Mossy rose gall is the result of two living organisms influencing each other. A small gall wasp lays eggs where the plant would normally grow a leaf or bud. A chemical reaction between the plant and the feeding residue of the larval wasps causes the normal leaf tissue to form the gall instead, which closes around the larvae and provides protection and also nutrients for the growing wasps. Up to 50 larval wasps will spend the winter in the gall before emerging as adults between May and August.
In some cases, birds and other small animals may enjoy tearing or drilling into the galls to get at the young wasps inside for a tasty winter snack.
They don’t generally cause any ill-effects to the plant, although they are more common in young or stressed plants. In the case of these roses at the south end of the Cheasty Woods, they’ve supported robust gall populations for several years and seem to be doing fine.