The photo below is of Witch-hazel cone gall, caused by an aphid bearing the same name. I find insects that cause galls to be really interesting. There are many different types, usually host-specific, with various shapes and sizes of galls formed. In most instances, the feeding of the insect causes an abnormal growth reaction in the plant that forms this gall where either the adult or the immature insect lives inside. In this case, a single aphid will feed on the leaf in spring, causing the gall to form. While inside the gall, the female aphid will produce young that eventually emerge in two forms, with two destinies: A wingless form stays on the host plant; the winged form goes to live on Birch trees. In autumn a second winged generation develops and flies back to the witch-hazel where eggs are laid on the twigs to start the process again the following spring.
When we see galls on leaves and twigs, do we stop to think about the lives of these insects, and the plants that support them? This world we share is much larger than we realize – it gets bigger the more we look up, or down, and we’re all in this together.
Your friendly neighborhood arborist,
ISA BCMA® OH-5129B
Insect notes summarized from Insects That Feed on Trees and Shrubs, by Warren T. Johnson and Howard H. Lyon, second edition.