Inquilines of the genus Periclistus Förster (Hymenoptera: Cynipidae) have lost the ability to induce their own galls, and are obligatorily dependent on completing their development within galls of Diplolepis. Periclistus induce gall tissues of their own from the tissues of the galls they inhabit ... Feeding by Periclistus causes each [Periclistus] larva to be surrounded within its own chamber, and as a result the inquiline-modified galls are structurally different from normal galls ...

Rosa spp.
Stem swellings
Abrupt stem swelling. Fig 204. Only guests have been reared from it.

204. From this very common stem gall on rose no maker has been reared. The guest fly, Periclistus sp. emerged Mar. 17 to Apr. 17. At Santa Margarita on Dec. 15 galls like the photo were found and along with them was a slight twig swelling containing one larval cell and a full-grown larva. This may be the normal structure which guests have modified into the woody polythalamous galls of the photo.