Today’s images come to us from Suzan D. Suzan posted the photos for identification on the UBC Botanical Garden forums in 2004. She cleverly called these “woses.” Original post.
Although the structure in the photo looks very much like a green rose, roses do not grow on willows. And while Suzan never found the insect, we believe this to be a gall caused by Rhabdophaga rosaria, the European rosette willow gall midge. Galls are abnormal growths found on plants that can be caused by parasites such as insects, mites, fungi or bacteria. Insects, particularly wasps and midges, are the most common cause of galls on plants. Plants form the galls in response to ovipositing or feeding by the insect, or from infection by another agent. The gall is an attempt to surround and isolate the invader. In the case of insects, the gall actually forms a protective chamber, where the larvae can develop safely away from predators.
Many types of galls exist. Many are shaped like balls or blisters. They are still unusual things to find in the garden, like this one from the UBCBG forums. In North America they are most commonly found on Fagaceae (oaks), but are also often found on Salicaceae (willows), Rosaceae (roses) and Asteraceae (asters). In general, they cause little damage to the plants and most are not considered to be significant pests.
Further reading on plant galls:
- Insect Galls, University of Florida Extension
- Gall-Making Insects – John A. Byers
- To Be Or Not To Be A Gall, Wayne’s Word http://waynesword.palomar.edu/pljuly99.htm