical; for some reason, this particular witchhazel must be more susceptible if it is the only one with galls. These aphids can cause unsightly (or interesting, depending on your point of view) damage, but it is not serious to the plant's overall health and no controls are recommended. At most, you could try to treat the leafless witchhazel in winter with dormant oil to help smother any aphid eggs present.
Other than removing the foliage, which would be a drastic and possibly futile attempt at control (especially this late in the year for this pest), or removing the shrub entirely, you could see if this can become an educational opportunity for passers-by on campus. If this plant is in a prominent location and that is why there is concern over its appearance, someone could post a small sign explaining what is happening - it's a fascinating life cycle and biology students (especially entomology or plant science) might appreciate the information. As a native prey insect, it supports a range of natural enemies like lady beetles, lacewings, various parasitoids, and probably multiple birds.
Thank you, Miri. The life cycle is fascinating. We’ll look for the closest river birch! I’ve also forwarded your answer to a Biology professor so hopefully some students will want to see the cone gall aphids. I’m glad to know they’re nothing to worry about, but extremely interesting.