As described in the Field Guide to Plant Galls Of California and Other Western States,
galls are growths of rose tissue activated by outside organisms such as fungi, bacteria or insects. Galls are quite specific, and gall-causing insects are often so specialized that they inhabit only a few plant species. Different rose galls are found on the crown and roots (caused by bacteria); leaves (often caused by tiny wasps); buds; new shoots; and stems. Click the PHOTOS
tab for examples of bud, stem and leaf galls.
Genetic sequencing of insect-caused galls shows that the insects somehow trigger growths that are very close to specific plant structure. Some galls are flower- like, some are seed-like, some are like new shoots. But all are really an insect-stimulated structure to house tiny infant forms of the insect.
Fasciation, strange ribbon-like stems on plants, may be galls. Occasionally, roses do have fasciation on canes.
The most common cause of galls above ground are very small Cynipid wasps of the genus Diplolepis.
Researchers think cynipid wasps are flying or being blown into suburban and urban gardens from wild roses in rural areas. They are also accidentally introduced along with their host plants.
The wasps are so difficult to find and their galls are so specific to particular roses that they are easier to identify from their galls. For example, Diplolepis spinosissimae
is named for the gall caused on Sweetbrier or eglantine rose.
Galls cannot be cured. Except for crown gall and major infestations of the rugosa shoot gall, most rose galls are a curiosity. Insecticides are usually useless because cynipid wasps are rarely seen. The best approach is the careful removal of the galled plant parts in the case of cane and leaf galls.
From "Rosa rugosa Thunb.,"
: One of the most conspicuous rose-feeding insects in Europe is the cynipid gall wasp Diplolepis rosae, which produces the familiar bedeguar or Robin’s pincushion gall commonly found on the native European Rosa species, mainly belonging to sect. Caninae. It has not been recorded in the wild from R. rugosa or from the native species of Sect. Cinnamomeae, R. majalis, despite considerable search effort (Stille 1984, 1985). However, Schröder (1967) reported one instance of D. rosae forming galls on cultivated R. rugosa and R. majalis (Table 4). Another Eurasian cynipid species, D. centifoliae,
attacks R. rugosa in Russian Far East (Kovalev 1965), but this association has not been recorded from Europe. In its exotic range in North America, galls are formed on R. rugosa by the native gall wasps D. spinosa, D. polita, D. radicum and D. fulgens,
that usually attack R. acicularis, R. woodsii Lindl., R. blanda Ait. and other native Rosa species, many of which belong to sect. Cinnamomeae as does R. rugosa (Shorthouse 1988, 1994, 1998, 2001).